Population conundrum, The

population conundrum, The

Eldredge, Niles

How Many People Can the Earth Support?, by Joel E. Cohen. New York and London: W.W.Norton & Company, 532 pp.

Two-thirds of the way through his admirable tour de force on human population, Joel Cohen tells a joke, repeated here for the insight it yields on the tone, content, and position that he strikes throughout his book:

An ecologist, an economist, and a statistician went on a deer hunt with bow and arrow. When a deer was spotted, the ecologist shot first, but his arrow landed five meters to the left of the deer. The economist shot next, but her arrow landed five meters to the right of the deer. The statistician looked at both arrows, looked at the deer, and jumped up and down shouting, “We got it, we got it]”

Though Cohen uses the story to chastise those who look only to averages rather than distributions, Cohen himself repeatedly seeks to box in the plausible range of answers to the questions: What has human population growth in the past been like? Where is it likely to go in the future? How many people can the Earth support? If Cohen is not himself looking at averages, he is very definitely staking out the statistician’s middle ground–eschewing the sometimes extreme apocalyptic visions of ecologists and the often far rosier view of economists, some of whom are on record that there can never be too many people on Earth.

Indeed, two pages after the joke we find a grim quotation from ecologists Gretchen Daily and Paul Ehrlich, proclaiming that we have already outstripped global carrying capacity, followed immediately by an assurance from economist Julian Simon and technologist Herman Kahn that the carrying capacity has continued to grow sufficiently so that by now the term “has no useful meaning.” The tension between biologically minded analysts of human population and those who approach the issues from the perspective of economics and, especially in Cohen’s pages, demography, constitutes a particularly fascinating subtext in this work. Though Cohen, as might be expected from the head of the Laboratory of Populations at Rockefeller University, leans toward the ecological approach and takes occasional slams at demography, for the most part he judiciously steers a middle course.


Cohen’s real aim is to reveal how enormously complicated his three seemingly simple questions on human population really are–how difficult it is to get any hard data and how complex notions such as carrying capacity truly are. (For instance, determining what level of material well-being is “acceptable” and to whom is integral to deciding how many people the Earth can support.) Cohen deeply distrusts simple-minded curve-fitting as a means of estimating future human population growth and palpably dislikes all forms of oracular pronouncements of certainty on any of these issues. Indeed, the joke I have quoted is mirrored in several “laws” of Cohen’s own devising –each wryly humorous and all intended to highlight the doubt and uncertainty that Cohen feels pervades his subject. For example, his Law of Prediction states, “The more confidence someone places in an uncomfortable prediction of what will happen in human affairs, the less confidence you should place in that prediction. If a prediction comes with an estimated range of error, then the narrower that range, the less you should believe it.”

Polemicists be warned: This book will frustrate any reader who is looking for absolute, definitive answers to Cohen’s three questions. That is the book’s main strength and contribution. Cohen has produced such a thorough and painstaking piece of scholarship that I would not be in the least surprised to find that it is (and is likely to remain for some time) the most complete survey of what other people–including an ancient Babylonian whose words open the book and John Stuart Mill, whose words close the main narrative–think of the issues.

But although Cohen’s text is rich with the verbal and graphic conclusions of others, it is thoroughly infused with his perspective. Cohen is constantly probing the credibility of all those he cites. Along the way, he keeps inviting his readers not to take his analytic word for it, but to check his own calculations. He means it when he says: Trust nobody on these issues.

Cohen himself offers no starting-from-scratch estimates of past, future, and ultimate population numbers. Instead he seeks to find a range of plausible answers based on everyone else’s assumptions and calculations. His conclusion can be stated briefly: “The possibility must be considered seriously that the number of people on the Earth has reached, or will reach within half a century, the maximum number the Earth can support in modes of life that we and our children and their children will choose to want.” He points out that population has doubled in the past 40 years, that credible estimates (based on assumptions of fertility) of future growth range between 7.8 billion and 12.5 billion people by the year 2050, and that estimates for the carrying capacity of the Earth, ranging between 1 billion and 1 trillion people , most credibly fall between 7.7 and 12 billion people.


Thus Cohen concludes that we have a problem on our hands–no surprise to many pessimistic pundits but all the more sobering given the relatively dispassionate care and thoroughness of Cohen’s analysis. The remedies? Cohen lists six “commandments” to guide efforts to reduce population growth: promote contraceptives; develop economies; save children; empower women; educate men; and, finally, do all of the above. The last is important because Cohen points out that some will be more effective than others in particular places depending on the cultural context.

