In today’s online world, the beliefs of people are shaped by aggregated opinions: the elicited, quantified and summarized judgements of many strangers. Ratings guide purchases, like guide shares, and polls guide votes. In this Review, we consolidate cross-disciplinary research to clarify how individuals draw inductive inferences about the world based on the opinions of others. We draw on philosophy to clarify what conceptually distinguishes aggregated opinion from other forms of evidence, draw on political science to describe its functional origins in collective judgement and decision-making, and draw on psychology to shed light on the mechanisms that drive how individuals conform to, learn from and ignore the collected opinions of others. Finally, we highlight future directions to address important gaps in the literature, such as exploring how the causal history of opinion shapes the inferences that people draw, and how the mechanisms that drive responses to aggregated opinion can be leveraged in tailored interventions that are responsive to people’s individual reasons for maintaining their beliefs.