Revisiting the concept of sexual motivation

Revisiting the concept of sexual motivation

Pfaus, James G

Sexual motivation seems like a simple and straightforward concept. It can be defined broadly as the energizing force that generates our level of sexual interest at any given time. It drives our sexual fantasies, compels us to seek out, attend to, and evaluate sexual incentives, regulates our level of sexual arousal, and enables us to copulate or engage in other forms of sex play. We can predict that high levels of sexual motivation will lead to high levels of sexual interest, and vice versa. In fact, we cannot predict otherwise. As a concept, sexual motivation is often circular, being inferred from the strength of a behavior or fantasy. The concept runs into problems of interpretation if the behavioral assessments are incomplete. It also runs into problems of definition when it is equated with vague, outdated phrases, such as libido, or when it is used to define a set of behaviors that are somehow distinct from sexual performance (as if any sexual performance is not also motivated). But if everything sexual is sexually motivated, then what value is there in the concept? Cannot expressions of sexual behavior be understood equally well as a cascade of sensory, vascular, and motor events that interact, leading to various sexual endpoints?

The value of a concept like sexual motivation lies in its ability to provide a framework-a heuristic-within which sexual behaviors can be more easily organized, catalogued, and understood at various levels of analysis. We are beginning to quantify brain activation in response to sexual stimuli and to understand how different sexual behaviors are organized with respect to the activity of different neurochemical pathways and hormonal systems. The complicated interaction of neural substrates for specific sexual responses makes it difficult to keep the whole behavior in perspective. Those studying sexual behavior have also begun to incorporate modern concepts of incentive motivation (Berridge & Robinson, 1998; Stewart, 1995; Toates, 1992), information processing (Everaerd, 1995), comparative neurobiology (Everitt & Bancroft, 1991; Pfaus, 1996), and evolutionary psychology (Abramson & Pinkerton, 1995; Buss, 1994; Symons, 1979). Using these concepts, one may or may not make similar predictions about different sexual behaviors, and it is not immediately obvious how they interact. Thus, it seems appropriate to revisit sexual motivation: to see how it has evolved in the past century of scientific study and to consider where it might be going in the next.

A Brief History of Sexual Motivation

The evolution of sexual motivation as a concept parallels that of more general concepts in motivational theory, navigating between several dichotomies, such as the push of internal states, or drives, versus the pull of external stimuli, or incentives; the fractionation of behavior into appetitive versus consummatory phases; the determining role of biological versus social factors; the distinction between proximal versus ultimate causes of behavior; and the notion of different underlying sexual strategies, such as polygamy versus monogamy.

Antiquity to the Modern Era

Authors of ancient texts, such as Ovid’s Ars Amatoria or Vatsayana’s Kama Sutra, took for granted the fractionation of sexual behavior into precopulatory and copulatory components; both Ovid and Vatsayana devoted large portions of their work to explaining the separate but interactive arts of seduction and lovemaking. Each behavior was imbued with a generative internal state or drive that could be activated by distal external cues, such as the sight of a lover or a particular odor, proximal external cues, such as a particular touch or taste, or by completely internal events, such as an erotic dream or effect of a drug. The “art” of each component inferred a kind of linearity in which more sophisticated expressions could be learned and practiced, and each could be expressed for its own sake, suggesting that each resulted in an intrinsic degree of pleasure or reward. The art of seduction to Ovid also included methods of inducing sexual interest, for example, with arousing experiences like sexual teasing, exposure to certain stressors, or ingestion of certain drugs that act to increase blood flow to the genitalia. Likewise, Vatsayana’s “Force of Desire” could be induced by erotic visual stimulation, scents, tastes, touch, and even love quarrels. Notably, although lovemaking came from a biological drive to reproduce, it did not require that end as its goal. Both texts identified a need to build up sexual tension in order to reduce it: sexual pleasure-and motivation for that pleasure–existed in the build-up and in the reduction. Individuals were also imbued with different amounts of this motivation; thus sexual motivation was not envisioned as a fixed property, but rather as a fluctuating and highly variable part of a person’s temperament.

Likewise, the Taoist “Art of the Bedchamber” books dating from the Han Dynasty in China (206 BC to AD 24) included separate chapters on the spiritual significance of sexual ecstacy; the many ways of foreplay; descriptions of the sexual act, including specific techniques and positions for increasing pleasure; therapeutic aspects of sex; what to look for in a partner; and recipes for foods and herbs that could strengthen or weaken the sexual impulses (Tannahill, 1992). These latter chapters are rich sources of medicinal psychopharmacology, and the use of plants or plant extracts that act as central nervous systems (CNS) stimulants, sedatives, or depressants to treat disorders of sexual desire, arousal, and orgasm are often recommended. Although Taosim emphasized the balance of yin and yang, it also noted that the mind (typically a man’s mind) would grow weak if deprived of sex, or if sexual activity occurred with too few partners. The more sex, the better. In fact, bisexuality was fashionable during the Han Dynasty, with men and women allowed to express their “masculine” and “feminine” sexual selves. Although this contrasted with the prudishness that came with the advent of Confucian thought, polygamy continued to be practiced, and the notion of sex for the sake of pleasure and spiritual growth continued to be the norm.

Thus, ancient texts from several different cultures divided sexual behavior into precopulatory and copulatory components, and contained specific prescriptions for attaining or enhancing sexual pleasure. Integration and balance were the keys. Sex was seen as a diverse and interactive process that had an impact on social and personal aspects of a person’s life, which, in turn, fed back to alter sexual responding.

An increasing tendency toward biological determinism grew in Europe during the Middle Ages, Reformation, and Industrial Age, as the physical body came under scrutiny by medical scientists. By the 19th Century, rudimentary information about the anatomy and physiology of sex organs, gonadal secretions, and pregnancy was being compiled, and the Western world had begun to view sexual behavior as serving the goal of reproduction. This led naturally to the concept of a drive to reproduce, which produced in men a “need” to expel semen. However, sexual behavior that deviated from the goal of reproduction, such as masturbation (“self-pollution”), homosexual behavior (“sexual inversion”), bestiality, or fetishism was viewed as either the symptom or cause of a mental disorder (e.g., Morel’s [1857] notion of physical or mental degeneration from sexual “misbehavior” [cited in Ellis, 19151). Heterosexual behavior for pleasure rather than reproduction, as with prostitutes or mistresses, was frowned upon by polite society as immoral but was not generally viewed as a sign of mental depravity. Indeed, some texts of the day (e.g., the phrenologist O.S. Fowler’s Sexual Science [1870]), seemed surprisingly modern in according pleasure the role of immediate sexual goal, in contrast to reproduction which served as the ultimate goal. Thus, although sexual motivation compelled individuals to copulate with the opposite sex, pleasure was retained by many thinkers as a driving force behind the motivation.

The Modern Era

The German physician Albert Moll (1897/1933) is credited with being the first modern sexologist to envision a dual system of sexual motivation (Everaerd, 1995). Moll conceived of two separate impulses, detumescence, the drive to relieve sexual tension, and contrectation, the instinct to approach, touch, and kiss a sexual incentive. These impulses could be directed at any sexual incentive that the individual found appropriate, and both were imbued with distinctive types of sexual pleasure. This dichotomy was strikingly similar to that in the emerging ethological literature between preparatory behaviors that bring animals into contact with goals and consummatory behaviors that animals display once contact is made (Craig, 1918; Woodworth, 1918). However, the theoretical line that divided these classes or phases of behavior was relatively thin, if it existed at all, Regarding Moll’s theory, although the impulse for detumescence could be viewed unambiguously as consummatory (presuming that blood has already engorged erectile tissues, creating a sexual “tension” that demands “release”), the impulse of contrectation did not fit easily into one of the two phases of behavior. By definition, touching and kissing a sexual incentive are consummatory behaviors; however, the impulse to do so could also be considered appetitive if it created an intention or motivation for the individual to approach a sexual incentive prior to any increase in sexual arousal. Thus, the preparatory or consummatory nature of contrectation depended on the absence, or presence, respectively, of a sexual incentive.

Perhaps the most influential “theory” of sexual motivation in the early modern period was promoted by Sigmund Freud in several works, notably Beyond the Pleasure Principle (1922) and The Ego and the Id (1927). These works spelled out Freud’s notion that pleasure is at the root of all human behavior and that its control by individuals or society forms the basis of all neuroses. It is a “biological” meta-theory, in that pleasure was envisioned to be controlled by biological drives connected to important, pleasure-giving erogenous zones of the body, and at the mercy of three cognitive meta-processes, the Id, Ego, and Superego. Two of these erogenous zones, the anogenital and orificial areas, were considered critical during infant development and remain important throughout the lifespan. During infancy, pleasure was viewed as diffuse and primary, with tension (e.g., from hunger) being relieved by goaldirected movements that led to successful consummation (e.g., suckling a nutrient source). Notably, suckling could be done for its own sake: The very act of doing so was innately pleasurable. This was also true of touching the anogenital area. Unfortunately, Freud chose the term infantile instead of hedonic to refer to this kind of pleasure (Jastrow, 1948), a misnomer that led to much political debate and little scientific progress (the notion of innate or instinctual pleasure developed as a truly scientific theory with the ethological work of Konrad Lorenz (e.g., Lorenz, 1950). The sexual drive, or “libido” was considered a primary biological drive for pleasure that centered in the genitalia. Masturbation and other forms of infantile autoeroticism were viewed as undergoing a transition to more “adult” forms of heterosexual behavior (e.g., intercourse) as society molded and constrained the Id’s version of a polymorphously perverse libido into a socially acceptable form, modulated by the Ego, against the self-restrictions imposed by the Superego. Thus, sex and social control over sex existed in a balance which was upset every time an individual was confronted by sexual incentives. Whether the Id or Ego won the battle depended on how strong the inhibition from the Superego was.

The notion of a biological system that facilitated sexual responding (Id) or led to its inhibition (Superego) was taken a step further by Wilhelm Reich, a psychoanalyst who broke early with Freud. In his book, The Function of the Orgasm (1942/1978), Reich proposed that a biological energy, called orgone, was created with sexual tension and released during orgasm. The inability to release this energy through its normal biological route led individuals to adopt neurotic behavioral styles (e.g., obsessive-compulsive behaviors, obedience to authority), and to take it out in other forms, often violently. Reich envisioned a sequence (1938/1979) in which mechanical tension (of the muscles) led to bioelectric charge (of the brain) which led to bioelectric discharge (of orgone), that resulted in mechanical relaxation. Reich (1945/1982) also envisioned that sexual arousal and anxiety were central antitheses of one another, although both had certain autonomic responses, such as increased sympathetic activation, in common. It was therefore easy for anxiety to take the place of sexuality, because they shared somatic and autonomic components. Thus, in Reich’s view, sex served a deep biological need for tension reduction. Pleasure was naturally associated with the reduction of sexual tension, but also in the generation of it. Sexual inhibition, imposed by the adoption of societal norms for sexual conduct, led to neurotic behaviors, anxiety, and violence.

