Here is some strong evidence about human polygamy (technically polygyny) based on DNA analysis. It links to the original article if you want it all nerded up.
Quote:
In a strictly monogamous population, one would expect to have an equal number of breeding females and males and, therefore, a breeding sex ratio of one female to one male. In a population where males tend to have more than one female mate, more females than males contribute to reproduction; for this reason the breeding ratio exceeds one. The authors of this study estimate that the breeding ratio varies between 1.1 and 1.4 according to population: 1.1 in Asia, 1.3 in Europe and 1.4 in Africa.
Modern man or Homo sapiens would, therefore, usually have been monogamous while exhibiting tendencies toward polygamy over the course of evolutionary history. These findings are consistent with studies in evolutionary psychology and anthropology that depict contemporary human populations.
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ETA: I'm curious about many things - where in Africa (tribal or Muslim or Christian areas, eg) these samples were taken, and how long the time scale is, i.e. 1,000 years or 100,000.
I also wonder if this method properly allows for the possibility of serial harem-polygyny, i.e. each male gets to breed with all the females in the group for a year or two before being ousted by the next male. That could create the genetic appearance of breeding parity, while still preserving polygyny.