The Emergence of 'Us and Them' in 80 Lines of Code: Modeling Group Genesis in Homogeneous Populations

Psychological explanations of group genesis often require population heterogeneity in identity or other characteristics, whether deep (e.g., religion) or superficial (e.g., eye color). We use game-theoretical agent-based models to explore group genesis in homogeneous populations and find robust group formation with just two basic principles: reciprocity and transitivity. These emergent groups demonstrate in-group cooperation and out-group defection, even though agents lack common identity. Group formation increases individual payoffs, and group structure is robust to varying levels of reciprocity and transitivity. Increasing population size increases group size more than group number, and manipulating baseline trust in a population has predictable effects on group genesis. An interactive online demonstration enables first-hand exploration of the parameter space (www.mpmlab.org/groups), and available source code (supplementary materials) provides a guide to implementing psychological agent-based models.