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For a long time, the story about the world\u2019s 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\u2019re headed toward demographic catastrophe.\n\nBut when I look at these numbers, I just find it strange. Why, as societies get richer, do their fertility rates plummet?\n\nMoney makes life easier. We can give our kids better lives than our ancestors could have imagined. We don\u2019t 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?\n\nI invited Jennifer D. Sciubba on the show to help me puzzle this out. She\u2019s a demographer, a political scientist and the author of \u201c8 Billion and Counting: How Sex, Death and Migration Shape Our World.\u201d She walks me through the population trends we\u2019re 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.Mentioned:\n\n\u201cWould You Have Four Kids if It Meant Never Paying Taxes Again?\u201d by Jessica Grose\n\n\u201cAre Men the Overlooked Reason for the Fertility Decline?\u201d by Jessica Grose\n\n\u201cIf We Want More Babies, Our \u2018Profoundly Anti-Family\u2019 System Needs an Overhaul\u201d by Jessica Grose\n\nBook Recommendations:\n\nExtra Life by Steven Johnson\n\nThe Bet by Paul Sabin\n\nReproductive States edited by Rickie Solinger and Mie Nakachi\n\nThoughts? Guest suggestions? Email us at ezrakleinshow@nytimes.com.\n\nYou can find transcripts (posted midday) and more episodes of \u201cThe Ezra Klein Show\u201d 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.\n\nThis episode of \u201cThe Ezra Klein Show\u201d 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\u2019s 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 Jessica Grose and Sonia Herrero.