Actually there are several puzzles about African polygamy:
- Why is it relatively high in tropical Africa? Oddly, Madagascar is the great exception: tropical, near Africa, but little or no polygamy. This makes me think that polygamy has deep roots, since Madagascar was settled by a separate population of Austronesians.
- There is a belt of high polygamy from West Africa to Tanzania, elsewhere in Africa it is present but at lower levels. Why does it vary? A paper by James Fenske* tests several hypotheses.
- Inequality among men. It turns out that current inequalities among men are not predictive, but historical indicators of inequality on the eve of colonial rule (taken from the Ethnographic Atlas) do predict polygamy today.
- Women supporting themselves by farming? It seems that polygamy is least common in those parts of Africa where women have historically been most important in agriculture. But I would add, even if it does not explain variation in Africa, this surely has to be part of the reason Africa as a whole has much polygamy compared to Eurasia.
- The slave trade, taking men away, leaving more women. Correlation with the slave trade is not robust. Angola sent many slaves but has lowish polygamy.
- Polygamy has declined over the past half century. Why? Fenske tests several hypotheses and concludes that falling child mortality explains much of the decline in polygamy across sub-Saharan Africa. The mechanism I suppose is that now women are more confident they can successfully raise children in a monogamous marriage. But is this cause or consequence? Monogamy means more paternal investment, which should lessen child mortality.
The puzzles multiply.