The idea that there is a biological “need” for sex (determined either by an ultimate need for genetic mutation and greater fitness, or by a proximal need to reduce the intensity of a sexual drive or tension) was, and remains, fraught with controversy. Sex has been regarded by many physiological psychologists as a primary drive, often equated in animals with hunger or thirst (e.g., Milner, 1970; Stellar, 1954). However, this notion was challenged by others, including Ellis (1915, 1933), Krafft– Ebing (1929), and Beach (1956). No tissue damage has been observed with sexual abstinence, nor does engaging in sexual activity necessarily maintain physical well-being. Sexual desire does not necessarily disappear in hypogonadal individuals or following castration (especially in primates). Homosexual copulation does not result in propagation of the species, and heterosexual copulation is often observed in certain primates and humans at times when reproduction is either impossible or unlikely. Neither masturbation nor paraphilic sexual behaviors serve a reproductive end, yet they occur with a relatively high frequency in different human societies. The notion of a build-up of “tension” during sexual arousal and its “release” during orgasm has never been demonstrated scientifically, although sympathetic, parasympathetic, and hormonal components of arousal and orgasm have been described. Beach (1956) noted that, although sexual behavior is indispensable for a species, it is not indispensable for an individual (“no one ever died for the lack of sex” [p. 31). Moreover, he argued that much of what was considered a primary drive for reproduction was actually a reflection of sexual appetite based on experience with the incentive characteristics of potential mates, partners, or other erotic stimuli. The Kinsey reports on the sexual behavior of human males (Kinsey, Pomeroy, & Martin, 1948) and females (Kinsey, Pomeroy, Martin, & Gebhard, 1953) reinforced the idea that sexual appetite had a wide range of expression in humans that deviated dramatically from sex for the sake of reproduction. Moreover, ethological studies of primate sexual behavior have revealed nonreproductive roles of sex in social control (e.g., de Waal, 1987). Thus, sexual behavior can serve a variety of social and physiological functions ranging from reward to reproduction. Models of sexual motivation must therefore take these underlying “causes” into account.

Models of Sexual Behavior

Three types of models of sexual behavior have emerged in the modern era. The first type consists of models of sexual function and dysfunction that specify the relationships among different physiological processes that underlie sexual arousal or copulation. The second are models of sexual responsiveness that specify the relationship between external sexual incentives and the physiological or psychological processes that they activate, such as desire, excitement, and arousal. The third are models that attempt to unite the two. To a large extent, these models overlap such that the real difference between them is the focus, either on internal events in the first type, external events in the second type, or some hierarchy of the two in the third type. Orthogonal to this is the data source, usually being reconstructions of human or animal sexual behaviors. However, models of human sexual behavior have been derived from clinical case studies, anecdotal reports, and questionnaire-based retrospectives, whereas models of animal sexual behavior have been based on direct observation of precopulatory and copulatory behaviors in laboratory and field settings. Thus, we have more direct information from animals regarding the motivation to obtain a sexual incentive, to initiate copulation, to regulate the amount of sensory stimulation received during copulation, and to avoid or terminate copulation, than we have from humans. Although human sexual behavior is best studied in humans, it remains impossible to do so using the same experimental rigor that we can apply to animals (Barfield, 1993).

One of the first modern models of sexual behavior was proposed by Beach (1956), who used the intercorrelations among consummatory measures of male rat copulatory behavior to derive a two-factor theory of male sexual motivation. In this conceptualization, copulatory behavior was initiated by a Sexual Arousal Mechanism (SAM) that increased the male’s physiological and psychological excitement in the presence of a sexual incentive such that a copulatory threshold was attained. Copulation was then maintained by an Intromission and Ejaculation Mechanism JEM) that further modified the male’s internal state such that an ejaculatory threshold was achieved. Ejaculation, in turn, fed back to inhibit both the SAM and IEM. Subsequently, Sachs (1978), Dewsbury (1979), and Pfaus, Mendelson, and Phillips (1990) used factor analysis of multiple correlations of male rat copulatory behavior to examine the structure of copulation (i.e., during the IEM). These studies revealed that copulation could be broken down further into conceptually distinct factors that reflected its initiation and rate, along with the amount of genital stimulation received prior to ejaculation. The similarity among the behavioral composition of these factors supported Beach’s contention that the IEM was relatively stable in males.

Beach (1956) also noted that the IEM was relatively stereotyped and species-specific in terms of the pattern of behaviors that it elicited (e.g., multiple mounts and vaginal intromissions in the male rat compared with continuous intromission in the male human), whereas the SAM was relatively labile and subject to the influence of learning. Other studies have examined more appetitive aspects of sexual behavior in rats and primates, especially sexual approach behaviors. Operant responses, such as crossing electrified grids (Moss, 1924), climbing over obstruction boxes (Warner, 1927), straight-alley running (Sheffield, Wulff, & Backer, 1951), bar-pressing (Everitt, Fray, Kostarczyk, Taylor, & Stacey, 1987; Jowaisas, Taylor, Dewsbury, & Malagodi, 1971; Sachs, Macaione, & Fegy, 1974; Schwartz, 1956), and maze learning (Hetta & Meyerson, 1978; Hull et al., 1991; Moses, Loucks, Watson, Matuszewich, & Hull, 1995; Warner et al., 1991; Whalen, 1961) were all studied in male and female rats using copulation with a sexually receptive or vigorous partner as the positive reinforcer. Although those behaviors were not copulatory per se (and indeed could be observed for other reinforcers, like food or water), it can be argued that the use of a sexual reward made them conditionally sexual. Everitt et al. (1987) took this a step further by training male rats to bar press for a secondorder stimulus (a small light within a testing chamber) that could be presented discretely for small amounts of time. Importantly, rats adjusted their behavior up or down depending on the schedule of second-order reinforcement, something they could not do bar-pressing for the female alone because she cannot be presented in discrete units. Thus, animals could learn to perform a vast array of tasks to obtain a primary sexual incentive (the actual partner) or a second-order sexual incentive (an object of learned sexual significance).

Animals can also learn Pavlovian associations between neutral objects and sexual incentives, such that the neutral objects become conditionally sexual and lead to conditioned sexual excitement and arousal. For example, male Japanese quail learn to associate buzzers or lights (the conditioned stimulus or CS) with the presentation of a receptive female (the unconditioned stimulus, or UCS) and will eventually display courtship behaviors in response to the CS alone (Domjan, Lyons, North, & Bruell, 1986; Farris, 1967). The same stimuli previously paired with copulation will also decrease the latency to initiate copulation in male quail (Domjan et al., 1986; Domjan, O’Vary, & Greene, 1988; Koksal, Domjan, & Weisman, 1994). Adult male rats can learn to associate neutral odors (almond, lemon) with copulation and will choose to ejaculate preferentially with receptive females that bear those odors (Kippin & Pfaus, 1999; Kippin, Samaha, & Pfaus, 1999; Kippen, Talianakis, Schattmann, Bartholomew, & Pfaus, 1998). A neutral odor (methyl salicylate, or wintergreen) paired with copulation can also increase serum testosterone and luteinizing hormone levels in male rats (Graham & Desjardins, 1980). Rats can also show sexual excitement in anticipation of receiving a sexual incentive. Male rats learn to run from level to level in bilevel chambers in anticipation of receiving access to a sexually receptive female (Mendelson & Pfaus, 1989; Pfaus et al., 1990). This level-changing behavior is enhanced by unconditioned olfactory cues present in the chamber, although in some strains of rat (e.g., Wistar rats), the olfactory cues are necessary for the expression of appetitive level changing (van Furth & van Ree, 1996). Finally, male rats can learn to associate environments and neutral tactile stimuli with copulation, such that presentation of the stimuli increase sexual arousal and lead to faster ejaculations (Zamble, Hadad, Mitchell, & Cutmore, 1985; Zamble, Mitchell, & Findlay, 1986), or result in conditioned place preferences for the environments associated with copulation (Agmo & Berenfeld, 1990; Hughes, Herbert, & Everitt, 1990; Mehrara & Baum, 1990; Miller & Baum, 1987; Paredes & Alonso, 1997). Indeed, male rats display more frequent female-directed behavior and initiate copulation faster in environments associated with the (presumably positive) reinforcing effect of opioid agonists, like morphine, compared with environments associated with saline injections (Mitchell & Stewart, 1990).

A decade after Beach, Masters and Johnson (1966) presented a physiological model of human sexual response. Their EPOR model emphasized a cascade of sexual responses that formed around the build-up and release of sexual excitement: a steep increase in sexual excitement (E) during sexual desire and arousal, to a less-steep plateau (P) during actual sexual stimulation, to the abrupt release of sexual excitement or tension during orgasm (O), to a prolonged period of relaxation known as resolution (R). This model applied to the sexual responses of both men and women, although individual differences existed in the actual response patterns, especially concerning the ability of women to experience multiple orgasms.

The models proposed by Beach (1956) and Masters and Johnson (1966) were useful because they specified the relationship and direction of different physiological mechanisms that generated both excitatory and inhibitory aspects of consummatory sexual behavior. However, with few exceptions, these models did not delineate the relationship between appetitive and consummatory components of sexual behavior. For example, according to Masters and Johnson, desire was anything that increased sexual excitement, with erection being its most obvious and unambiguous behavioral manifestation. Although Beach accepted the notion that sexual drive could be inferred from some learned appetitive responses (e.g., the running speed of male rats in an alleyway that leads to a sexually receptive female), he doubted the validity of other appetitive responses (e.g., crossing electrified grids to obtain a sex partner) if they did not correlate with copulatory measures or were not diminished by castration.

Other theories of sexual behavior, especially human sexual behavior, emphasized the role of acculturation and experience (Geer & O’Donohue, 1987). For example, Margaret Mead (1949) noted the diverse ways that precopulatory and copulatory activity is initiated by men and women across several different cultures and discussed many social endpoints, not only sexual ones, that such behaviors serve in those societies. Hardy (1964) proposed an appetitional theory of sexual motivation in which he de-emphasized certain biological mechanisms, especially hormonal determinants, in favor of cognitive-affective experiences. To Hardy, sexual behavior served a greater goal of hedonic reward, which could be achieved in a variety of culturally determined sexual circumstances, from making a necessary instrumental response to obtain a sexual incentive to genital stimulation and orgasm. Hardy emphasized that sexual motivation is based on learned expectations of affective (hedonic) change. Such expectations were aroused by real external incentives, or imagined internal fantasies, that had been associated by experience with some form of sexual gratification. Sexual motivations were categorized into approach, avoidant, and ambivalent, which reflected positive, negative, or mixed affect concerning a particular sexual fantasy, incentive, or behavior. Hardy also noted that a degree of habituation could occur to sexual stimuli in each category, such that the initial experience was usually more intense than subsequent experiences. Finally, Hardy argued that sexual incentives that are more immediate will be considered more attractive and responded to with greater vigor than those more distant. However, these aspects applied to human sexual behavior, not necessarily animal sexual behavior. Hardy believed strongly that extrapolation from animal studies to humans should be limited in light of interspecies differences in the expression of sexual behavior, and because of the relative independence of human sexual response (as it was conceived of in the early 60s) on genetic or endocrine status compared to experiential factors.

