When someone installs rooftop solar panels, neighbors are increasingly likely to do the same, especially if the panels are visible from the street. It’s part of a phenomenon called behavioral contagion, which can either have positive effects, as with solar panels, or negative effects, as when people become more likely to drink excessively when they spend more time with heavy drinkers. In “Under the Influence,” economist Robert H. Frank talks about how peer influences alter the policy choices we face under COVID-19 and climate change, and how we can better harness the power of contagion for good.