Increased complexity of social category markers leads to diverse rule-based categorizations and reduced intergroup...

Social categories are key to human social life, often leading to intergroup bias and stereotypes. While traditional studies use binary social markers, real-world markers are more complex. This study explored whether such complexity results in complex social categories or less cognitively demanding binary rules. Participants (n = 784, prolific) played a multiplayer video game where they competed with other players for stars and could remove other players from their path by zapping them, with manipulated player appearances: binary colour, multidimensional shape and colour or a continuous colour gradient. In all conditions, participants showed an intergroup bias, zapping players who were similar to them less than dissimilar players. This effect was reduced when social markers were complex. However, the group pattern was shown to be driven by participants' idiosyncratic binary rules in their behaviour, revealed using clustering analysis, with rule diversity increasing with social marker complexity. Notably, some participants showed differential treatment of the most dissimilar players, while others singled out intermediate-colour players, resulting in less salient social categories overall. These findings provide a mechanistic understanding of social category formation and the way exposure to complex markers of social identity scales up to shape collective intergroup bias.