Whalen (1966) attempted to synthesize elements of Hardy (1964) and Beach (1956). He argued that sexual motivation in animals and humans could be reduced to common components that consisted of both hormonal and experiential factors. Following Winoker (1963), Whalen defined six basic elements of sexual behavior: (a) Sexual Identification, or the gender role played by an individual; (b) Object Choice, or the persons or things that an individual directs sexual attention to; (c) Sexual Gratification, or the reward or pleasure derived from sexual activities; (d) Sexual Arousal, or the momentary level of sexual excitation; (e) Sexual Arousability, or an individual’s characteristic rate of approach to orgasm as a result of sexual stimulation; and (f) Sexual Activity, as sexual behavior observed directly or inferred from the content of fantasies. Whalen regarded sexual arousal and arousability as being unique indicators of sexual motivation, with arousal as a transient state between nothing and orgasm, and arousability as a more solid physiological feature of each individual (similar perhaps to Ovid’s notion of temperament). For example, as sexual arousal increases along a linear dimension, some individuals may achieve orgasm faster than others. An individual who needs less stimulation to achieve orgasm was said to have more arousability than someone who needs more stimulation to achieve orgasm. Knowledge of the level of arousal required for orgasm was therefore crucial in this analysis, both to provide more accurate estimates of arousability and to keep the entire notion from being circular. Whalen provided some animal and human evidence that arousal and arousability decay at different rates after ejaculation, supporting the contention that these two elements are distinct. Most importantly, the level of arousal was determined by external factors, such as unconditioned or conditioned incentives and expectancies, whereas arousability was determined by internal factors, such as hormonal activity and genitosensory feedback. However, arousability could also be conditioned by experience, especially the pleasure produced by genitosensory stimulation (similar perhaps to the mechanisms by which neutral odors paired with copulation can come to elicit increases in serum androgen levels (e.g., Graham & Desjardins, 1980). Whalen did not believe that inferences about sexual motivation could be made on the basis of sexual identification or object choice because these aspects of sexuality were theoretically independent of underlying motivational variables. For example, a person’s choice of hair color, body shape, or gender as components of sexual attraction does not allow any inference about level of arousability or frequency of sexual contact. Object choice also varies across cultures or even experiments: simply because dogs that have been masturbated by an experimenter will subsequently focus appetitive sexual behavior toward that experimenter (Beach, 1950) does not mean that their more general motivation to obtain sexual gratification has been altered.

Like the models proposed by Beach, Hardy, and Whalen (and also the more general model of affect proposed by William James [1890]), Byrne’s Sexual Behavior Sequence model (1977) and Barlow’s Working Model of Sexual Dysfunction (1986) specified an interactive role for both unconditioned sexual stimuli, especially autonomic activation, and conditioned sexual stimuli in the generation of sexual arousal, affect, and cognition. Both authors believed that arousal, affect, and cognition could shape particular sexual responses based on excitatory or inhibitory feedback. For example, in the case of arousal, genital stimulation could enhance, but ejaculation inhibit, genital blood flow. The degree of affective processing of this behavior (e.g., as appetitive or aversive) would feed back to shape the rate of further blood flow. A dimension of erotophobia-erotophilia was also proposed by Byrne from which he attempted to predict the degree of sexual responding to erotic cues. For example, an individual of high erotophilia could have a high degree of subjective sexual arousal that would lead to highly positive reactions to sexual stimulation. Actual sexual stimulation, in turn, could feed forward to increase subjective sexual arousal further. Thus, a complex set of feedback systems in sexual behavior existed to allow arousal, affect, and cognition to interact in generating a net sexual response to any given stimulus.

Barlow (1986) elucidated five factors or dimensions that differentiated sexually functional from dysfunctional men. First, an affective factor existed along a positive to negative dimension in which sexually functional individuals displayed positive affect to sexually arousing stimuli, whereas sexually dysfunctional individuals displayed more negative affect. Second, an arousal factor existed in which sexually functional individuals experience more subjective arousal and control over their arousal than do sexually dysfunctional individuals. Third, the demand characteristics of sexual performance-related stimuli distracted sexually dysfunctional individuals, but enhanced the arousal of sexually functional individuals. Fourth, the demand characteristics of nonsexual performance-related stimuli had the opposite effect of distracting sexually functional individuals but having no effect on sexually dysfunctional individuals. The fifth factor, anxiety, could arouse sexually functional men, but inhibit arousal in sexually dysfunctional men. In Barlow’s model, the perception of either internal or external sexual stimulation initiated a process of increased attention on sexually arousing stimuli and evaluation as either rewarding (in the case of functional men) or aversive (in the case of dysfunctional men). These evaluations then fed back to either increase or decrease, respectively, autonomic arousal. Ongoing feedback was critical in this model, as continued attention to sexual cues would increase arousal and initiate sexual approach, whereas continued attention to nonsexual cues would decrease arousal, and/or lead to aversive responses.

Bancroft (1989) published his theory of a Psychosomatic Circle of Sex. Individual feedback loops were specified and linked together along a sequence of five interactive dimensions that relay information processing throughout the central and autonomic nervous systems. The first two dimensions involve the brain: (a) cognitive processes in the cortex which influence (b) emotional processes in limbic and hypothalamic systems. In this case, cognitions span a range of conditioned responses, including cultural beliefs and attitudes, attentional mechanisms, learned sexual incentives, and learned inhibition over sexual responding. The affective dimension came from the interaction of cortical structures with limbic/hypothalamic structures. Together, these systems influence the spinal cord and autonomic processes related to the mechanics of sexual response: (c) sexual reflex centers in the spinal cord; (d) parasympathetic genital responses such as erection; and (e) other sympathetic autonomic responses consistent with sexual arousal (e.g., increased heart rate). In the models of Byrne, Barlow, and Bancroft, the perception of arousal (genital and peripheral) was sufficient to initiate awareness of a sexually motivated state. The positive or negative outcome of that state was then dependent on interpretation from cognitive feedback.

Toates (1992) proposed a similar kind of interactive system for male sexual behavior based on an incentive systems model (Toates & O’Rourke, 1978). The stimulation of sexual arousal depends on both the value of sensory information coming from the external sexual incentive and the state of arousability of the nervous system, a value that is set by both hormone action on target tissues in the brain and periphery, and any positive feedback from the system that modulates incentive value. In a systems analysis, several “comparators” exist to determine incentive value based on the memory of previous interactions with similar incentives, or to determine the level of sexual arousability. Both comparators can feed back on one another to increase or decrease net arousal. An ejaculatory mechanism for males was also proposed that activates an inhibitory feedback system. In turn, this system reduces arousability two ways, by inhibiting arousal and by devaluing the incentive. Interestingly, Toates took for granted that sexual motivation serves the goal of fertilization (p. 97) and did not raise the issue of sexual pleasure or nonreproductive sexual behavior in primates or other animals.

An important aspect of systems analysis is information processing from different modalities. Current sensory information (both external and internal) must be compared to previous experience at many levels of analysis, and ongoing changes in experience must register immediately so that the requisite excitatory or inhibitory neural systems can be activated. The processing and evaluation of sexual incentives, fantasies, sex play, and tactile sexual stimulation occur almost simultaneously at cognitive, emotional, endocrine, and spinal levels. Everaerd, Geer, and their colleagues (Dekker & Everaerd, 1988; Everaerd, 1995; Geer & Fuhr, 1976; Geer & Head, 1990; Geer, Lapour, & Jackson, 1993; Geer & McGlone, 1990; Janssen & Everaerd, 1993; Laan, Everaerd, van der Velde, & Geer, 1995) have applied hypotheses from information processing theory, particularly those related to attention, encoding and identification, response selection, and response production, to the study of sexual arousal. As in other systems analyses, these processes are thought to interact throughout a sexual experience. The ability to attend to features of a sexual stimulus requires both innate and conditional mechanisms that can select them from an array of other external or tactile stimuli. Encoding and identifying sexual stimuli are viewed as a second layer of processing. Some stimuli may be perceived at a subconscious level until they reach a critical threshold for the activation of sexual arousal or excitement, after which they are regarded consciously as sexual. Such encoding and identification mechanisms may be innate and pre-programmed, as with a male rat attending to the odors of an estrous female or a sexually functional human male experiencing erection while being stimulated by an attractive partner, whereas cognitive mechanisms must also exist to allow conditioned stimuli (e.g., neutral odors associated with copulation, fetish objects, etc.) to elicit a threshold sexual response. Encoding also requires memory for both Pavlovian and operant associations. This creates an economy of behavior, such that less and less attention is required for a stimulus to be perceived. Some of this may take the form of subliminal priming, which then feeds back to focus the attentional mechanisms more exclusively on sexual stimuli. However, stimuli may be encoded several ways depending on experience. For example, if an individual’s social conditioning has been negative or aversive with regard to sexual expressions, a sexual stimulus may evoke feelings of anxiety rather than feelings of arousal. Finally, experience in response selection and production allows sexual responses to be determined more quickly or efficiently in a given situation. Experience with efficient sexual responding then can feed back to facilitate ongoing sexual behavior. Although the incorporation of concepts from information processing to sexual motivation is useful theoretically, specific mechanisms, such as distractors that facilitate or inhibit attention, evaluation, or behavioral reaction to sexual stimulation, have only begun to be tested.

Where Are We Now?

At the end of the 20th century, we find ourselves with more information about sex than ever before, but with a chaotic framework within which to place what we know. We understand much about reproductive physiology and endocrinology. We know that there are as many sexual preferences as there are people and that humans sometimes find themselves caught sexually between curiosity and conformity. We know that sexual behavior for humans and other animals is influenced or regulated by hormones and environmental circumstances. We know that the value of sexual incentives can vary as a function of hormonal milieu, environmental circumstance, sensory processing, and feedback from sexual stimulation, arousal, and anxiety. We know that neutral stimuli associated with sexual stimulation can come to elicit sexual interest conditionally. We know that humans and other animals will perform all sorts of operant tasks to obtain a sexual partner. We know that humans and other animals can learn to inhibit their sexual interest. We know that different classes of drug can amplify both excitatory and inhibitory processes that underlie sexual excitement, desire, arousal, and performance, thus giving us a window into the neurochemistry of sex (Pfaus & Everitt, 1995). We assume that sexual behavior reflects deep processes of evolution that have molded strategies for successful reproduction in different animals. We also assume that sexual responding is rewarding, either intrinsically or for its secondary social reinforcement properties. We believe that our sexual preferences are immutable, having a hard-wired genetic basis in the brain, and/or bias determined by early sexual experience (e.g., LeVay & Hamer, 1994; Storms, 1981). We have made great strides in understanding the psychophysiology of sexual arousal in both men and women. In fact, because the physiology of erection and ejaculation is similar in many male mammals, we are able to make inferences about drug effects on measures of sexual arousal in rats that have predictive validity in humans (Barfield, 1993; Everitt & Bancroft, 1991; Meisel & Sachs, 1994; Pfaus, 1996).

