Our society’s 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. Research has never been kind to this thesis. It’s 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? In his 2017 book, “The Hungry Brain,” 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. Paired 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’s research: They don’t make our bodies burn more calories, they make our brains crave less food. So 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. Mentioned: “Relationship between food habituation and reinforcing efficacy of food” by Katelyn A. Carr and Leonard H. Epstein “Dietary obesity in adult rats: Similarities to hypothalamic and human obesity syndromes” by Anthony Sclafani and Deleri Springer “Why Have Americans Become More Obese?” by David M. Cutler, Edward L. Glaeser and Jesse M. Shapiro “Persistent metabolic adaptation 6 years after “The Biggest Loser” competition” by Erin Fothergill, Juen Guo, Lilian Howard et al. “The future of weight loss” by Stephan Guyenet Under a White Sky by Elizabeth Kolbert Book recommendations: Burn by Herman Pontzer Salt Sugar Fat by Michael Moss The Secret of Our Success by Joseph Henrich Thoughts? Guest suggestions? Email us at [email protected]. You can find transcripts (posted midday) and more episodes of “The Ezra Klein Show” 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. “The Ezra Klein Show” is produced by Emefa Agawu, Annie Galvin, Jeff Geld, Rogé 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.