In the past it has sometimes been assumed in health care that all adverse events involve individual incompetence and therefore blameworthiness, an assumption that is likely to hinder the development of comprehensive and honest incident reporting systems. At the same time, a full understanding of adverse events in healthcare systems requires that distinctions are drawn between a variety of error types, each of which has different origins and demands different strategies for remediation. In this paper a range of cognitive biases identified by psychologists is described. Examples are given of these biases, which are naturally employed in trying to understand our own behaviour and that of others, and therefore affect our understanding of adverse events. It is suggested that awareness of these biases, which form part of our normal thinking, should help to avoid a narrow focus on individual culpability and facilitate a more sophisticated approach to the investigation of adverse events.