The left-handed riddle

The left-handed riddle

A steady minority of about 10 per cent, more men than women, use their left rather than right hand for writing and other tasks. Left-handedness is also more common in twins than singletons. Through the ages, left-handedness has been attributed to sorcery, genes (heredity), social programming (mimicking other lefties), brain damage (due to pregnancy or birth problems) and, more recently, to sex hormones (prenatal testosterone levels). While some studies have suggested an unusually high proportion of developmental problems and immune disorders among lefties, left-handedness is also notorious among the most gifted of humans. Highly talented individuals in many fields have been left-handed: in music, Paul McCartney, J.S. Bach; in art, Pablo Picasso, Michelangelo, Paul Klee and Leonardo Da Vinci; among actors, Harpo Marx and Charlie Chaplin; in sports, Babe Ruth, Martina Navratilova, John McEnroe; in the sciences, Albert Einstein. And of famous monarchs, Queen Victoria, Julius Caesar and Alexander the Great were also lefties; so is U.S. President, George Bush. Experts are busily trying to unravel the riddle of left-handedness, how it occurs and why it has persisted as an unchanging human trait through history. Nothing about left-handedness is simple or clear cut. It will likely take years before the puzzle of the left hand is solved to everybody’s satisfaction. But handedness research is a burgeoning field in which Canadians are world leaders.

Exploding sinister leftie myths

Always in the minority, left-handers have historically suffered slurs of deviousness and witchcraft. “A left hand that is too gifted and agile is the sign … of a perverse and devilish disposition” is how one anthropologist summed up ancient beliefs. Roget’s Thesaurus equates left-handedness with “unskillfulness.” Although not its primary definition, Webster’s Dictionary says it can mean “insincere, indecisive, perhaps malevolent” as in “a left-handed compliment.” Doubts about left-handers are reflected in the terms for left. The English word derives from the Celtic “lyft” (meaning weak), the French “gauche” means “awkward” and the Latin, “sinister” stands for evil. In some cultures, the right hand is used for eating, the left for toilet hygiene after defecating, contributing to the sullied view of the left hand. Symbolically, the right hand is the more trustworthy, traditionally used for bestowing blessings, making the sign of the cross, taking oaths and giving greetings. Ancient custom dictates that wedding rings be worn on the left hand, in the belief that the precious metal will protect the body’s “weaker” side from temptation. The Bible displays anti-left prejudice in the Vision of judgement (Matthew), which depicts the King blessing those on his right, telling them they will “inherit the kingdom,” while instructing those on the left to “depart … cursed …into everlasting fire.”

Passed down through the centuries, the unmerited ill repute of lefties lingers on. The political left is often relegated to the “revolutionary fringe,” while those on the fight are considered more “solid.” The fact that Jack the Ripper and the Boston Strangler were both left-handed has done little to enhance the image of left-handers. As if ancient folklore hadn’t made life tough enough for lefties, since the turn of this century some psychologists and neurologists have theorized that left-handedness is due to brain damage caused by difficult births. Perhaps it’s time to counter these ideas by suggesting that those on the left are “nearer the human heart” and thus more valued ! New scientific research is eradicating the myth of left-handers as inept, abnormal or devious. The latest thinking attributes lefthandedness to biology, leaning towards a genetic explanation. The fact that there has always been a steady minority of lefties since antiquity points strongly to an inherited tendency. Experts argue that it can’t be too serious a drawback, because such a supposedly deleterious trait would have been selected out through evolution, rather than surviving unaltered. Historic records reveal an unchanging proportion of about 10 per cent of left-handers through the ages. Ancient cave drawings, Egyptian hieroglyphics and Babylonian friezes consistently depict a few left-handers among the predominantly right-handed users of weapons and tools. Amid historic disapproval and despite the compelling pressures of a right-handed world, left-handers have persistently flown in the face of convention, stubbornly refusing or unable to switch hands and do the “right” thing. As summed up by one eminent scientist, “the condemnation of an entire group of humans on the basis of hand preference alone amounts to the attempted suppression of what researchers now think is a basic biological trait.”

The latest studies show no differences between the cognitive or mental powers of right- and left-handers. One renowned Michigan State University child psychologist (a left-hander himself) points out that, “many lefties are unusually talented in mathematics, music, spatial ability, engineering, architecture, creative originality and elaborate thinking – a benefit to any social group.” Left- or mixed-handedness also occurs more frequently than average in those who excel in modem confrontation sports. In fact, in baseball left-handers are often used to great advantage ! How does a brain turn left(ie)?

