In Empty Planet: The Shock of Global Population Decline (Crown, Feb.), Ibbitson and Darrell Bricker predict an unexpected crisis of underpopulation on the horizon.

The UN forecasts that the world’s population will grow to 11.2 billion by 2100, up from 7.6 billion now. But you disagree. Why?

We talked to demographers who think global population will top out between eight and nine billion people around the middle of the century, then go down. We could be back to seven billion by the end of this century, which will not be dominated by population growth, as many think, but by population decline.

What’s the evidence for that?

In the developed world, almost two dozen countries are losing population. Developing countries are headed that way. China’s fertility rate is well below the replacement rate of 2.1 children per woman. Brazil has gone below replacement rate; India and Mexico are reaching it. Africa is the last region on Earth with high fertility, and even there fertility declines are accelerating—faster, according to many demographers, than the UN is accounting for.

What’s causing birth rates to plummet?

More than anything else, urbanization. Once people move from the farm into the city, children stop being an asset, hands to work the field, and start being a liability, just another mouth to feed. With urbanization comes improved education for women. When women get education and autonomy, they stop having so many babies. We interviewed women at a dinner party in Belgium, a university in Seoul, a favela in São Paulo, a slum in Delhi. We discovered an amazing commonality: women want control over their futures, and that involves having fewer children than their parents did.

So we probably won’t be eating soylent green. What else is good, and bad, about the population bust?

It’s nothing but good environmentally. It will mitigate global warming and help the oceans recover. Economically, there’s nothing but bad. First, you have fewer young people paying taxes to sustain old people. Second, economies are driven by consumption by growing families. With fewer young people and smaller families, the drop in consumption will be a challenge to economic growth.

Is there anything to be done about population decline?

You can encourage people to have more kids with child-support programs, parental leave, and daycare, but those are expensive and have only marginal impacts. The alternative is immigration. This is a huge advantage for countries that are already immigrant-based, like the United States and Canada; they can replace the babies that aren’t born with immigrants.