False beliefs can spread within societies even when they are costly and when individualsshare access to the same objective reality. Research on the cultural evolution of misbeliefshas demonstrated that a social context can explain what people think, but not whether italso explains how people think. We shift the focus from the diffusion of false beliefs to thediffusion of suboptimal belief-formation strategies, and identify a novel mechanism wherebymisbeliefs arise and spread. We show that, when individual decision-makers have access tothe data-gathering behaviour of others, the tendency to make decisions on the basis ofinsufficient evidence is amplified, increasing the rate of incorrect, costly decisions. Weargue that this mechanism fills a gap in current explanations of problematic, widespreadmisbeliefs such as climate change denial.