We examine whether investor behavior can be influenced by the social norms to which they are exposed. Specifically, we test two competing hypotheses regarding the influence of social trust on the disposition effect related to mutual fund investment. On the one hand, a higher level of social trust may elicit stronger investor reactions by increasing the credibility of the performance numbers reported by funds. This results in higher flow-performance sensitivity, which mitigates investors’ tendency to sell winners and hold onto losers. On the other hand, societal trust may reduce concerns about expropriation, thereby weakening investors’ need to react to poor performance. The resulting lower flow-performance sensitivity increases the disposition effect. Based on a proprietary dataset of complete account-level trading information for all investors in a large mutual fund family in China, we find compelling evidence 1) of a significant disposition effect among fund investors; 2) that a higher degree of social trust is associated with higher flow-performance sensitivity; and 3) that (high) trust-induced flows mitigate the disposition effect. Our results suggest that, in addition to cognitive biases, investor behavior is also strongly influenced by social norms.