Evolution of polygamous marriage by maximization of inclusive fitness

The majority of human societies practice polygynous marriage, in line with the typical mating pattern found in mammals. Polygyny in humans is often associated with the transfer of wealth to a male's sister's offspring, and it has been suggested that this "mother's brother phenomenon" is adaptive when paternity confidence is low. Polyandry, on the other hand, while virtually unknown in mammals, is practiced by a few human societies, and it has been suggested that this is adaptive if the co-husbands are genetically related. The evolution of human marriage strategies, therefore, can be studied in the framework of kin selection and game theory, as strategic transmission of wealth by males and strategic paternity allocation by females can evolve to maximize inclusive fitness. Here I analyse the stability of polygynous and polyandrous marriage using a game theoretical model previously developed to study monogamy. I show that the "mother's brother phenomenon" depends on the degree of resource depletion through division, whereas the paternity threshold commonly discussed in the anthropological literature is largely irrelevant. Resource depletion through division is also the major determinant of the stability of polyandry, whereas relatedness between co-husbands is not essential. Finally, I show that when females control the transfer of their own resources, monogamy is stable under more general conditions than previously believed.