The 19th century French physician, Paul Broca, was the first to link left-handedness scientifically to brain organization. His studies on aphasics (people unable to speak or express language owing to brain damage from strokes) showed that loss of verbal ability was usually linked to left-sided brain injury. Many nerve (brain) pathways and motor systems (for muscle control) cross over so that the brain’s left side controls actions on the body’s right side and vice versa. Broca concluded that the brain’s left hemisphere is responsible for speech and verbal skills, also probably favouring the opposite (right) hand for writing. Since lefties use their left hand to write, Broca assumed that their language centres would be situated in the brain’s opposite or right hemisphere. Broca was partly correct.

True, almost 95 per cent of right-handers are dominant for language on the brain’s left side, with emotional and spatial functions (map-reading or face recognition, for instance) usually controlled by the right hemisphere. But while left-handers differ from right-handers, they are not the reverse. Like right-handers, the majority (60 to 70 per cent) of left-handers also have left hemispheric speech control, roughly 20 per cent having right-brained speech control, the remainder of lefties processing language on both sides of the brain. Lefties seem to have less rigid and less predictable brain laterality and are more variable than right-handers in the brain’s division of functions. Ambidextrous (bi-handed) individuals with bilateral speech control, who use both right and left sides of the brain to process language, may even have the edge in creativity. The bi-sided cerebral organization may confer more integrated perception by the simultaneous use of verbal and non-verbal thought modes.

A McMaster University researcher has recently reported that the corpus callosum (tract of nerve fibres connecting the brain’s left and right sides) is approximately 11 per cent larger in ambidextrous (bi- or both-handed) individuals, perhaps allowing more efficient inter-hemispheric communication. However, when they sustain brain injury, left-handers may experience language difficulties whichever side of the brain is damaged.

No clear link to pregnancy or birthing problems

Since the middle of this century, reports have trickled in suggesting a higher-than-usual incidence of prolonged labour, prematurity, low birth weight and multiple births among lefthanders. Researchers have also reported left-handers more frequently than usual among the developmentally disordered (especially males) – the mentally retarded, autistics, dyslexics (with reading problems), spastics, epileptics, the hyperactive and the generally clumsy. Some studies have also linked lefthandedness to psychiatric and social problems such as alcoholism, schizophrenia, depression and criminality. Others allege that left-handers tend to mature more slowly, are shorter than average and more accident-prone. Such findings prompted speculation that left-handedness might be pathological possibly resulting from damage to the brain during pregnancy or birth. Slight anoxia (lack of oxygen) during problem pregnancies or a difficult labour might affect brain organization, leading to left-handedness. Among others, Paul Bakan of BC’s Simon Fraser University proposed that damage to the left side of the fetal brain might weaken nerve pathways on the body’s right side, causing otherwise natural right-handers to use their left hands. More recent research from the University of British Columbia suggests that older mothers (aged 40 and up) have more stressful births and tend to produce more leftie offspring. In a survey of 2,000 college students, the UBC researchers found that left-handers were more than twice as likely to have been born to women over 40 than younger women.

The latest scientific views oppose the notion that left-handedness usually stems from birth or prenatal stress. Birth or pregnancy difficulties do not explain most cases of left-handedness. Only a small subset (perhaps one per cent) of lefties become left-handed because of problem births or pregnancies. In the same way, a few natural lefties may be forced into right-handedness by marginal birth injury! (Some researchers object to classifying any left-handers as pathological, since natural right-handers forced into left-handedness by birthing difficulties are no more abnormal than left-handers who “turn right” because of perinatal problems.) The most recent studies also discount a link between left-handedness and childhood disabilities such as autism, dyslexia or stuttering. (However, there seems to be some association between left-handedness and epilepsy, both perhaps a result of early brain damage.)

Genetic theories: is left-handedness inherited?

Many experts now believe there is a genetic or family tendency for handedness, although it’s not always clear how much is inherited or learned behavior. Lefties are more likely than others to produce left-handed children, although many dextral (right-handed) parents have sinistral (left-handed) children. Children with leftie relatives have a three to four times greater-than-usual chance of being left-handed. Recent studies show that about nine per cent of children with two fight-handed parents are left-handed compared to 19 per cent of those with one left-handed parent and 26 per cent of those with two left-handed parents. The latest data indicate that the real prevalence of left-handedness among humans has been under-estimated and may be as high as 14-15 per cent.

