With assists from politicians and social media, people are increasingly dividing themselves into social and political factions. Models can hint at how it happens—and maybe offer ways to mitigate it. In 2016, when Vicky Chuqiao Yang first started to work on computer simulations of US politics, she was fascinated by the realization that the left–right standoff widely described as “polarization” is not one thing. “There are two kinds of polarization that the media and the public often get confused,” says Yang, an applied mathematician at the Santa Fe Institute in New Mexico. One type is issue polarization: “how much people disagree on policies like ‘What should be the tax rates?’, or ‘What should be the laws to regulate guns?’.” Americans have always been polarized in some ways. But using models, a cadre of researchers is trying to understand why social polarization is on the rise and—perhaps more importantly—what we can do about it. Image credit: Dave Cutler (artist). Those divisions have been widening of late. But they aren’t nearly as incendiary as social or “affective” polarization, which is about anger, distrust, resentment, tribal identity, and mutual loathing (see Fig. 1). As a team of prominent social scientists warned last year, social polarization in conjunction with legislative gridlock and hyper-partisan media have created an “American sectarianism” that threatens democracy itself (1). Researchers are trying to understand why social polarization is on the rise and—perhaps more importantly—what we can do about it. Can we find solutions by focusing on racial anxieties, conspiracy theories, and social media echo chambers that endlessly reinforce a single viewpoint? Or do we also need to look for more fundamental forces at work? These are the kinds of questions that have brought Yang and a host of other modelers into the long-established field of opinion dynamics: the …