NPR. Here are two seemingly contradictory truths about the people of planet Earth. The first is that

over the past century, the world's population has exploded, going from around

2 billion people to 8 billion people today. Truth number two is even as the Earth's population

has boomed, the average fertility rate has declined. And if things keep going that way,

a few decades from now, the world's population will actually

start to shrink. And just as the world's population spiked in recent centuries, it could also

crash. And our guests today say that we're living near the very edge of this crash.

This is the indicator from Planet Money. I'm Adrienne Ma. Today on the show, our guests are

economists Dean Spears and Mike Jerusalem. They've Adrienne Ma. Today on the show, our guests are economists

Dean Spears and Mike Jerusalem. They've co-authored a forthcoming book called After the Spike,

Population, Progress, and the Case for People. And when we come back, they'll explain why,

population-wise, the human party will soon peak, and why that's a problem.

For economists who study population growth and decline, the fertility rate is a very important metric.

The fertility rate is the average number of children a woman

has in a lifetime. And here, two is kind of the magic number. That's what demographers call

the replacement rate. And when the global fertility rate falls below two for long enough,

that leads over time to something called depopulation. Depopulation is the name for what happens when birth rates are low enough that the population shrinks,

decade by decade and generation by generation.

This is Dean Spears, an economist at the University of Texas at Austin.

And while fertility rates vary from country to country,

on a global scale, they've actually been declining

for centuries. And in fact, the vast majority of people on Earth today live in a country where

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