<p>Young adults face a rising tide of mental illness and loneliness. We propose that an overlooked barrier for social connection is how people perceive each other’s empathy. Here, our longitudinal study of an undergraduate student community (<i>N</i> = 5,192) reveals that undergraduates who perceive their peers as empathic report better current and future well-being. Yet we document an ‘empathy perception gap’: people systematically see others as less empathic than others see themselves. Students who perceived their peers as less empathic were less willing to take social risks and grew more isolated over time. To disrupt this cycle, we conducted two field experiments that presented students with data on their peers’ self-reported empathy and behavioural nudges to encourage social risk taking. These interventions reduced the empathy perception gap, increased social behaviours and expanded social networks months later. This work offers a promising, scalable strategy to cultivate social well-being, simply by presenting people with data about each other.</p>