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