How Zero Population Growth Works

A crowd of people waits to cross the street in midtown Manhattan on Oct. 31, 2011, in New York City, the day the U.N. declared the world population had hit 7 billion.
A crowd of people waits to cross the street in midtown Manhattan on Oct. 31, 2011, in New York City, the day the U.N. declared the world population had hit 7 billion.
Spencer Platt/Getty Images

In 1968, Stanford University biology professor Paul Ehrlich published a controversial best-seller titled "The Population Bomb." In it, Ehrlich – and his wife Anne, uncredited – prophesied imminent environmental catastrophe and human suffering caused by overpopulation.

"The battle to feed all of humanity is over," Ehrlich pronounced in the book's dramatic opening line. "In the 1970s the world will undergo famines – hundreds of millions of people are going to starve to death in spite of any crash programs embarked upon now" [source: Ehrlich and Ehrlich].

In 1968, there were 3.56 billion people living on Earth [source: U.S. Census Bureau]. Earlier in the decade, American women were giving birth to more than 3.5 babies on average, and the global fertility rate was five babies per woman worldwide [source: Suzuki].

When Ehrlich did the math back in the 1960s, he saw worldwide population growing at an exponential rate. Global food production, on the other hand, was stagnated by droughts and decreasing fertility of farmland [source: IFPRI]. Combining too many people with too little resources, the obvious result for Ehrlich would be a world ravaged by food shortages, mass famine and even the collapse of civilization itself.

The same year Ehrlich published "The Population Bomb," he cofounded Zero Population Growth (ZPG), a grassroots organization that joined forces with the burgeoning environmental movement to raise awareness about the threat of overpopulation.

The central tenet of the zero population growth movement is not that humans should stop having babies. Rather, we should stop having more babies than we want. When women have control over their fertility – through education, contraception and improved women's rights – birth rates inevitably go down.

The goal of zero population growth is to reach a sustainable global birth rate at or below "replacement level." This is the fertility rate at which population is maintained, but not grown. Replacement level is affected by many factors, notably the average life expectancy. The longer people live, the fewer babies you need to replace them. In the U.S., the replacement level fertility rate would be 2.1 babies per woman. In certain developing countries where there are higher death rates and shorter lifespans, it could be 3.0 babies per woman.

From the day it was published, "The Population Bomb" drew angry criticism from groups opposed to contraception and skeptical of Ehrlich's doomsday rhetoric. Nearly 50 years later, there are twice as many people on Earth (7.2 billion), and it's hard to ignore the fact that England has not devolved into a "small group of impoverished islands, inhabited by 70 million hungry people," as Ehrlich predicted, or that 4 billion people have not, in fact, died of starvation [source: Chivers].

But do Ehrlich's failed predictions mean that zero population growth is itself a dumb idea? Keep reading to explore the real threat of overpopulation, proposed solutions and whether it's an exercise in futility.