Do people conform to social norms at least partly to signal their social preferences? Using a vignette experiment, we find that parents who do not marry off their under-age daughters in Malawian villages where child marriage is prevalent are perceived as less altruistic, reciprocal, and trustworthy. If parents indeed “harm to signal” in this setting, could alternative signals encourage them to abandon the practice, by offering them other means of showcasing pro-sociality? Randomly assigning public donation drives across 412 villages, we find that those who do not support child marriage are no longer perceived as less pro-social than others in treated high-prevalence villages. Consistent with a new signaling equilibrium, child marriage and teenage pregnancies decrease by nearly 30% in those villages, one year after the intervention.