Kin selection underpins family dynamics in rural China

The number of co-resident members is an important characteristic of human families and thus plays an important role in human evolution. However, an inclusive fitness model of how many members should co-reside has not been explicitly tested. Here we use an evolutionary game model and a decade of family dynamic data from a rural population of southwestern China (1,110 households) to show that the existence of an evolutionary stable family size can be understood by considering the average local relatedness and average inclusive fitness of the family. Empirical data analyses show that average relatedness [Formula] decreased with increasing family size, and [Formula] was associated with helping behavior (especially for the elders) in the family, so the family tended to keep a higher level of average relatedness by adjusting the number of co-resident members. These findings are helpful to understand the variation in family size in different cultures and to demonstrate the importance of kin selection in regulating family dynamics.