Various genetic models have been proposed to explain the inheritance of handedness. One suggests that the gene (or genes) for left-handedness reside in the pseudoautosonial (non sex-linked) arm of the X chromosome. Another proposes that left-handedness occurs when a certain gene – the “right-shift” factor – is missing. Most genetic models leave room for the influence of nature or learned behaviour, and for occasional injury to the brain before or at birth (forcing some natural right-handers to turn leftie). One stumbling block to genetic theories is the fact that although left-handedness is twice as common in twins, identical twins (who invariably have identical genes) are not always same-handed. Only in about half of the identical twin pairs where there’s one leftie is the other also left-handed – a paradox that remains to be investigated. Clearly, besides genes, other influences contribute to hand preference. (Some experts attribute the lower incidence of left-handers among females to greater passivity and conformism among girls than boys, a stronger wish to please by adapting to the rfight-handed norm.) Recent intriguing reports come from Irish researchers who photographed fetuses in the uterus, showing that 90 per cent of all unborn infants suck their fight hand and only about 10 per cent suck their left – roughly the same proportion as lefties in the general population. These findings underscore the idea that lefties inherit their left-handed tendency rather than acquiring the habit by watching others. They are the first to document a hint of handedness before birth. The researchers suspect that these newborn “southpaws” will continue to favour their left hand later in life – a theory that remains for future follow-up. But experts stress the difficulty of assessing handedness in very young children, before motor skills develop.

No proof that lefties die younger

A University of British Columbia and California State University research team recently concluded that – whatever the ultimate cause of left-handedness – lefties are apt to die younger than right-handers. In a recent letter to the New England Journal of Medicine, the researchers claimed a shortened life span for lefties, pointing to the small number of left-handers left among the over-65 year olds. Whereas 13 per cent of 20 year olds are left-handed, by age 50, only five per cent are left-handed, and by age 80 there are hardly any lefties left. Why the disappearance of lefties with advancing years? The UBC team suggested that life in a right-handed world is tough for lefties and leads to more fatal accidents, so that left-handers die on average nine years earlier than equivalent right-handers. The researchers base their argument partly on an analysis of death certificates in two southern California counties, which asked the next of kin about left-handedness in relatives who had died. Also looking up the records of Major League baseball players, the UBC team deduced that leftie players died younger than right-handers. The researchers surmized that left-handers die of accidents more often than right-handers, suggesting that left-handedness is a serious risk. Besides facing the possible drawbacks of birth damage and childhood disorders the UBC researchers hypothesized that left-handers have more accidents in a world where most tools, machines, and vehicles are designed for right-handers. Recent studies show no higher accident rates in right-than left-handers.

Other experts strongly dispute the validity of the UBC claims for more leftie accidents and decreased survival rates. They criticize the studies because of biased samples and a loose, imprecise evaluation of left-handedness. (Simply asking people whether a dead relative was left-handed is not necessarily accurate.) Others, who also examined the death records of Major League baseball players, found no life span differences between right- and left-handers. (In fact, some suggested that leftie players lived longer!) The New England Journal of Medicine received a flurry of letters disputing the idea that lefties die young. One pointed out that “the Framingham Heart Study, one of the most thorough scientific investigations ever conducted, found no difference in mortality rates between left-and right-handed people.” The U.S. National Health Examination Follow-up Study actually found some association between left-handedness and a longer life! Many experts discount any link between left-handedness and a shortened life span. Psychologists give a simpler explanation for the UBC findings, based on former efforts to correct left-handedness. At the beginning of this century, left-handers were commonly pressured by teachers and parents to change hands. With today’s more relaxed approach, lefties are no longer urged to switch. Thus, while uncorrected, younger left-handers (born after the early 1900s) would be counted as lefties, older, corrected left-handers (who have switched hands) might be classified as right-handers, perhaps explaining why only three per cent or less of those now over age 75 are left-handers.

Possible sex hormone and immune system connection

In the early 1980s, two well-known U.S. researchers observed that left-handers seem particularly prone to migraine headaches and certain immune system disorders – particularly Hashimoto’s thyroiditis (viral thyroid infection), myasthenia gravis (a paralytic muscle disorder) and ulcerative colitis (an inflammatory bowel disease). What do such curious findings add up to? The late Dr. Norman Geschwind and Dr. Albert Galaburda of Harvard University speculated that one clue to this oddly assorted data lay in intra-uterine (pre-birth) influences, particularly that of the male sex hormone, testosterone. Produced at an early embryonic stage by “XY” or male fetuses, testosterone guides male gender differentiation and primes specific brain pathways. Re researchers theorized that fluctuations in the levels of fetal testosterone may influence the developing brain and make it take on a slightly different configuration. Dr. Geschwind speculated that excess fetal testosterone at a critical point may retard development of the left hemisphere, allowing the right to dominate, thereby predisposing to left-handedness. An excess of fetal testosterone might thus be one reason for lefthandedness (in males).

