Not leaving home: grandmothers and male dispersal in a duolocal human society.

Models suggest that dispersal patterns will influence age- and sex-dependent helping behavior in social species. Duolocal social systems (where neither sex disperses and mating is outside the group) are predicted to be associated with mothers favoring sons over daughters (because the latter are in reproductive competition with each other). Other models predict daughter-biased investment when benefits of wealth to sons are less than daughters. Here, we test whether sex-biased investment is occurring in the duolocal Mosuo of southwestern China. Using demographic and observational data from Mosuo, we show support for both hypotheses, in that 1) males are more likely to disperse from their natal household if their mother dies, but females are not; 2) a large number of brothers increases the likelihood that both females and males disperse, whereas a large number of sisters only increases female dispersal; 3) mothers help daughters reproduce earlier and reduce death risk of daughter's children, but not sons or sons' children; 4) data on multiple paternity show that female reproductive success does not suffer from multiple partners, and in males multiple mates are associated with higher reproductive success, indicating that mothers can benefit from investing in their sons' mating effort; and 5) gift decisions reveal similar kin effects to those shown in the demographic data, with mothers helping adult daughters and adult sons equally, but helping only her daughter's children, not her son's children. Mosuo mothers may invest resources for parental investment in their daughters and their offspring, while investing in their sons mating effort.