When Stanford biologist Paul Erlich published the population bomb, there were about 3.5 billion people on the planet, and it looked like the only thing that might reverse the trend was the moment when we literally ran out of food to keep everybody alive. Since 1968, we have added nearly

5 billion people to the count. We're still adding people. But something few people predicted

decades ago is also true. We are likely within a few years to hit the maximum number of humans ever,

and then see that number start to fall and keep falling. From KERA in Dallas,

this is Think. I'm Chris Boyd. Maybe you hear that and feel relieved, but there are reasons

to be concerned for the future of humanity unless we can find a way to

stabilize the global population. And that turns out to be a surprisingly challenging thing to do.

Dean Spears is founding executive director of the Research Institute for Compassionate Economics

and Associate Professor of Economics at the University of Texas at Austin. His book, written with

Michael Jeruso, is called After the Spike, Population, Progress,

and the Case for People.

Dean, welcome to think.

Thank you so much for having me.

We know that people in most parts of the world, childbearing adults, are now reproducing

at a level below the replacement rate.

Do we know why this is happening?

That's right.

So most people around the world now live in a country where the birth rate is below the two

kids for two adults level that would stabilize the population.

And that means that global depopulation is the

most likely future, where each generation will be smaller than the generations before. Fundamentally,

that's going to change a number of things, and it prompts the questions, why are birth rates

...