Social conformity is a heuristic when individual risky decision-making is disrupted.

When making risky choices in social contexts, humans typically combine social information with individual preferences about the options at stake. It remains unknown how such decisions are made when these preferences are inaccessible or disrupted, as might be the case for individuals confronting novel options or experiencing cognitive impairment. Thus, we examined participants with lesions in insular or dorsal anterior cingulate cortex, key regions implicated in risky decision-making, as they played a gambling task where choices were made both alone and after observing others' choices. Participants in both lesion groups showed disrupted use of standard utility-based computations about risky options. For socially situated decisions, these participants showed increased conformity with the choices of others, independent from social utility-based computations. These findings suggest that in social contexts, following others' choices may be a heuristic for decision-making when utility-based risk processing is disrupted.