For a long time, the story about the world’s population was that it was growing too quickly. There were going to be too many humans, not enough resources, and that spelled disaster. But now the script has flipped. Fertility rates have declined dramatically, from about five children per woman 60 years ago to just over two today. About two-thirds of us now live in a country or area where fertility rates are below replacement level. And that has set off a new round of alarm, especially in certain quarters on the right and in Silicon Valley, that we’re headed toward demographic catastrophe.But when I look at these numbers, I just find it strange. Why, as societies get richer, do their fertility rates plummet?Money makes life easier. We can give our kids better lives than our ancestors could have imagined. We don’t expect to bear the grief of burying a child. For a long time, a big, boisterous family has been associated with a joyful, fulfilled life. So why are most of us now choosing to have small ones?I invited Jennifer D. Sciubba on the show to help me puzzle this out. She’s a demographer, a political scientist and the author of “8 Billion and Counting: How Sex, Death and Migration Shape Our World.” She walks me through the population trends we’re seeing around the world, the different forces that seem to be driving them and why government policy, despite all kinds of efforts, seems incapable of getting people to have more kids.Book Recommendations:Extra Life by Steven JohnsonThe Bet by Paul SabinReproductive States edited by Rickie Solinger and Mie NakachiThoughts? Guest suggestions? Email us at ezrakleinshow@nytimes.com.You can find transcripts (posted midday) and more episodes of “The Ezra Klein Show” at nytimes.com/ezra-klein-podcast. Book recommendations from all our guests are listed at https://www.nytimes.com/article/ezra-klein-show-book-recs.This episode of “The Ezra Klein Show” was produced by Rollin Hu. Fact checking by Michelle Harris, with Kate Sinclair and Mary Marge Locker. Mixing by Isaac Jones. Our senior editor is Claire Gordon. The show’s production team also includes Annie Galvin, Jeff Geld and Kristin Lin. Original music by Isaac Jones. Audience strategy by Kristina Samulewski and Shannon Busta. The executive producer of New York Times Opinion Audio is Annie-Rose Strasser. Special thanks to Sonia Herrero.