What don’t we know? We still know very little about orgasm. We have no agreed-upon measures of orgasm. It is unclear if orgasm in human males is a separate process that co-occurs with ejaculation. We do not know the role(s) that it might serve in human females, nor do we have anything more than hypotheses about its evolution. We do not yet understand how it acts to inhibit sexual desire or arousal, other than rudimentary actions in the autonomic nervous system related to ejaculation and muscle relaxation. Likewise, we understand very little about sexual desire. Is it motivation? How is it related to arousal, arousability, and performance? Is it inherently rewarding or pleasurable? Most importantly, how can we measure it?

Our concept of sexual motivation is correspondingly fuzzy. Some still view sexual motivation as something distinct from sexual performance. Without any way of measuring desire adequately or unambiguously, we have little ability to determine how much of desire is motivated by internal or external factors. We do not know exactly how desire or learning modulates arousal, except that it does (O’Donohue & Plaud, 1994). We believe that sexual behavior unfolds linearly like a “Cascade,” and in a particular sequence of actions, but we really do not understand how the individual events are linked together. Is sexual motivation a state in which behavior is focused on sexual incentives or pleasure, or is it an intervening set of variables that creates such a state? Is it driven by hormones and incentives? Is it modulated by central inhibitory processes that are both temperamental and learned? Is it played out in a sequence of events related to tension and release? Is it real?

Synthesis

Motivations are always inferred from the strength of behaviors that define the particular motivational system. In this regard, the physiological psychologists are correct: Sex is no different from systems that regulate other physiological drives, such as hunger, thirst, and thermoregulation, and there is good evidence that the neurochemistry of sex shares many features with that of feeding and other motivational systems (Blackburn, Pfaus, & Phillips, 1992). We may infer that an animal is hungry by how much of its attention is spent searching for food, how much it is willing to work for food if an operant is imposed, or by the latency for it to initiate feeding when food is available (e.g., Toates, 1992). The same is true for sexual motivation. Animals with high sexual motivation have greater sexual excitement and desire, and lower thresholds for the initiation of sexual arousal, copulation, and orgasm. For such a system to work efficiently, each of these events must be linked to one another in feedback loops, each of which are linked in parallel to different kinds of associative memory systems that modulate the feedback according to past experience (both short-term and long-term). These events must also be linked to physiological and psychological comparators that are constantly evaluating the current level of stimulation against some ideal. Together, these processes give rise to expectations and set a threshold level of activation for each event. The logic of this kind of system should hold for all animals, such that homologous behaviors would exist that define the same events or endpoints.

Appetitive Versus Consummatory Behaviors

For all animals, sexual behavior occurs as a sequence, or cascade, of behavioral events. A majority of the sexual behaviors studied in the laboratory are measured immediately before, during, or after sexual arousal or copulation. For copulation to occur at all, however, animals must be capable of responding to a variety of internal and external cues: They must be able to respond to hormonal and neurochemical changes that signal their own sexual arousal and receptivity; to identify external stimuli that predict where potential sex partners can be found; to actively seek out or work to obtain sex partners; to distinguish pheromonal, visual, or tactile cues, or behavioral patterns, of potential sex partners from those that are not sexually receptive; and to pursue desired sex partners once sexual contact has been solicited. Most of these responses occur during an appetitive phase of behavior, before copulation is ever initiated, and can be used to make inferences concerning sexual desire, distinct from consummatory (usually copulatory) performance (e.g., as in Perper, 1985). Indeed, many of these responses are made while the animal is alone, so that the presence of, or interaction with, a partner will not confound the behavioral interpretation. An animal’s ability to perform these responses also requires a great deal of behavioral flexibility and the capacity to learn which stimuli or responses are the most predictable or efficacious, respectively.

Rectification of Terms

Before considering a model of sexual motivation that can account for both the relationship and flow of different events, some terms must be defined. For the sake of argument, sexual incentives are anything with which an animal initiates sexual behavior. These would typically be the recipients of copulatory behavior, but could include second-order stimuli associated with copulation that animals either perform operants to obtain (e.g., Everitt, 1990; Everitt et al., 1987) or that modulate copulation or masturbation directly (e.g., Zamble et al., 1986). Sexual drive is the product of all internal events that push an animal to attend to sexual incentives or sexual activity. Sexual excitement is any psychomotor stimulation elicited in anticipation of a sexual incentive or sexual activity. Sexual arousal is the momentary level of genital blood now (parasympathetic) but can also include indices of sympathetic activation of the heart, galvanic skin response, or breathing rate, in response to a sexual incentive. Sexual arousability is defined four ways, as a characteristic rate of approach or latency to interact with a sexual incentive, a rate of or latency to achieving full sexual arousal, a rate of or latency to initiate copulation, and/or a rate of approach or latency to ejaculation or orgasm. As will be discussed, the first instance would be unambiguously appetitive, whereas the third and fourth instances would be unambiguously consummatory. Sexual identification, activity, and object choice are defined as in Whalen (1966), as is sexual gratification with the addition of reward derived from the act of obtaining a secondary sexual reinforcer. Thus, an animal with high sexual drive, arousal, and arousability would be expected to show a high level of appetitive sexual excitement, work hard to gain access to a sexual incentive, obtain genital blood flow quickly, and ejaculate or show orgasm with a low threshold of stimulation. It is also expected that the value of a sexual incentive will be modulated directly by drive and arousability, but also by ongoing feedback from arousal. It is possible for an animal to have a high sexual drive but low or inhibited appetitive arousability, such that it attends to or fantasizes about sexual incentives but does not readily approach them.

The Incentive Sequence Model

The appetitive and consummatory distinction has remained a key factor in modern incentive theory as it applies to behaviors as disparate as feeding, aggression, temperature regulation, drug self-administration, and electrical brain stimulation (e.g., Berridge, 1996; Berridge & Robinson, 1998; Bindra, 1969, 1974; Hollis, 1984; Ikemoto & Panksepp, 1996; Robinson & Berridge, 1993; Stewart, de Wit, & Eikelboom, 1984; Toates, 1992). Its application to sexual behavior (e.g., Everitt, 1990, 1995; Pfaus, 1995; Pfaus & Everitt, 1995; Pfaus et al., 1990, Pfaus, Smith, & Coopersmith, 1999) forms the basis of the Incentive Sequence Model (Pfaus, 1996). In this model, appetitive and consummatory responses made during any goal-directed or incentive behavior are conceived of as overlapping Venn diagrams (Figure 1) in which the behavioral stream moves sequentially from left to right, that is, from an appetitive phase to a consummatory phase. The diagrams are overlapping rather than discontinuous to emphasize that the division between appetitive and consummatory phases is not necessarily fixed: Some responses can be placed into both phases. This is especially true in situations where appetitive responses are displayed after the animal has come into contact with the primary incentive (i.e., when the consummatory phase occurs in bouts). Both the appetitive and consummatory phases consist of two subdiagrams, one contained within the other, which reflect preparatory and anticipatory responses, respectively. Although both types of responses are learned (as opposed to instinctive appetitive responses that do not fall within these subcategories [e.g., imprinting]), they are distinguished on the basis of necessity: Both anticipatory and preparatory responses are made in anticipation of an incentive or event, but preparatory responses must be made to obtain it, whereas anticipatory responses are not necessary to obtain it. The model preserves the sequential nature of sexual responding inherent in Reich (1938/1979), Beach (1956), Masters and Johnson (1966), and Byrne (1977), and also emphasizes the feedback relationship between events, as proposed by Beach (1956), Byrne (1977), Barlow (1986), and Bancroft (1989).

Example 1: Rats

In both male and female rats (Figure 2), instrumental responses (e.g., operant bar-presses), running a maze, crossing electrified grids, conditioned place preference, or partner preference would be considered preparatory, whereas measures of conditioned sexual excitement (e.g., locomotion, investigation, grooming, certain adjunctive behaviors) would be considered anticipatory but not necessarily preparatory (unless such behaviors actually got the animal closer to the incentive physically or temporally-something that depends on the type of testing chamber used and the definition of what constitutes the incentive). In contrast, the consummatory phase consists of the species-specific, and usually sexually differentiated, responses made in direct contact with the primary sexual incentive (i.e., partner). In this scheme, operant responding for second-order sexual reinforcers would not be considered consummatory, whereas pursuit of a pacing female would be, despite both behaviors being considered preparatory. In female rats, the only unambiguous consummatory response is lordosis. In male rats, mounts, intromissions, and ejaculations would be considered unambiguous consummatory responses. Note that sexual excitement in this scheme (as reflected by psychomotor stimulation) is distinct conceptually and temporally from sexual arousal (as reflected by genital blood flow), although the two can occur simultaneously. Note also that the unambiguous appetitive responses are the same for female and male rats. Although there may be important sex differences in neural or endocrine control of these responses, the behavioral metric of each is identical. Thus, female and male rats can be compared directly on these measures, something that cannot be done when comparing consummatory behaviors.

The area of interaction of the two diagrams contains precopulatory behaviors that comprise appetitive, anticipatory, and preparatory actions displayed after contact has been made with the primary sexual incentive. In male rats, noncontact erections (psychogenic sexual arousal), pursuit of the female, the postejaculatory interval, and copulatory rate prior to sexual exhaustion would occupy this interactive area. In female rats, proceptive behaviors like solicitations, pacing behaviors, hops and darts, active and passive rejection responses, and the female’s own postejaculatory interval would occupy this interactive area.

