In a new book, “Breaking the Social Media Prism,” he offers a surprising theory about polarization and life online, drawn from “hundreds of millions of data points that describe the behavior of thousands of social media users over multiple years.” There’s a widespread belief that social media traps people in bubbles by serving limited or wrong information from the outside in. But Bail thinks that the polarizing influence of social media works from the inside out: people project identities into the digital landscape, like sonar pings, and refine their sense of self and of the world according to the response that they get back.