Cohen spends some time showing that the “demographic transition”–in which birth rates eventually fall to a point equal to, or even lower than, death rates–is by no means an automatic, lasting consequence of industrialization and economic development. Likewise, counter to a position that I have recently been taking, Cohen concludes that the latest single-fix solution–the education and economic empowerment of women–is no greater a guarantee of achieving a demographic transition than is the older notion of economic development.

At the end of the book, Cohen does offer some suggestions of his own, aimed at establishing an infrastructure for problem solving. But as he says at the very beginning, his real purpose is to show that “interactions among populations, economies, environments and cultures” informs and enriches the dialogue on human carrying capacities and will enable people to come to conclusions on what to do about the problem on their own. Herein lies Cohen’s main beef with demography and economics: the failure of both disciplines to appreciate some of the contextual complexities of the population problem.

At several junctures in the text, Cohen charges that demographers pay virtually exclusive heed to the internal dynamics of population growth and little or no attention to “ecological, economic, cultural and other factors.” Somewhat later, remarking that “demographers fear to tread where ecologists rush in,” he speculates that “the present theoretical framework of demography focuses attention on the composition and growth of populations and diverts attention from their absolute size” and suggests that similar problems bedevil economics. Although I am not certain I understand him completely, I find it remarkable that so many economists seem to view environmental degradation, species loss, and the depletion of essentially nonrenewable resources very differently from ecologically minded biologists, as if the world were really put here originally for our use so that it really doesn’t matter very much how we approach the global system as long as we achieve measurable economic growth.


Cohen presents an instructive overview of the varied meanings of the term “carrying capacity” in both pure and applied ecology. He is especially compelling in his arguments that none of the concepts apply in a straightforward manner to the human condition. But I confess I was perturbed by some of the ecological and evolutionary biology I encountered in this book, specifically questionable assumptions about what species and populations are (Cohen, like many evolutionary biologists, seems not to make a distinction between them) and what characteristic roles such entities play in the material world.

For example, little of the tabulated information and none of his discussion indicates that carrying capacity in ecology is rooted in a context of its own: Species are invariably broken up into local populations, which are parts of and play concerted roles in local ecosystems. There is, typically, a (fluctuating) number of individuals per species per local ecosystem, the number being determined by factors such as the availability of food. Thus the total number of individuals of any given species at any one time is a simple sum of the number of populations it has. It is the populations within ecosystems, and not species per se, whose numbers are regulated.

Except with humans. Cohen agrees with the common perception that what he calls the “local agricultural evolution” that occurred in several discrete places starting some 10,000 years ago had a sudden and dramatic effect on the number of humans. “At a minimum,” he concludes, “the average rate of population growth increased by a factor of 13 after the local agricultural evolution, compared to the earlier period.” Why? Cohen does not point out that with this initial agricultural revolution came an unprecedented ecological change: Agriculturist human beings were no longer de facto members of the local ecosystem–that is, they no longer lived in relatively small bands whose numbers were governed by the productivity and other aspects of local ecosystems. Conversion of countryside to botanical monocultures changed the rules dramatically, as did the domestication of animals.

Though there were subsequent population “evolutions,” none changed our ecology so radically. The reason, in my view, is why there is indeed an ultimate cap on the total number of humans on the planet is that, once again, the current population of 5.7 billion Homo sapiens has begun to interact more or less as a unit with the global environmental system. No other species in the (minimally) 3.5 billion-year history of life on Earth has ever played such a concertedly unified and direct ecological role.

Ironically, then, over the past 10,000 years, Homo sapiens has emerged as the first species that can be realistically treated as a single enormous population–freed as all but a relict handful of hunter-gathering peoples are from the confines and controls of life in the local ecosystem. This newfound global-interactive condition has developed rapidly, particularly in the last century, and thus the bulk of Cohen’s analysis, devoted as it is mainly to the past few centuries and especially to the present, remains essentially unaffected by such considerations. Still, given Cohen’s drive to get demographers and economists to consider the complexities of environmental context, it would be nice as well to see ecologists and evolutionary biologists doing the same.

But such objections are minor compared with the marvelous scholarly achievement and genuinely important contribution that Cohen has given us. How Many People Can The Earth Support? is by far the most sober and serious consideration available of what must be the premier millennial issue confronting Homo sapiens. By all means read it yourself–with or without a calculator. You can trust Joel Cohen.

Niles Eldredge is a curator in the Department of Invertebrates at the American Museum of Natural History, in New York City, and author of Reinventing Darwin (John Wiley and Sons, 1995) and Dominion: Can Nature and Culture Coexist? (Henry Holt, 1995).

Copyright Issues in Science and Technology Spring 1996

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