Our understanding of consistent individual differences in behaviour — or ‘traits’ — in non-human species is limited. Forkosh et al. placed 168 mice in groups of 4 in ‘social boxes’ and used automatic location tracking to record each animal’s behaviour over 4 days. They used a linear discriminant analysis of 60 behavioural dimensions to identify four ‘identity domains’ (IDs) with high between-individual discriminative ability and high within-individual stability over time, even when mice were placed in different groups. ID scores correlated with scores on multiple behavioural assays and with transcriptomic variance in certain brain regions.