Example 2: Humans

The arrangement of appetitive and consummatory behaviors is nearly identical in humans (Figure 3), although the motor acts that constitute the classes of behavior are different. For both men and women, the appetitive phase would manifest itself in terms of sexual desire, with conceptually distinct, but not exclusive, subclasses of fantasy, sexual excitement, and preparatory behaviors (both instrumental and preference responses). The multivariate nature of sexual desire here is consistent with previous interpretations (e.g., Levine, 1988). In contrast to rats, however, the consummatory behaviors directed toward genital stimulation (both in terms of masturbation and copulation) and orgasm would be relatively similar for men and women, as would many of the overlapping precopulatory behaviors (e.g., solicitation, arousal, foreplay, and the refractory periods between successive copulations that constitute a measure of copulatory rate). Note here that sexual excitement and arousal are also considered conceptually and temporally distinct, as they are in rats. Although sexual arousal is generally appetitive, it is placed into the overlapping precopulatory category because it can be induced by fantasy or visual stimulation that is distal to the observer. This means that contact is made “psychologically” with an incentive and is the only way to configure the motivational system of an individual masturbating in the presence of erotic visual cues or fantasy representations.

Feedback Systems and Learning

The link between appetitive subclasses is predicted to occur in positive feedback loops, as depicted in Figure 4 for male rats. In male rats, the odors of sexually receptive estrous females evoke psychomotor stimulation (Damsma, Pfaus, Wenkstern, Phillips, & Fibiger, 1992) and noncontact penile erection (Sachs, Akasofu, Citron, Daniels, & Natoli, 1994). These systems must be activated in sequence for males to approach, mount, and intromit with a receptive female. Ongoing sensory feedback at each stage, from distal to proximal, appetitive to precopulatory, should propel the male faster toward the consummatory goal of mounting and intromitting. Consummatory behaviors are linked to themselves and to appetitive behaviors in both positive and negative feedback loops. For example, in male rats intromission leads to dismount and genital grooming. In female rats, intromission leads to the offset of lordosis and the onset of pacing (Pfaus et al., 1999). Pacing behavior requires males to chase or otherwise search for the female. This chasing behavior is especially obvious prior to ejaculation, when females impose more temporal distance between themselves and the male (McClintock, 1984; Pfaus et al., 1999). Thus, sensory feedback from intromission, which is necessary to reach the ejaculatory threshold in the male, is then linked positively to precopulatory chasing behavior. In contrast, ejaculation is linked negatively to appetitive and precopulatory behaviors. During the absolute refractory phase, males do not respond sexually to solicitations or other proceptive behaviors displayed by females, and they will not chase the females.

Similarly, in humans, appetitive sexual responses are linked positively to precopulatory arousal, which is further linked positively to the initiation of masturbation, foreplay, and/or copulation. Sensory feedback from copulatory stimulation is both positively and negatively linked to itself and to appetitive responses. Orgasm then provides negative feedback on both appetitive and consummatory responses.

It is easy to conceptualize a nervous system in which positive feedback exists between sensory modalities and motor outputs. One pathway simply has to excite or disinhibit another, and the flow of information will then allow different behavioral endpoints to reach threshold at the required rate. On the other hand, inhibition must be imposed actively on these systems for behavior to have a natural culmination. In systems models of motivation, satiety is viewed as a behavioral endpoint in which further behavior is inhibited. The threshold for satiety must therefore switch on an inhibitory system that provides negative feedback on the previously reinforced behavior. Somewhat paradoxically, satiety could be considered an ultimate positive reinforcer, and yet it violates operant rules of positive reinforcement in that it inhibits rather than facilitates the expression of behavior. Sexual satiety may be achieved during orgasm, an event that can be considered the most obvious and immediate reward or goal of copulation. The feedback to consummatory behaviors would be inhibitory, with further sexual stimulation being perceived as aversive. However, feedback to appetitive behaviors may not be as obvious. It is known, for example, that satiety achieved by food intake inhibits feeding. However, Weingarten (1984) has shown that satiety does not inhibit certain preparatory behaviors displayed in the presence of secondary reinforcers, such as an animal approaching a feeding niche when a CS+ for food is switched on. It is not known whether conditioned sexual behaviors, especially those that are not genitally based, can be elicited following sexual satiety. It is known, however, that sexual excitement and certain precopulatory behaviors such as anogenital investigation in male rats are more resistant to the inhibiting effects of ejaculation or castration than consummatory measures of copulation (Beach, 1956; Centeno, Jacques, & Pfaus, 1999).

For animals to optimize their sexual responding, they must be able to make predictions based on internal and external cues and circumstances. Thus, nonassociative processes, like habituation, dishabituation, and sensitization, and associative processes like Pavlovian or operant learning modulate the strength of appetitive, precopulatory, and consummatory feedback systems. Habituation can occur to the perception of incentives that are presented over and over again, which then weakens responses directly or devalues the incentive. Dishabituation, or an abrupt restoration of normal responsiveness to incentives, can occur following shock- or drug-induced arousal, or after a sufficient change (natural or drug-induced) has occurred in the environment or circumstance under which habituation developed. For example, under natural circumstances, dishabituation may be accomplished by a change in activity with the same incentive. Likewise, habituation can be blocked or delayed by engaging in the same activity with a different incentive (Pavlov, 1927). Sensitization, or the process by which the connection between incentives or reaction to an incentive strengthens over time, would be expected to enhance the value of an incentive, such that responding for that incentive would get stronger and stronger. As obvious examples, unresolved or unrequited desire for a distant or unobtainable incentive, or an obsessive fantasy about engaging in exotic, “naughty,” or highly arousing sexual behavior with an incentive (Bem, 1996), can serve to sensitize the behavior and increase the value of the incentive (and also the reward experienced from engaging in the behavior). Likewise, simple sexual primes may serve to sensitize responses to full sexual incentives. However, engaging in sexual activity under the influence of psychomotor stimulant drugs like amphetamine or cocaine, which sensitize their own behavioral responses, and which amplify both the value of the incentive and the reward experienced by interacting with the incentive, can lead to faster habituation of responding in nondrug circumstances.

Associative learning provides a second layer of control over these feedback systems. Pavlovian contingencies (McIntosh, 1983; Pavlov, 1927; Rescorla, 1980) between stimuli allow primary and secondary associations to form between neutral stimuli that are paired explicitly with sexual activity or reward. This allows an enormous amount of environmental control over the expression of sexual behavior (e.g., Domjan et al., 1986; Kippin et al., 1999; Zamble et al., 1985), which can occur in both an excitatory and inhibitory manner. As with dishabituation, disinhibition can occur following a sudden change in the environment or circumstance under which conditioned inhibition developed. For example, alcohol has long been assumed to stimulate human sexual desire through a process of cognitive disinhibition, in which the depressant effects of alcohol inhibit the activity of learned sexual inhibition (Abel, 1980; Wilson, 1977). In fact, male rats that have been trained not to copulate with nonreceptive females will attempt to mount those females following low to moderate doses of alcohol (Pfaus & Pinel, 1989). Finally, operant or instrumental contingencies between stimulus and response (Skinner, 1938; McIntosh, 1983) can also serve to excite or inhibit sexual responding but under schedules of reinforcement in which particular responses or different rates of responding are controlled. The linking of sexual reward with particular sexual responses follows operant rules, as do culturally based sexual practices and taboos.

Arousability

Arousability signals the animal’s willingness and ability to engage in behavior and is one of the most direct measures of motivation. We can assess it three ways in our model, as a rate of approach to an incentive (appetitive), as a degree of sexual arousal (precopulatory), and as a latency to initiate copulation or a rate of approach to ejaculation or orgasm (consummatory). Whalen (1966) regarded arousability as indicative of internal factors (e.g., hormone function) that regulate sexual drive. We might expect other internal mechanisms (e.g., the activity of monoamine or neuropeptide systems in the brain) to alter arousability, especially if they are linked to attentional, perceptual, or learning mechanisms. We also expect there to be individual differences in arousability that have important implications for sexual motivation. If we envision arousability as an inverted U-shaped curve that defines the relationship between two variables, for example, arousal and performance, we can observe differences in optimality that may track differences in incentive stimulus intensity, perceptual ability, memory, strength of learning, and neuroendocrine function. In turn, these elements can be controlled or otherwise examined directly. Such differences in arousability might form an important component, along with learning, of alternate sexual practices such as paraphilic behavior. It is conceivable that an individual of low arousability may require or gravitate toward stimuli of high incentive value in order to achieve the necessary attentional, arousal, or performance levels. This is much like an animal that is sated or otherwise not hungry requiring food-related stimuli of high incentive value to eat (McFarland & Sibley, 1975; Toates, 1992). In contrast, an animal of high arousability would be expected to react to stimuli of less intensity, and may achieve attentional, arousal, and performance thresholds quickly. Being able to differentiate animals on this basis may allow neurobiological correlates of sexual arousability to be identified. In turn, identification of substrates for certain paraphilias or even harmful sexual activities might aid in the development of successful interventions.

Appetitive arousability can be inferred from virtually all the tasks in the unambiguous appetitive portion of the model, including rates of anticipatory behaviors, such as conditioned psychomotor stimulation in rats (e.g., Mendelson & Pfaus, 1989; Pfaus et al., 1990) and possibly also in humans, and rates of preparatory behaviors, such as navigating mazes or the speed with which operant responses are learned, in both rats and humans. The imposition of operant tasks for sexual rewards in humans has not received much scientific attention but has a colorful anecdotal history. It does not stretch the imagination to consider a range of dating or courtship behaviors that, upon close scrutiny, meet Skinner’s (1938) criteria of an operant. The ability to assess these kinds of behaviors directly would also aid us greatly in measuring sexual desire, because the willingness to work for a sexual incentive or reward can be modulated up and down by the imposition of operant schedules of reinforcement (e.g., Teitelbaum, 1966). Moreover, in rats, we have the ability to assess identical measures of appetitive arousability in both males and females, without having to compare consummatory behaviors that are sexually dimorphic. This opens up the possibility to compare homologous behaviors and their neural or endocrine substrates between rats and humans.

Precopulatory arousability is already being assessed on the basis of genital blood flow and cardiovascular activation in response to sexual incentives (e.g., Barlow, 1986; Palace & Gorzalka, 1990, 1992; Rosen & Beck, 1988; Schaefer & Colgan, 1977). Although animal models of contact (physiological) and noncontact (psychological) erection are not commonly interpreted in this light, it would be interesting to see how such measures in a male rat relate to appetitive and consummatory measures of arousability. It would also be important to develop models of vaginal blood flow in the rat.

As with precopulatory arousability, consummatory arousability has been studied, but mostly in the context of premature or delayed ejaculation (or orgasm). Mount, intromission, and ejaculation latencies in rats have been studied in detail with respect to drug effects (see Bitran & Hull, 1987; Dornan & Malsbury, 1989; Pfaus & Everitt, 1995, for reviews) and stimulation parameters (e.g., the enforced interval effect, Larsson, 1956; or the effects of sexual exhaustion, Beach & Jordan, 1956; Rodriguez-Manzo & Fernandez-Guasti, 1995; Teifer, 1969). Consummatory arousability in female rats may be inferred from the rate of solicitation or pacing behaviors (with which females control their sexual contact with males), or the length of time the female remains sexually receptive. Although driven by well-known hormonal mechanisms (Pfaff, Schwartz-Giblin, McCarthy, & Kow, 1994; Pfaus et al, 1999), these behaviors may also be affected by ongoing sexual stimulation (such as vaginocervical stimulation from intromission) or altered by experience and expectancy (Paredes & Alonso, 1997).

