Many anthropologists have noted a correlation between the incidence of polygyny and the predominance of women in agriculture. The more women are responsible for producing food, the likelier men will have second or third wives. This correlation is especially evident in sub-Saharan Africa, where food is produced mostly by mothers hoeing garden plots and where fathers are highly polygynous (see Goody 1973 for a review of the literature).
This correlation has exceptions, even in sub-Saharan Africa. Goody (1973) points to the West African savannah where humans have adapted to this semi-arid zone by developing a very different system of food production. Here, women plant grain and help with the harvest but men do the rest of the farm work. Nonetheless, polygyny rates are as high as in other regions where women produce most of the food.
How do these men manage to feed all of their wives and children? Well, they don’t, at least not entirely. In this region, as in West Africa in general, women obtain food for themselves and their families through trade, i.e., by tending stalls in village markets and by gathering wood for sale. There is also food provisioning from unmarried sons. Since young men have to wait some fifteen years to get married (because so many women are taken by older polygynous males), the unmarried sons in a family are called upon to help their father provide for his wives and offspring.
This system is not without disadvantages. Ideally, a woman should specialize in activities that are compatible with infant care and transport, i.e., that can be done within a relatively small land area. This is why women tend to specialize in food gathering and hoe gardening. Collecting and selling wood requires much longer walking distances. The risks are higher not only for any infants being carried but also for the mother.
Nor does this system work very well for young men. Some of them die during the fifteen or so years they spend waiting for a bride. Their genetic contribution to the next generation is limited to kin selection (by helping out their polygynous fathers), illicit sex, and abduction of women during warfare.
If humans could design their societies, things might be done differently. The polygyny rate could be reduced to give all men early access to marriage and to equalize the burden of parental investment for men and women. This would make everything a lot fairer
Of course, societies are not designed. They come about ad hoc. And fairness has nothing to do with it either. When humans stumble into a new environment, they try to adapt while also trying to satisfy their existing inclinations and predispositions, these being psychological adaptations to the previous environment. In time, new adaptations will arise through natural selection, but again this takes time—all the more so if a way is found to sustain the existing ones.
Goody, J. (1973). “Polygyny, economy and the role of women,” in J. Goody (ed.). The Character of Kinship, Cambridge: Cambridge University Press, pp. 175-190.