Fig. 1: Schematic of the model.

a The multi-level evolutionary process. Societies (green) comprise families (blue) in which family members (black) live together. The grey frame represents a single generation. When the population of a society reaches twice the initial population, a society splits and another society is removed from the system at random to keep the number of societies fixed—i.e. we adopt the hierarchical Moran process. b The life cycle of families. The black squares, circles, and triangles represent members of different generations. The size of the blue circle reflects the amount of wealth that each family possesses. The dashed arrows show the separation of siblings, and the arrows show the production of wealth through family labour. Each family has a strategy parameter s. With the probability of 1−s, siblings are separated before production, and then they produce independently and reproduce the next generation, as in a nuclear family. On the other hand, with the probability of s, siblings produce together, after which they build their own home and reproduce the next generation, as in an extended family. When we extend the model to consider marriage, its process is added just before production.