Emptying Planet

Paul Ehrlich's legacy

On March 13, 2026, Paul Ehrlich, who was the Bing Professor of Population Studies at Stanford, died at the age of 93. He was a prominent biologist who made numerous significant contributions, but he was best known for his work on the dynamics of population growth. In his 1968 book “The Population Bomb,” co-authored with his wife Anne, Ehrlich predicted that overpopulation would lead to famine, civil unrest, even societal collapse. In 1970, he predicted that within a decade “100-200 million people will be starving to death.” Ehrlich became a household name in the US. He was a regular guest on the Johnny Carson show.

His alarmist ideas about population growth became very influential in the 1970s, partly because they tapped into deep-seated existential angst at the time. There was bipartisan support for the notion that population growth was out of control. President Richard Nixon even endorsed a gradual transition to zero population growth. Governments around the world started to organize conferences on the topic of overpopulation. Some started calling for radical measures such as forced sterilization of women after they had given birth to a certain number of children. Abroad, Ehrlich’s ideas have a dark legacy. The World Bank gave the Indian government loans to fund a sterilization program between 1972 and 1980. India in fact adopted a program of mass sterilization. Eight million people were sterilised. People were forced to show proof of sterilization to obtain salaries or medical treatment. China launched the one-child policy in 1979.

The NYTimes obituary delicately states: “Such bold predictions, some of which turned out to be premature or in error, prompted rivals in business and academia to question the validity of his claims.” In fact, none of his predictions turned out to be premature.

First, we just became much better at producing food. In fact, our farmers are now productive enough to feed a world with many more. As my colleague Chad Jones pointed out, we could imagine a world of 100 billion people all living in cities with the same density as Paris. We would only use 5% of habitable land. If we all stop eating burgers, we could feed everyone if we were all as productive as the Dutch currently are in agriculture. If we don’t give up burgers, we could feed everyone if we were 30% more productive than the Dutch currently are in agriculture.

Second, fertility rates started to decline in advanced economies around the time the Ehrlichs wrote their book. And the fertility rates then continued to decline around the world, with the exception of sub-Saharan Africa.¹ The total fertility rate for the planet, which measures the number of children per woman of childbearing age, is now 2.17, below the replacement rate of 2.21. As a result, not enough babies are being born to keep the population number steady. Instead, the global population will start to shrink around 2055 (see Jesus Fernandez-Villaverde’s excellent slide deck on this topic). And the planet will start to empty. Only 10 years ago, the consensus view was that the world population would stabilize at 10 to 12 billion. The UN thinks it will take much longer because their projections assume that the fertility rates will recover in the years ahead and revert back to the replacement rate, but there doesn’t seem to be a solid rationale behind this assumption.²

The global fertility numbers are striking, especially once you look outside of Europe and North America. Chile now has a fertility rate of 1.14. China recorded a fertility rate of 1 in 2025, well below that of rapidly aging countries like Japan and Italy. As Dean Spears and Mike Geruso put it in their excellent book After the Spike, “No future is more likely than that people worldwide choose to have too few children to replace their own generation. Over the long run, this would cause exponential population decline.”

Ehrlich could not have been more wrong. Our kids and grandkids are likely to live in an emptying planet. I’ll borrow Chad Jones’ beautifully simple example. Suppose each household has 1 child (a fertility rate of 1). Population halves each generation (25 years). 1 billion in 3 generations (75 years), 125 million in 6 generations (150 years) and 8 million in 10 generations (250 years). This is a misleading model, but it’s quite useful to illustrate population dynamics. For the world population to stabilize at any level, the global fertility rate has to revert back to the replacement rate.

It’s stunning to see how slowly the demographic projections caught up with these demographic changes. As recently as 2012, the UN predicted that the world population would not peak in the 21st century. Only in 2022 did the UN project a peak in the 21st century. As argued by Jesus Fernandez-Villaverde, these revised estimates may still be too optimistic.

While Ehrlich primed the world to prepare for overpopulation, all this time we should have been getting ready to deal with an emptying planet. The economic implications are significant. Negative population growth is a headwind for economic growth. Every 1 percentage point decline in the population level translates one for one into a 1 percentage point decline in the size of GDP, unless everyone becomes 1% more productive. And the size of total GDP, not GDP per capita, is what matters for the government’s ability to keep its promises to its bondholders.

In advanced economies, governments have been using government debt to shift the increasing burden of caring for the old and the sick to future generations, especially in those rapidly aging countries with unfunded pension systems —think Japan and Italy. That makes electoral sense, but it’s not sustainable as well as questionable when you think about intergenerational equity.

Ehrlich’s Stanford obituary quotes one of his colleagues as saying: “For me, Paul’s biggest influence was demonstrating that one can be both a great scientist and a great advocate for social progress.” To me, his career signals the enormous pitfalls of advocacy for scientists. Ehrlich’s dire predictions were actually just based on simple and naive extrapolations of population growth rates without any consideration of technological advances or the fundamental determinants of population growth. His predictions were not scientific at all.

Based on a hunch, he tried to convince a generation that having kids was a selfish and destructive act. He couldn’t have been more wrong.

1. The fertility rate is the average number of children would be born to a woman if she were to live through ages 15-44.

2. “The rebound in future fertility for low-fertility countries is consistent with an expectation of continued progress toward gender equality and women’s empowerment and improving social and economic opportunities for young people and families.“ WPP 2024, Summary of Results, page 23, BoxII.1