Slate | January 9, 2013
By Jeff Wise

The world’s seemingly relentless march toward overpopulation achieved a notable milestone in 2012: Somewhere on the planet, according to U.S. Census Bureau estimates, the 7 billionth living person came into existence.

Lucky No. 7,000,000,000 probably celebrated his or her birthday sometime in March and added to a population that’s already stressing the planet’s limited supplies of food, energy, and clean water. Should this trend continue, as the Los Angeles Times noted in a five-part series marking the occasion, by midcentury, “living conditions are likely to be bleak for much of humanity.”

A somewhat more arcane milestone, meanwhile, generated no media coverage at all: It took humankind 13 years to add its 7 billionth. That’s longer than the 12 years it took to add the 6 billionth—the first time in human history that interval had grown. (The 2 billionth, 3 billionth, 4 billionth, and 5 billionth took 123, 33, 14, and 13 years, respectively.) In other words, the rate of global population growth has slowed. And it’s expected to keep slowing. Indeed, according to experts’ best estimates, the total population of Earth will stop growing within the lifespan of people alive today.

And then it will fall.

This is a counterintuitive notion in the United States, where we’ve heard often and loudly that world population growth is a perilous and perhaps unavoidable threat to our future as a species. But population decline is a very familiar concept in the rest of the developed world, where fertility has long since fallen far below the 2.1 live births per woman required to maintain population equilibrium. In Germany, the birthrate has sunk to just 1.36, worse even than its low-fertility neighbors Spain (1.48) and Italy (1.4). The way things are going, Western Europe as a whole will most likely shrink from 460 million to just 350 million by the end of the century. That’s not so bad compared with Russia and China, each of whose populations could fall by half. As you may not be surprised to learn, the Germans have coined a polysyllabic word for this quandary: Schrumpf-Gessellschaft, or “shrinking society.”

American media have largely ignored the issue of population decline for the simple reason that it hasn’t happened here yet. Unlike Europe, the United States has long been the beneficiary of robust immigration. This has helped us not only by directly bolstering the number of people calling the United States home but also by propping up the birthrate, since immigrant women tend to produce far more children than the native-born do.

But both those advantages look to diminish in years to come. A report issued last month by the Pew Research Center found that immigrant births fell from 102 per 1,000 women in 2007 to 87.8 per 1,000 in 2012. That helped bring the overall U.S. birthrate to a mere 64 per 1,000 women—not enough to sustain our current population.

Moreover, the poor, highly fertile countries that once churned out immigrants by the boatload are now experiencing birthrate declines of their own. From 1960 to 2009, Mexico’s fertility rate tumbled from 7.3 live births per woman to 2.4, India’s dropped from six to 2.5, and Brazil’s fell from 6.15 to 1.9. Even in sub-Saharan Africa, where the average birthrate remains a relatively blistering 4.66, fertility is projected to fall below replacement level by the 2070s. This change in developing countries will affect not only the U.S. population, of course, but eventually the world’s.

Why is this happening? Scientists who study population dynamics point to a phenomenon called “demographic transition.”

“For hundreds of thousands of years,” explains Warren Sanderson, a professor of economics at Stony Brook University, “in order for humanity to survive things like epidemics and wars and famine, birthrates had to be very high.” Eventually, thanks to technology, death rates started to fall in Europe and in North America, and the population size soared. In time, though, birthrates fell as well, and the population leveled out. The same pattern has repeated in countries around the world. Demographic transition, Sanderson says, “is a shift between two very different long-run states: from high death rates and high birthrates to low death rates and low birthrates.” Not only is the pattern well-documented, it’s well under way: Already, more than half the world’s population is reproducing at below the replacement rate.