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How Many People Can the Earth Support? by Joel E. Cohen

Horn of Plenty?

How Many People Can the Earth Support?
by Joel E. Cohen
Norton. 532 pp. $30.00

For decades, debate about overpopulation has been frozen at two poles. In one camp are what have been called “neo-Malthusians,” pessimists who believe that the world already holds more people than it can sustain, and that as environmental degradation proceeds, excess population will likely be pruned by disease, disorder, starvation, and war. In the other camp are people whom some critics have labeled “Cornucopians,” those who observe that in many locations rapid population growth has been consistent with broad improvement in the conditions of life; they maintain that thanks to the self-correcting mechanism of markets, and to human ingenuity, scarcity can be fended off indefinitely.

In this debate, the neo-Malthusians’ position is by far the more popular, even if intellectually theirs is much the harder row to hoe. Paul Ehrlich, perhaps the most vocal exponent of this camp and the author of the runaway best-seller, The Population Bomb (1968), has been repeatedly proved wrong in his predictions; the 1970’s, 80’s, and half the 90’s have come and gone without a sign of anything resembling his frightening scenarios of destruction. On the other side, perhaps the most outspoken of the Cornucopians is the economist Julian Simon, the author of, among other books, The Ultimate Resource (1981). Simon’s contention that scarcity does not set “limits to growth” has yet to be contradicted by the final arbiter: history itself.

Nevertheless, the past cannot guarantee the future, and the earth’s continuing ability to support a burgeoning humanity remains an issue of burning import, with implications for other contentious matters ranging from environmental regulation to the role of government in family planning. All the more reason, then, to welcome this new book by Joel E. Cohen, the head of the Laboratory of Populations at Rockefeller University and the author of a significant body of work on topics lying at the intersection of ecology, mathematics, and social affairs.



Cohen’s argument in How Many People Can the Earth Support? is divided into two major parts. First, he lays out in great detail what is and—just as pertinently—what is not known about past population growth, and how we reached our present record level of 5.7 billion souls; along the way, he examines and criticizes the various projections of growth produced by demographers at institutions like the United Nations and the World Bank. In the second part, Cohen discusses the meaning of “carrying capacity,” or the number of organisms that can be supported by a given resource base. Bringing together his conclusions from both sections, he then analyzes a range of estimates for how many human beings the planet can sustain.

Cohen sums up his fundamental finding simply enough: the “future of human population growth is uncertain.” Rather an underwhelming conclusion, one would think—were it not for Cohen’s powerful demonstration of how strongly such a posture of intellectual humility runs against the tide. In his ground-clearing analysis, written with critical intelligence, clarity, and wit, he not only puts before us the widely varying estimates that demographers have hitherto set forth but also exposes the unarticulated or insufficiently defended assumptions upon which their projections rest.

Absent a major catastrophe, writes Cohen, the only thing of which we can be sure is that the number of human beings will continue to grow for some time to come. By how much, how fast, and where cannot be stated with any confidence. Among other uncertainties, population growth depends on how many children future generations will want to have, and demographers have never enjoyed notable success in predicting the contours of family size.

Uncertainty about global carrying capacity is, necessarily, just as great. Human beings are not like other animals, who in the face of limits on their resources must migrate or die. As Cohen and many others before him have pointed out, we can develop technologies that extend the availability of resources, or substitute new for exhausted ones. Moreover, limits to carrying capacity cannot be ascertained without knowing both how people want to live and how capable they are of achieving their goals. Such desires and abilities, of course, change over time and vary in different circumstances, again making estimates of carrying capacity highly contingent.



Given this indeterminacy, what kinds of policies would Cohen have us adopt? He evaluates the now-traditional arsenal of social planners: increased access to contraception, economic development, “empowering women, educating men; doing everything at once.” Although he endorses them all, he does not repose much confidence in our ability to know which if any of these methods works best, or to understand the tradeoffs among them. He therefore suggests such modest preliminary steps as better collection of data, refined economic accounting, more research into the interactions of population, culture, and economics, greater rationalization of the systems that make difficult distributive decisions, and a more expansive sense of the ethical demands of “mutual aid.”

These suggestions make sense as far as they go. But if, as is implicitly conveyed by Cohen’s book, population problems ultimately revolve around such questions as the equitable distribution of resources, trust in government, and the shape of daily life, then they recapitulate all the stuff of political dispute for as long as there has been politics. While social scientists have long sought to exclude values from “rational” discourse, Cohen’s book shows that this cannot be done in any rational discussion of population. A growth rate that might horrify a Chinese bureaucrat in charge of family-planning might be found desirable by, say, an Israeli government minister of housing and development.

There is not one “population problem.” Rather, there are “population problems,” as many and varied as the lives that people seek in common. For providing a wealth of illustrations of this fundamental insight—not to mention for debunking a good deal of shaky and shoddy scholarship—Cohen’s How Many People Can the Earth Support? deserves to be widely read.

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