The tendency to favour members of one’s own group, also known as in-group bias, is ubiquitous in social mammals from rats to humans. This preference for in-group members (which often translates into aggression against out-group members) plays a major role in mammalian social life, and governs how individuals and groups perceive and behave toward others. Understanding how such social dynamics emerge, evolve, and impact behaviour is crucial for evolutionary theory and for the design of more mammal-like agents. Yet current swarm robots are typically extremely social and cooperative, and do not differentiate between individual robots. Here, we introduce two cognitive and social layers to a swarm engaged in a foraging task and a communication game: robot individuation (a partner-specific memory) and evolving sociality (a partner-specific tendency to interact based on previous interactions). We test whether social asymmetries such as in-group bias can spontaneously emerge from repeated local interactions without implicit group markers or pre-assigned identity tags, and study its strength and stability over varying conditions: cognitive (memory size and degree of decay), social (group size and innate sociality) and environmental (spread of resources). We show that implementing these two mammal-like features leads to the spontaneous emergence of in-group bias, which is robust across ecologies. These results shed light on the evolution of social asymmetries, and pave the way for modelling the complex social dynamics of mammals.