Homologies

The Incentive Sequence Model accommodates a full range of incentive responses and specifies their placement within general subclasses of appetitive, precopulatory, and consummatory phases of behavior. This organizing scheme allows different behaviors between species to be compared within the same subclass. For example, operant bar-pressing in rats to gain access to a primary or secondary sexual incentive would be considered preparatory. Analogous operant behaviors in humans aimed at achieving the same end (e.g., gift-giving in natural settings, or contrived operant tasks in a laboratory setting) would also be considered preparatory. To the extent that animal and human studies show that the two putatively analogous behaviors respond similarly to hormone manipulations, drug treatments, and so forth, then the behaviors themselves can be considered homologues and may therefore reflect similar neuroendocrine or neurochemical substrates. In this way, lists of analogous behaviors between species can be compiled. It is likely that there will be more overlap between rats and humans in appetitive behaviors compared with certain unambiguous consummatory behaviors (e.g., lordosis) for which human homologues do not exist.

Where Do We Go From Here?

The strength of any model lies in its ability to be used to make accurate predictions. The Incentive Sequence Model provides a framework within which different motivated behaviors exist in an excitatory and/or inhibitory relationship to one another. Those relationships can be stated explicitly and tested directly. This can be done relatively easily in rats, but not so easily in humans. We can measure a variety of appetitive, precopulatory, and copulatory behaviors in rats, but we remain restricted in what we can observe in humans. Notwithstanding the obvious ethical concerns surrounding direct observation of human copulation, we have few paradigms that allow a direct examination of sexual desire and arousability, other than measures of arousal (e.g., mercury– in-rubber strain gauges, plethysmographs, rigiscans) that are linked to the well-controlled presentation of sexual stimuli (Barlow, 1986; Palace & Gorzalka, 1990, 1992; Rosen & Beck, 1988). We can impose operants on rats for sexual rewards, but we relegate such effects in humans to tabloid news and popular magazines. We can measure attraction and solicitation in female primates to study the link between arousability, hormone level, and social condition in the control of sexual desire (Wallen, 1982, 1995), but we depend on questionnaires and retrospectives in human females for this information. Thus, there is little evidence yet to allow any direct comparisons of motivational systems between species except for penile blood flow and a few appetitive or consummatory behaviors that we might consider intuitively homologous.

As mentioned above, sexual desire and orgasm remain elusive subjects of study in humans, yet these are two of the three sexual events from which motivation is usually inferred. Knowing how many times a week, or asking subjects to rate the intensity on a Likert scale is a start, but only barely. How else can we assess them? We can correlate orgasm likelihood or intensity with ratings on reliable questionnaires that assess global sexual functioning (e.g., the CMSH-SFQ [Corty, Althof, & Kurit, 1996], the Sexual Experience Survey [Anderson, Bancroft, & Wu, 19921, the Rust Sexual Inventory [Rust & Golombok, 1986], the Derogatis Sexual Functioning Inventory [Derogatis & Melisaratos, 1979], etc.) or with ratings of physiological arousability, such as the Arousal Predisposition Scale (Coren & Mah, 1993). We can also correlate these variables with physiological measurements of arousal, cardiovascular response, anogenital muscle activation, EEG activity, or plasma concentrations of neurochemicals during masturbation-induced orgasm (Chambliss et al., 1982; Cohen, Rosen, & Goldstein, 1976; Fox, 1976; Henson, Rubin, & Henson, 1982; Kruger et al., 1998). Until such time as copulatory orgasms can be studied directly, we will have to make do with retrospective and correlative analyses. We are thus in dire need of a reliable measurement device specific for orgasm, perhaps something based on the logic of the McGill Pain Questionnaire (Melzack, 1975).

The study of operant contingencies between sexual incentive, sexual reward, and sexual activity could provide us with important information regarding sexual desire, not to mention the formation of preferences and the link between preparatory behaviors, arousal, and performance. How could this be accomplished? A rudimentary operant sexual reward system already exists in the form of the Internet. The sale of erotic, pornographic, and other sexually related images has burgeoned into its own economy, surpassing that of some nations. People are willing not only to part with money, but with personal information, including credit card numbers, on unsecured servers to obtain these images. The logic of this could be translated into a laboratory-based, computerized operant presentation system, much like the laboratorybased computerized gambling systems that are currently in use (e.g., Richards, Zang, Mitchell, & de Wit, 1999). A false economy could be imposed, such that individuals have to pay more and more credits to obtain erotic images that vary along a predictable or controlled dimension. The effects of drugs, priming stimuli, or the correlation of willingness to work with sexual arousal, satisfaction, or performance measures could be examined to form a multivariate operational definition of sexual desire. Hypotheses regarding the formation of operant sexual responses in animals (e.g., Everitt et al., 1987) could also be tested directly in humans using this kind of presentation system.

Conclusions

Sexual motivation may be a real thing, but we must infer it from our observations of sexual behavior and, in particular, behaviors that allow us to infer sexual arousability and the direction and strength of neuronal feedback systems. In the Incentive Sequence Model, sexual motivation is fractionated along appetitive, precopulatory, and consummatory phases. Appetitive responses that bring animals into contact with sexual incentives, precopulatory behaviors that stimulate arousal and orient animals toward genital or copulatory contact, and consummatory behaviors of masturbation or copulation are specified in a general organizing scheme that can apply to different species and can aid in the study of responses to different incentives. The role of nonassociative and associative learning, context, priming, and reward in the modulation of internal feedback systems can be placed into the framework of the model, as can the role of internal mechanisms (e.g., neuroendocrine and neurochemical systems) in the control of arousability to different incentives. It is hoped that the ability to make accurate predictions about the effect of a drug or experience on human sexual desire based on effects in laboratory animals (e.g., Cantor, Binik, & Pfaus, 1999) will stimulate more cross-talk between clinicians and animal researchers.

Acknowledgements

Preparation of this paper was supported by grants from the Natural Sciences and Engineering Research Council of Canada (OGP-0138878), the Medical Research Council of Canada (MT-13125), the U.S. National Institute on Drug Abuse (1121 DA11898), and Fonds pour la Formation de Chercheurs et l’Aide A la Recherche du Quebec (CE-98). The author is indebted to Drs. Jim Blackburn, John Bancroft, Michael Baum, Walter Everaerd, Barry Everitt, Erick Janssen, Tod Kippin, Ellen Laan, Martha McClintock, Eileen Palace, Raul Paredes, Ray Rosen, Ben Sachs, Jane Stewart, and Paul Vasey, for fruitful discussions that helped formulate the ideas expressed in this paper.

References

ABEL, E. L. (1980). A review of alcohol’s effects on sex and reproduction. Drug and Alcohol Dependence, 5, 321-332.

ABRAMSON, P. R., & PINKERTON, S. D. (1995). With pleasure: Thoughts on the nature of human sexuality. New York: Oxford University Press.

AGMO, A., & BERENFELD, R. (1990). Reinforcing properties of ejaculation in the male rat: Role of opioids and dopamine. Behavioral Neuroscience, 104, 177-182.

ANDERSON, R. A., BANCROFT, J., & Wu, F. C. (1992). The effects of exogenous testosterone on sexuality and mood of normal men. Journal of Clinical Endocrinology and Metabolism, 75,1503-1507.

BANCROFT, J. H. (1989). Human sexuality and its problems. Edinburgh: Churchill Livingstone.

BARFIELD, R. J. (1993). Animal models of human sexual behavior. In A. J. Riley, M. Peet, & C. Wilson (Eds.), Sexual pharmacology (pp. 73-86). Oxford: Clarendon Press. BARLow, D. H. (1986). Causes of sexual dysfunction. Journal of Consulting and Clinical Psychology, 54, 140-157.

BEACH, F. A. (1950). Sexual behavior in animals and man. The Harvey Lectures, 43, 259-279.

BEACH, F. A. (1956). Characteristics of masculine “sex drive.” Nebraska Symposium on Motivation, 4, 1-32.

BEACH, F. A., & JORDAN, L. (1956). Sexual exhaustion and recovery in the male rat. Quarterly Journal of Experimental Psychology, 8, 249-254.

BEM, D. J. (1996). Exotic becomes erotic: A developmental theory of sexual orientation. Psychological Review, 103, 320-335.

BERRIDGE, K C. (1996). Food reward: Brain substrates of wanting and liking. Neuroscience & Biobehavioral Reviews, 20, 1-25.

BERRIDGE, K. C., & ROBINSON, T. E. (1998). What is the role of dopamine in reward: Hedonic impact, reward learning, or incentive salience? Brain Research Reviews, 28, 309369.

BINDRA, D. (1969). A unified interpretation of emotion and motivation. Annals of the New York Academy of Sciences, 159, 1071-1083.

BINDRA, D. (1974). Neuropsychological interpretation of the effects of drive and incentive-motivation on general activity and instrumental behaviour. Psychological Review, 75, 1-22.

BITRAN, D., & HULL, E. M. (1987). Pharmacological analysis of male rat sexual behavior. Neuroscience and Biobehavioral Reviews, 11, 365-389.

BLACKBURN, J. R., PFAUS, J. G., & PHILLIPS, A. G. (1992). Dopamine functions in appetitive and defensive behaviours. Progress in Neurobiology, 39, 247-279.

BUSS, D. M. (1994). The evolution of desire. New York: Basic Books.

BYRNE, D. (1977). The imagery of sex. In J. Money & H. Muspah (Eds.), Handbook of sexology (pp. 327-350). Amsterdam: Elsevier.

CANTOR, J. M., BINIK, Y. M., & PEAUS, J. G. (1999). Chronic fluoxetine inhibits sexual behavior in the male rat: Reversal with oxytocin. Psychopharmacology, 144, 355-362.

CENTENO, S., JACQUES, A., & PFAUS, J. G. (1999). Appetitive and consummatory sexual behaviors of male rats following castration and androgen replacement. Manuscript submitted for publication.

CHAMBLISS, D. L., STERN, T., SULTAN, F. E., WILLIAMS, A. J., GOLDSTEIN, A. J., LINES– BERGER, M. H., LIFSHITZ, J. L., & KELLY, L (1982). The pubococcygeus and female orgasm: A correlational study with normal subjects. Archives of Sexual Behavior, 11, 479-490.

COHEN, H. D., ROSEN, R. C., & GOLDSTEIN, L. (1976). Electroencephalographic laterality

changes during human sexual orgasm. Archives of Sexual Behavior, 5, 189-199.

