Social grouping is omnipresent in the animal kingdom. Considerable research has focused on understanding how animal groups form and function, including how collective behaviour emerges via self-organising mechanisms and how phenotypic variation drives the behaviour and functioning of animal groups. However, we still lack a mechanistic understanding of the role of phenotypic variation in collective animal behaviour. Here we present a common framework to quantify individual heterogeneity and synthesise the literature to systematically explain and predict its role in collective behaviour across species, contexts, and traits. We show that individual heterogeneity provides a key intermediary mechanism with broad consequences for sociality (e.g., group structure, functioning), ecology (e.g., response to environmental change), and evolution. We also outline a roadmap for future research.
