In 1980, the economist Julian Simon took to the pages of Social Science Quarterly to place a bet against his intellectual rival, the biologist Paul Ehrlich. The Population Bomb, Ehrlich’s 1968 bestseller, had argued that the staggering growth of the human species threatened to jeopardize life on Earth. Simon insisted that, contrary to Ehrlich’s predictions, humanity would not self-destruct by overusing the planet’s resources. Instead, Simon believed that humans would innovate their way out of scarcity. Human ingenuity, Simon wrote, was “the ultimate resource.”

Their wager was specifically about the changes in the prices of a suite of commodities over a ten-year period, but it represented much more. The infamous bet was a battle between two larger camps: the catastrophists, who thought that humans were breeding themselves into extinction, and the cornucopians, who believed markets and new technologies would work together to lower prices no matter how big the population became. Ehrlich ultimately lost that bet at a time when global economic conditions favored Simon’s optimistic view of the functioning of markets. Countries also avoided catastrophe as the soaring growth of the world’s population in the twentieth century did not lead to mass famine but to growing prosperity and rising standards of living.

Nearly half a century later, this debate persists in a new form. Many environmentalists still share Ehrlich’s original concern and worry that population growth and consumption continue to vastly outpace the planet’s ability to cope with unrelenting extraction and pollution. The challenge to that view, however, comes from a different place today. The problem, a new kind of catastrophist insists, is not too many people but too few. Although the last century saw an astounding six billion people added to the total world population, today two out of three people live in countries that have fertility rates below the replacement level—the rate of births per woman required to sustain natural population growth. The average number of children born per woman has been falling so rapidly that the UN Population Division estimates that 63 countries or territories have already hit their peak population size. Although the overall human population may eventually rise to around ten billion by about 2060 or 2080 (according to various estimates), it will fall thereafter—and precipitously, with each generation smaller than the last.

Simon’s cornucopian vision, with all its faith in ingenuity, was fueled by a seemingly endless supply of new people, bringing fresh minds and innovative ideas. Although they share much of Simon’s worldview, the economists Dean Spears and Michael Geruso have seen their faith eroded by steep plunges in fertility rates around the world. In After the Spike: Population, Progress, and the Case for People, they show that the world is at a critical juncture: down one path, humanity could experience a stunning and stunting depopulation; alternatively, societies could find a way to stabilize population levels by encouraging people to have more children. Only this latter route will allow societies to maintain and strengthen the sources of their flourishing.

At a time when much pronatalist rhetoric veers into xenophobia and misogyny, Spears and Geruso offer a welcome intervention. They acknowledge the reality of climate change and the centrality of individual rights even as they stress that depopulation is a real problem and a threat to human well-being. They hold these seemingly opposed thoughts side by side. As they write: “It would be better if the world did not depopulate. Nobody should be forced or required to have a baby (or not to have a baby).” (italics in the original)

The authors privilege a moral argument over an economic one, insisting that a world with more people is in and of itself a better one. But that emphasis provides only a weak guide for action. Simon argued decades ago for continued population growth because he thought such growth meant that more human beings could lead productive and meaningful lives. Spears and Geruso concur. But instead of rehashing that utilitarian reasoning, they could have provided a map to guide societies down what they consider the better path of population stabilization, which would require people to have more babies than they are having now.

That inability to offer a more concrete way forward may stem from the broad scale of the authors’ vision. They choose to meet environmentalists at the planetary level, worrying about the carrying capacity of Earth.........

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