In sub-Saharan societies, female-dominated agriculture is associated with low paternal investment and high polygyny rates. Why? The short answer is that year-round tropical agriculture enables women to meet their food needs and those of their children without a male provider. Paternal investment thus tends to fall to zero and men are free to maximize their reproductive fitness by mating with as many women as possible.
Long answer: this difference between tropical and non-tropical humans was already present before agriculture. Even during the hunter-gatherer stage, men provided more food for their mates and offspring with increasing distance from the equator, apparently because the longer winters made food gathering impossible for women during much of the year. Thus, when agriculture became the new mode of food production, men were much more predisposed to exploit its possibilities outside the tropics.
For instance, most livestock were domesticated in Europe and Asia, the guinea fowl being the only food-producing animal to have been domesticated in sub-Saharan Africa. Livestock domestication thus seems to have been limited not only by technical constraints (availability of wild animals with the right characteristics) but also by psychological constraints (relative predispositions of men and women to take part in food production).
Wherever men had previously provisioned their families with food from hunting, they were more inclined to do the same by domesticating game animals—often the same ones they used to hunt.
I discuss some of these points in my latest article: "Sexual Selection and Human Geographic Variation."
Frost, P. (2008). Sexual selection and human geographic variation, Special Issue: Proceedings of the 2nd Annual Meeting of the NorthEastern Evolutionary Psychology Society. Journal of Social, Evolutionary, and Cultural Psychology, 2(4), pp. 169-191.