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Our society\u2019s dominant narrative is that body size is a product of individual willpower. We are skinny or fat because of the choices we make: the kinds of food we buy, the amounts we eat, the exercise regimens we follow.\n\nResearch has never been kind to this thesis. It\u2019s a folk narrative we use to punish people, not an empirical account of why residents of most rich countries are getting heavier over time. But, then, what account does fit the data?\n\nIn his 2017 book, \u201cThe Hungry Brain,\u201d Stephan Guyenet, a neurobiologist, argues that weight gain is less about willpower than it is the product of an evolutionary mismatch between our brains, our genetics and our environments. Now a new class of weight loss drugs is raising the possibility that we can change our brains to fit this new environment.\n\nPaired with diet and exercise, Ozempic and Wegovy caused anywhere from about a 15 percent to 18 percent loss of body weight over the course of just over a year in people classified as obese or overweight. And they do this in a way that aligns exactly with Guyenet\u2019s research: They don\u2019t make our bodies burn more calories, they make our brains crave less food.\n\nSo I asked Guyenet on the show to talk me through his model of weight gain, the research on these new drugs and the strange implications of living with old brains in a new world.\n\nMentioned:\n\n\u201cRelationship between food habituation and reinforcing efficacy of food\u201d by Katelyn A. Carr and Leonard H. Epstein\n\n\u201cDietary obesity in adult rats: Similarities to hypothalamic and human obesity syndromes\u201d by Anthony Sclafani and Deleri Springer\n\n\u201cWhy Have Americans Become More Obese?\u201d by David M. Cutler, Edward L. Glaeser and\n\nJesse M. Shapiro\n\n\u201cPersistent metabolic adaptation 6 years after \u201cThe Biggest Loser\u201d competition\u201d by Erin Fothergill, Juen Guo, Lilian Howard et al.\n\n\u201cThe future of weight loss\u201d by Stephan Guyenet\n\nUnder a White Sky by Elizabeth Kolbert\n\nBook recommendations:\n\nBurn by Herman Pontzer\n\nSalt Sugar Fat by Michael Moss\n\nThe Secret of Our Success by Joseph Henrich\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, and you can find Ezra on Twitter @ezraklein. Book recommendations from all our guests are listed at https://www.nytimes.com/article/ezra-klein-show-book-recs.\n\n\u201cThe Ezra Klein Show\u201d is produced by Emefa Agawu, Annie Galvin, Jeff Geld, Rog\xe9 Karma and Kristin Lin. Fact-checking by Michelle Harris and Kate Sinclair. Mixing by Sonia Herrero. Original music by Isaac Jones. Audience strategy by Shannon Busta. The executive producer of New York Times Opinion Audio is Annie-Rose Strasser. Special thanks to Pat McCusker and Kristina Samulewski.