<p>The gut microbiome has profound influences on brain activity and complex behaviours. Research across diverse animal species, in both natural environments and laboratory settings, has identified biological mechanisms that underlie gut–brain interactions. An emerging central theme is that the gut microbiome is shaped by, and actively contributes to, sociability throughout the lifespan. In this Review, we highlight recent literature revealing the effects of the microbiome on early neurodevelopment, immune modulation, stress responses and microorganism-mediated metabolism that affect social behaviour. Studies investigating the cellular and molecular pathways that underlie microbial influences on social behaviour have implicated brain regions and circuits that mediate critical aspects of animal behaviours, including bonding, mating, defence, aggression and social learning. Gut microbiome–brain research using animal models of social deficits and ecological studies in the wild, as well as investigations of human conditions comorbid with impaired social behaviour, could offer new and natural avenues for improved quality of life in individuals and social groups.</p>