According to the Geschwind-Galaburda theory, excess testosterone may also dampen growth of the thymus gland which is responsible for certain immune system defences (involving white blood cells). A slowly maturing thymus could lead to increased immune disorders later in life. Thus, exposure of male fetuses to above-normal amounts of testosterone might explain some puzzling aspects of left-handedness – the preponderance of males over females and the possibly greater risk of immune disorders. However, although ingenious, die role of fetal hormones in left-handedness remains much debated and the theory has many loopholes.

Curiously though, a search for mathematical geniuses at Baltimore’s Johns Hopkins University in the 1980s lent slight support to the Geschwind-Galaburda hypothesis. The Baltimore researchers wanted to identify mathematically gifted grade seven students using the Scholastic Aptitude Test for mathematics. They found that 260 of the top mathematics students were boys, only 20 girls: a ratio of 13 to one. (The ratio diminished with decreasing mathematical ability, until, for average mathematical ability, boys and girls levelled out.) The researchers argued that social conditioning, which generally encourages boys but discourages girls from studying math, might contribute to the discrepancy, also suggesting a direct biological link between male sex and mathematical aptitude “boys do better”). Heating of the Geschwind theory, the Baltimore team reexamined their gifted math students for handedness finding twice the expected proportion of left-handers. Moreover, above average numbers evidently suffered from immune-system disorders ! At present, there’s no confirmed link between immune disorders and handedness. Hard facts are few and far between, and reports from many countries find no consistent elevation of immune disorders among lefties. Critics charge that many of the claims about left-handers and immune disease are based on small, poorly-designed studies, being statistical artefacts rather than facts. A 1992 joint British and University of Waterloo survey calls the Geshwind-Galaburda theory “incorrect in consistently linking left-handedness to immune disorders,” finding “no overall relationship” to handedness in those with asthma, diabetes, lupus erythematosis, rheumatoid arthritis or other immune system defects. There might, however, be an elevated incidence of left-handedness in those with thyroid disorders, migraine and possibly hay fever. By contrast, allergic skin conditions such as eczema, urticaria and myasthenia gravis seem to be more common in right- than left-handers.

Modern “compromise” theories about lefties

Combining the various divergent theories, experts now believe that there are two or more subgroups of left-handers. Most are genetic or biologically based but a small subset of lefties may be “pathological” in origin, perhaps caused by marginal brain injury (where something goes wrong during pregnancy or birth). The exact proportion of “natural” or genetic lefties versus those possibly arising from brain damage remains unclear. Some researchers propose a 50:50 split. Most believe that left-handedness is predominantly inherited.

Wiser to persist than switch

If left-handedness were to carry with it an added risk of immune disorders, developmental handicaps, accidents and other unlucky events, would it be desirable to try and “correct” it (a difficult task)? The psychologists say no. Early this century, teachers and psychologists favoured changing a child’s writing hand from left to right, and some parents and teachers still force children to shift hands. But most experts condemn the practice. For one thing, it’s only successful in the very young and only about a third of the time. Even then, it usually only works for a targeted activity such as writing or drawing. There is also some evidence that forcing a child to switch hands causes undue stress and may lead to stuttering and other stress-related disorders.

Finally, the child may lose dexterity in both hands, and never be able to manage fine eye-hand coordination with either hand. It is also possible that imposing right-handed patterns on a left-handed person’s activities might make the person more accident-prone. Although sinistrals have been considered inordinately abundant among the less fortunate, they are also over-represented at the higher, most gifted end of the scale. Studies find left-handers more common than right-handers among great mathematicians, musicians, actors, artists, engineers and architects. Students of the left hand argue that if left-handedness and mathematical or artistic genius originated from the same mechanism, then curing left-handedness might also “cure” or remove much human talent. Whether their different neuropsychological brain organization also influences the gifts of left-handers remains for research to answer.

In conclusion: While the riddle of left-handedness remains unsolved, many former myths have been laid to rest. Left-handers are not necessarily more prone to accidents or immune disorders and not likelier to have had bad births than right-handers. Nor is there any solid proof that lefties have a shorter life span. Experts generally now believe that there’s a strong genetic link to left-handedness. Most lefties can probably thank their ancestors for their hand preference.

COPYRIGHT 1992 Strategic Inc. Communications Ltd.

COPYRIGHT 2008 Gale, Cengage Learning