According to social identity theory and self-categorization theory, we largely define ourselves through our membership in groups. A recent Nature study mapped both the social networks and gut microbiome of 1,787 adults living in 18 remote villages in Honduras. This article clearly showed that social networks can shape a biological feature of their members, microbial composition, alongside their established effects on social behavior and identity. Serotonin, dopamine, GABA, and short-chain fatty acids: Your gut microbes help produce and regulate these mood-shaping molecules. Your gut microbes may be quietly co-authoring your personality.