In this talk, I will present a theoretical model linking ecology to male paternal investment. The basic argument is that there are three fundamental potential inputs into offspring fitness: genes, care and energy. The ecological niche to which the population/species is adapted determines three important relationships: 1) the impacts of variation in each of those inputs on offspring fitness; 2) the degree to which those inputs are complements or substitutes; and 3) the extent to which provisioning of care trades-off against the provisioning of energy to offspring. The model predicts male paternal investment will be greatest when both care and energy have large and complementary impacts on offspring fitness, and when the amount of energy a single parent can provide trade-offs sharply with the care that parent can provide. It applies the model to large scale cross-species variation in paternal investment and in the forms that it takes.
The model is then applied to humans. It is proposed that there is a modal human economy of food production and redistribution that evolved as part of the hunting and gathering lifeway. Three fundamental social relationships organized the system of redistribution: 1) Intergenerational transfers from older to younger individuals, both from parents to offspring and from grandparents to grandoffspring; 2) Risk-reduction reciprocity among members of the same and different generations; and 3) complementarity between men and women in resource production and childcare. This economy favored the evolution of male paternal investment and the establishment of long term pair bonds between men and women.
As human economies changed over the last ten thousand years, both among and within cultural diversity in mating and parental investment has grown substantially. In particular, variance in earnings among men has grown substantially in some economies, resulting in stable polygyny (in which some men mate with more than one woman while other mate with none) or polygynandry (in which both men and women have more than one mating partner, simultaneously or serially). The same principles explaining diversity in nonhuman mating systems help explain the intraspecific diversity in humans.