Economists are increasingly using experiments to study and measure discrimination between groups. In a meta-analysis containing 447 results from 77 studies, we find groups significantly discriminate against each other in roughly a third of cases. Discrimination varies depending upon the type of group identity being studied: it is stronger when identity is artificially induced in the laboratory than when the subject pool is divided by ethnicity or nationality, and higher still when participants are split into socially or geographically distinct groups. In gender discrimination experiments, there is significant favouritism towards the opposite gender. There is evidence for both taste-based and statistical discrimination; tastes seem to drive the relatively strong discrimination with artificial identity, while statistical motivations moderate it. Relative to all other decision-making contexts, discrimination is much stronger when participants are asked to allocate payoffs between passive ingroup and out-group members. Students and non-students appear to discriminate equally.We discuss possible interpretations and implications of our findings.