20 years ago I had an Idea. Maybe someone who knows something about the field can tell me what is wrong with it in 20 seconds (including maybe someone else had the idea 40 years ago).
This thought was stimulated by reading Structural Anthropology a collection of essays by Claude Levi-Strauss. There are two questions. One is why are some cultures monogynous and others polygynous ? The other is why do the Bororo divide their tiny villages into 3 endogamous clans ?
OK first question. Why in some cultures men can marry more than one woman and in others only one ? One possbile explanation is polygyny occurs when the gender ratio is many women for each man. This can happen if lots of men get killed by other men. So women share husbands or go single wasting their uteruses (the Moll Flanders problem described by Daniel Defoe some time ago).
Could be the explanation, but I would like to talk about another. Levi Strauss was very interested in a very simple mathematical model which pointed out that hunter gatherers typically live in tiny groups (have too to avoid killing off all the game within walking distance). Someone else (really some two else) calculated that these groups were about as small as could be sustained given risk that a generation would be all male or all female and thus the last (he didn’t explain this model very clearly and I didn’t look it up). OK see how much worse this problem is if monogynous. If people live in small groups and are mostly endogamous (must have some flow with other villages/bands to avoide inbreeding but I assume this is pretty low). If each man is allowed to get only one woman pregnant, the number of woman who reproduce each generation is the lesser of the number of woman and the number of men. If each man is allowed to get as many women pregnant as are available then the number of women who reproduce each generation is the number of women. Polygyny might be required in people who live in small mostly endogamous villages to deal with random fluctuations in the sex ratio.