COREN, S., & MAH, K. B. (1993). Prediction of physiological arousability: A validation of the Arousal Predisposition Scale. Behavioral Research & Therapy, 31, 215-219.

CORTY, E. W, ALTHOF, S. E., & KUM, D. M. (19996). The reliability and validity of a sexual functioning questionnaire. Journal of Marital and Sex Therapy, 22, 27-34.

CRAIG, W. (1918). Appetites and aversions as constituents of instincts. Biological Bulletin of Woods Hole, 34, 91-107.

DAMSMA, G., PFAUS, J. G., WENKSTERN, D., PHILLIPS, A. G., & FIBIGER, H. C. (1992). Sexual behavior increases dopamine transmission in the nucleus accumbens and striatum of male rats: Comparison with novelty and locomotion. Behavioral Neuroscience, 106, 181191.

DEKKER, J., & EVERAERD, W (1988). Attentional effects on sexual arousal. Psychophysiology, 25, 45-54.

DEROGATIS, L. R., & MELISARATOS, N. (1979). The DSFI: A multidimensional mesure of sexual functioning. Journal of Sex & Marital Therapy, 5, 244-281.

DE WAAL, F. B. M. (1987). Tension regulation and nonreproductive functions of sex in captive bonobos (Pan paniscus). National Geographic Research, 3, 318-335.

DEWSBURY, D. A. (1979). Factor analysis of measures of copulatory behavior in three species of muroid rodent. Journal of Comparative and Physiological Psychology, 93, 868878.

DomjAN, M., LYONS, R., NORTH, N. C., & BRUELL, J. (1986). Sexual Pavlovian conditioned approach behavior in male Japanese quail (Coturnix japonica). Journal of Comparative Psychology, 100, 413-421.

DomjAN, M., O’VARY, D., & GREEN, P. (1988). Conditioning of appetitive and consummatory sexual behavior in male Japanese quail. Journal of the Experimental Analysis of Behavior, 50, 505-519.

DoRNAN, W A., & MALSBURY, C. W. (1989). Neuropeptides and male sexual behavior. Neuroscience & Biobehavioral Reviews, 13, 1-15.

ELLIS, H. (1915). Analysis of the sexual impulse (Vol. III). Studies in the psychology of sex. Philadelphia: EA. Davis & Co.

ELLIS, H. (1933). The psychology of sex. New York: Emerson Books.

EVERAERD. W. (1995). Information processing approach and the sexual response in human studies. In J. Bancroft (Ed.), The pharmacology of sexual function and dysfunction (pp. 175-184). Amsterdam: Elsevier.

EvERiTT, B. J. (1990). Sexual motivation: A neural and behavioral analysis of the mechanisms underlying appetitive and copulatory responses of male rats. Neuroscience & Biobehavioral Reviews, 14, 217-232.

EVERITT, B. J. (1995). Neuroendocrine mechanisms underlying appetitive and consummatory elements of masculine sexual behavior. In J. Bancroft (Ed.), The pharmacology of sexual function and dysfunction (pp. 15-31). Amsterdam: Elsevier.

EVERITT, B. J., & BANCROFT, J. (1991). Of rats and men: The comparative approach to male sexuality. Annual Review of Sex Research, 2, 77-118.

EVERiTT, B. J., FRAY, P., KOSTARCZYK, E., TAYLOR, S., & STACEY, P. (1987). Studies of instrumental behavior with sexual reinforcement in male rats (Rattus norvegicus): 1. Control by brief visual stimuli paired with a receptive female. Journal of Comparative Psychology, 101, 395-406.

FARRIS, H. E. (1967). Classical conditioning of courting behavior in the Japanese quail, Coturnix coturnix japonica. Journal of the Experimental Analysis of Behavior, 10, 213217.

FOWLER, 0. S. (1870). Sexual science. Philadelphia: National Publishing Co.

Fox, C. A. (1976). Some aspects and implications of coital physiology. Journal of Sex and Marital Therapy, 2, 205-213.

FREUD, S. (1922). Beyond the pleasure principle. New York: Albert & Charles. FREUD, S. (1927). The ego and the id. London: Hogarth Press.

GEER, J. H., & FUHR, R. (1976). Cognitive factors in sexual arousal: The role of distraction. Journal of Consulting and Clinical Psychology, 44, 238-243.

GEER, J. H., & HEAD, S. (1990). The sexual response system. In J. T. Cacioppo & L. Tassinary (Eds.), Principles of psychophysiology (pp. 599-630). New York: Cambridge University Press.

GEER, J. H., LAPOUR, K. L., & JACKSON, S. R. (1993). The information processing approach to human sexuality, In N. Birbaumer & A. Ohman (Eds.), The structure of emotion: Psychophysiological, cognitive, and clinical aspects (pp. 135-155). Toronto: Hogrefe– Huber.

GEER, J. H., & McGLONE, M. S. (1990). Sex differences in memory for erotica. Cognition and Emotion, 4, 71-78.

GEER, J. H., & O’DONOHUE, W. T. (1987). Theories of human sexuality. New York: Plenum Press.

GRAHAM, J. M., & DESJARDINS, C. (1980). Classical conditioning: Induction of luteinizing hormone and testosterone secretion in anticipation of sexual activity. Science, 210, 1039-1041.

HARDY, K. R. (1964). An appetitional theory of sexual motivation. Psychological Review, 71, 1-18.

HENSON, D. E., RUBIN, H. B., & HENSON, C. (1982). Labial and vaginal blood volume responses to visual and tactile stimuli. Archives of Sexual Behavior, 11, 23-31.

HErrA, J., & MEYERSON, B. J. (1978). Sexual motivation in the male rat. Acta Physiologica Scandinavica, 453 (Suppl.), 1-67.

HOLLIS, K. L. (1984). The biological function of Pavlovian conditioning: The best defense is a good offense. Journal of Experimental Psychology: Animal Behavioral Processes, 10, 413-425.

HUGHES, A. M., EVERITT, B. J., & HERBERT, J. (1990). Comparative effects of preoptic area infusions of opioid peptives, lesions and castration on sexual behavior in male rats: Studies of instrumental behavior, conditioned place preference, and partner preference. Psychopharmacologia, 102, 243-256.

HULL, E. M., WEBER, M. S., EATON, R. C., DUA, R., MARKOWSKI, V. P., LUMLEY, L., & MOSES, J. (1991). Dopamine receptors in the ventral tegmental area affect motor, but not motivational or reflexive, components of copulation in male rats. Brain Research, 554, 7278.

IKEMOTO, S., & PANKSEPP, J. (1996). Dissociations between appetitive and consummatory responses by pharmacological manipulations of reward-relevant brain regions. Behavioral Neuroscience, 110, 331-345.

JAMES, W. (1890). Principles of psychology (Vol. 1 & 2). New York: Holt.

JANSSEN, E., & EvERAERD, W. (1993). Determinants of male sexual arousal. Annual Review of Sex Research, 4, 211-245.

JAsTRow, J. (1948). Freud: His dream and sex theories. New York: Pocket Books. JOWAiSAs, D., TAYLOR, J., DEWSBuRy, D. A., & MALAGODI, E. F. (1971). Copulatory behavior of male rats under an imposed operant requirement. Psychonomic Science, 25, 287290.

KINSEY, A. C., POMEROY, W. B., & MARTIN, C. E. (1948). Sexual behavior in the human male. Philadelphia: W.B. Saunders.

KINSEY, A. C., POMEROY, W. B., MARTIN, C. E., & GEBHARD, P. H. (1953). Sexual behavior in the human female. Philadelphia: W.B. Saunders.

KIPPIN, T.E., TALIANAKIS, S., SCHATTMANN, L., BARTHOLOMEW, S., & PFAUS, J.G. (1998). Olfactory conditioning of sexual behavior in the male rat (Rattus norvegicus). Journal of Comparative Psychology, 112, 389-399.

KIPPIN, T. E., & PFAus, J. G. (1999). The development of olfactory conditioned ejaculatory preferences in the male rat: I. Nature of the unconditioned stimulus. Manuscript submitted for publication.

KiPPIN, T. E., SAmAHA, A., & PFAus, J. G. (1999). The development of olfactory condi

tioned ejaculatory preferences in the male rat: II. Parametric manipulation of training session number and duration. Manuscript submitted for publication.

KoKsAL, F., DomiAN, M., & WEISMAN, G. (1994). Blocking of the sexual conditioning of differentially effective conditioned stimulus objects. Animal Learning and Behavior, 22, 103-111.

KRAFT-EBING, R. VON (1929). Psychopathia sexualis (F. J. Rebman, Trans.). New York: Physicians and Surgeons Book Co.

KRUGER, T., ExToN, M. S., PAWLAK, C., VON ZUR MUHLEN, A., HARTMANN, U., & SCHEDLOWSKI, M. (1998). Neuroendocrine and cardiovaxcular responses to sexual arousal and orgasm in men. Psychoneuroendocrinology, 23, 401-411.

LAAN, E., EvERAERD, W., VAN DER VELDE, J., & GEER, J. H. (1995). Determinants of subjective experience of sexual arousal in women: Feedback from genital arousal and erotic stimulus content. Psychophysiology, 32, 444-451.

LARSSON, K. (1956). Conditioning and sexual behavior in the male albino rat. Stockholm: Almquist.

LEVAY, S., & HAMER, D. H. (1994). Evidence for a biological influence in male homosexuality. Scientific American, 270, 44-49.

LEVINE, S. (1988). Intrapsychic and individual aspects of sexual desire. In S. R. Lieblum, & R. C. Rosen, (Eds.), Sexual desire disorders (pp. 1-44). New York: Guilford. LORENZ, K. (1950). The comparative method in studying innate behaviour patterns.

Symposium of the Society for Experimental Biology, 4, 221-268.

MASTERS, W. H., & JOHNSON, V. E. (1966). Human sexual response. Boston: Little, Brown.

MACINTOSH, N. J. (1983). Conditioning and associative learning. New York: Oxford University Press.

MCCLINTOCK, M. K. (1984). Group mating in the domestic rat as a context for sexual selection: Consequences for the analysis of sexual behavior and neuroendocrine responses. Advances in the Study of Behavior, 14, 2-50.

McFARLAND, D. J., & SIBLEY, R. M. (1975). The behavioural final common path. Philosophical Transactions of the Royal Society of London, 270, 265-293.

MEAD, M. (1949). Male and female. New York: Morrow.

MEHRARA. B. J., & BAUM, M. J. (1990). Naloxone disrupts the expression but not the acquisition by male rats of a conditioned place preference response for an oestrous female. Psychopharmacology (Berlin), 101, 118-125.

MEISEL, R. L., & SACHS, B. D. (1994). The physiology of male reproduction. In E. Knobil and J. D. Neill (Eds.), The physiology of reproduction, Vol. 2 (pp. 3-105). New York: Raven Press.

MELZACK, R. (1975). The McGill Pain Questionnaire: Major properties and scoring methods. Pain, 1, 277-299.

MENDELSON, S. D., & PFAus, J. G. (1989). Level searching: A new assay of sexual motivation in the male rat. Physiology & Behavior, 45, 337-341.

MILLER, R. L., & BAum, M. J. (1987). Naloxone inhibits mating and conditioned place preference for an estrous female in male rats soon after castration. Pharmacology, Biochemistry & Behavior, 26, 781-789.

MILNER, P. (1970). Physiological psychology. New York: Holt, Reinhart & Winston. MITCHELL, J. B., & STEWART, J. (1990). Facilitation of sexual behaviors in the male rat in the presence of stimuli previously paired with systemic injections of morphine. Pharmacology, Biochemistry & Behavior, 35, 367-372.

MOLL, A. (1897). Untersuchungen uber die libido sexualis [Analysis of the sexual libido]. Berlin: Fischer’s Medicin Buchhandlung.

MOLL, A. (1933). Libido sexualis: Studies in the psychosexual laws of love verified by clinical sexual case histories. New York: American Ethnological Press.

MOSES, J., LOUCKS, J. A., WATSON, H. L., MATUSZEWICH, L., & HULL, E. M. (1995). Dopaminergic drugs in the medial preoptic area and nucleus accumbens: Effects on motor

activity, sexual motivation, and sexual performance. Pharmacology, Biochemistry & Behavior, 51, 681-686.

Moss, F. A. (1924). A study of animal drives. Experimental Psychology, 7, 165-185. O’DONOHUE, W., & PLAUD, J. J. (1994). The conditioning of human sexual arousal. Archives of Sexual Behavior, 23, 321-344.

PALACE, E. M., & GORZALKA, B. B. (1990). The enhancing effects of anxiety on arousal in sexually dysfunctional and functional women. Journal of Abnormal Psychology, 99, 403-411.

PALACE, E. M., & GORZALKA, B. B. (1992). Differential patterns of arousal in sexually functional and dysfunctional women: Physiological and subjective components of sexual response. Archives of Sexual Behavior, 21, 135-159.

PAREDEs, R. G., & ALONSO, A. (1997). Sexual behavior regulated (paced) by the female induces conditioned place preference. Behavioral Neuroscience, 111, 123-128.

PAvLov, I. P (1927). Conditioned reflexes (G. V Anrep, Trans.). Oxford: Oxford University Press.

PERPER, T. (1985). Sex signals: The biology of love. Philadelphia: ISI Press.

PFAFF, D. W., SCHWARTZ-GIBLIN, S., McCARTHY, M. M., & Kow, L.-M. (1994). Cellular and molecular mechanisms of female reproductive behaviors. In E. Knobil & J. D. Neill (Eds.), The physiology of reproduction, ( 2nd ed., pp. 107-220). New York: Raven Press.

PFAus, J. G. (1995). Neural mechanisms of sexual motivation and performance in females. In J. Bancroft (Ed.), The pharmacology of sexual function and dysfunction (pp. 37-48). Amsterdam: Elsevier.

PFAus, J. G. (1996). Homologies of animal and human sexual behaviors. Hormones and Behavior, 30, 187-200.

PFAus, J. G., & EVERiTT, B. J. (1995). The psychopharmacology of sexual behavior. In F. E. Bloom & D. J. Kupfer (Eds.), Psychopharmacology: The fourth generation of progress (pp. 743-758). New York: Raven Press.

PFAus, J. G., MENDELSON, S. D., & PHILLIPS, A. G. (1990a). A correlational and factor analysis of anticipatory and consummatory measures of sexual behavior in the male rat. Psychoneuroendocrinology, 15, 329-340.

PFAus, J. G., & PINEL, J. R J. (1989). Alcohol inhibits and disinhibits sexual behavior in the male rat. Psychobiology, 17, 195-201.

PFAus, J. G., SMITH, W. J., & COOPERSMITH, C. B. (1999). Appetitive and consummatory sexual behaviors of female rats in bilevel chambers: 1. A correlational and factor analysis and the effects of ovarian hormones. Hormones and Behavior, 35, 224-240.

REICH, W. (1979). The bion experiments. New York: Farrar Straus Giroux. (Original work published in 1938)

REICH, W (1978). The function of the orgasm. New York: Farrar Straus Giroux. (Original work published in 1942)

REICH, W. (1982). The bioelectrical investigation of sexuality and anxiety. New York: Farrar Straus Giroux. (Original work published in 1945)

RESCORLA, R. A. (1980). Pavlovian second order conditioning: Studies in associative learning. Hillsdale, NJ: Lawrence Erlbaum.

RICHARDS, J. B., ZHANG, L., MITCHELL, S. H., & DE WIT, H. (1999). Delay or probability discounting in a model of impulsive behavior: Effect of alcohol. Journal of the Experimental Analysis of Behavior, 71, 121-143.

ROBINSON, T. E., & BERRIDGE, K. C. (1993). The neural basis of drug craving: An incentive-sensitization theory of addiction. Brain Research Bulletin, 18, 247-291. RODRIGUEz-MANzo, G., & FERNANDEz-GuAsTi, A. (1995). Opioid antagonists and the sex

ual satiation phenomenon. Psychopharmacology (Berlin), 122, 131-136.

ROSEN, R. C., & BECK, J. G. (1988). Patterns of sexual arousal. New York: The Guilford Press.

RuST, J., & GOLOMBOK, S. (1986). The GRISS: A psychometric instrument for the assessment of sexual dysfunction. Archives of Sexual Behavior, 15, 157-165.

SACHS, B. D. (1978). Conceptual and neural mechanisms of masculine copulatory behavior. In T. E. McGill, D. A. Dewsbury, & B. D. Sachs (Eds.), Sex and behavior: Status and prospectus (pp. 267-295). New York: Plenum.

SACHS. B. D., MACAIONE, R., & FEGY, L. (1974). Pacing of copulatory behavior in the male rat: Effects of receptive females and intermittent shocks. Journal of Comparative and Physiolgocial Psychology, 87, 326-331.

SACHS, B. D., AKASOFU, K., CITRON, J. H., DANIELS, S. B., & NATOLI, J. H. (1994). Noncontact stimulation from estrous females evokes penile erection in rats. Physiology & Behavior, 55, 1073-1079.

SCHAEFER, H. H., & COLGAN, A. H. (1977). The effect of pornography on penile tumescence as a function of reinforcement and novelty. Behavior Therapy, 8, 938-946.

SCHWARTZ, M. (1956). Instrumental and consummatory measures of sexual capacity in the male rat. Journal of Comparative and Physiological Psychology, 49, 328-333. SHEFFIELD, F D., WULFF, J. J., & BACKER, R. (1951). Reward value of copulation with

out sex drive reduction. Journal of Comparative and Physiological Psychology, 44, 3-8. SKINNER, B. F. (1938). The behavior of organisms. New York: Appleton-Century-Crofts. STELLAR, E. (1954). The physiology of motivation. Psychological Review, 61, 5-22. STEWART, J. (1995). How does incentive motivational theory apply to sexual behavior?

In J. Bancroft (Ed.), The pharmacology of sexual function and dysfunction (pp. 3- 11). Amsterdam: Elsevier.

STEWART, J., DE WIT, H., & EIKELBOOM, R. (1984). Role of unconditioned and conditioned drug effects in the self-administration of opiates and stimulants. Psychological Review, 91, 251-268.

STORMS, M. (1981). A theory of erotic orientation development. Psychological Review, 88, 340-353.

SYMONS, D. (1979). The evolution of human sexuality. New York: Oxford University

Press.

TANNAHILL, R. (1992). Sex in history. London: Scarborough House.

TEIFER, L. (1969). Copulatory behavior of male Rattus norvegicus in a multiple-female exhaustion test. Animal Behavior, 17, 718-721.

TEITELBAUM, P. (1966). The use of operant methods in the assessment and control of motivational states, In W K. Honig (Ed.), Operant behavior: Areas of research and application (pp. 565-608). New York: Appleton-Century-Crofts.

TOATEs, F. (1992). Motivational systems. Cambridge: Cambridge University Press. TOATEs, F., & O’RoURKE, C. (1978). Computer simulation of male rat sexual behavior. Medical and Biological Engineering and Computing, 16, 98-104.

vAN FURTH, W. R., & vAN REE J. M. (1996). Appetitive sexual behavior in male rats: 1. The role of olfaction in level-changing behavior. Physiology & Behavior, 60, 999-1005. WALLEN, K. (1982). Influence of female hormonal state on rhesus sexual behavior

varies with space for social interaction. Science, 217, 375-376.

WALLEN, K. (1995). The evolution of female sexual desire. In P. R. Abramson & S. D. Pinkerton (Eds.), Sexual nature sexual culture (pp. 57-79). Chicago: University of Chicago Press.

WARNER, R. K., THOMPSON, J. T., MARKOWSKI, V. P., LoucKs, J. A., BAZZETT, T. J., EATON, R. C., & HULL, E. M. (1991). Microinjection of the dopamine antagonist cis-flupenthixol into the MPOA impairs copulation, penile reflexes, and sexual motivation in male rats. Brain Research, 540, 177-182.

WARNER, W. H. (1927). A study of sex drive in the white rat by means of an obstruction method. Comparative Psychology Monographs, 4, 1-67.

WEINGARTEN, H. R (1984). Meal initiation controlled by learned cues: Basic behavior propoerties. Appetite, 5, 147-158.

WHALEN, R. E. (1961). Effects of mounting without.intromission and intromission without ejaculation on sexual behavior and maze learning. Journal of Comparative and Physiological Psychology, 54, 409-415.

WHALEN, R. E. (1966). Sexual motivation. Psychological Review, 73, 151-163.

WILSON, G. T. (1977). Alcohol and human sexual behavior. Behavioral Research & Therapy, 15, 239-252.

WINOKUR, G. (1963). Aspects of sexual behavior: A classification. In G. Winokur (Ed.), Determinants of human sexual behavior (pp. vii-viii). Springfield, IL: Charles C Thomas. WOODWORTH, R. S. (1918). Dynamic psychology. New York: Columbia University Press. ZAMBLE, E., HADAD, G. M., MITCHELL, J. B., & CUTMORE, T. R. H. (1985). Pavlovian con

ditioning of sexual arousal: First- and second-order effects. Journal of Experimental Psychology: Animal Behavior Processes, 11, 598-610.

ZAMBLE, E., MITCHELL, J. B., & FINDLAY, H. (1986). Pavlovian conditioning of sexual arousal: Parametric and background manipulations. Journal of Experimental Psychology: Animal Behavior Processes, 12, 403-411.

Copyright Society for the Scientific Study of Sex 1999

Provided by ProQuest Information and Learning Company. All rights Reserved