Robert Wright in The Moral Animal, a popular presentation of evolutionary psychology, argues that monogamy tends to be associated with societies that are either very poor, so a man cannot support two families, or have fairly even income distributions, so that half of one man is almost always worth less to a woman than all of another .
My guess would be that those societies either have polygamy on the end of society that can afford it, or higher rates of women dying in child birth. The former matches the higher bidding price women have with polygamy, explaining why multiple men need to go in to afford a single wife. The latter case is really speculative, just spit balling at why a society might have more men than women. If multiple spouses are allowed, unmarried women are very likely to be soaked up as they bid down the price of a wife. Someone is going to want spare concubines, so something has to keep the number of single women low. Junior wives that are basically servants is one possibility, but then that gets back to polygamy.
One thing I never quite understood about societies with two husbands per wife is "what happens to the unmarried women?"
Do they just end up as household laborers or something, like unsexed junior wives?
My guess would be that those societies either have polygamy on the end of society that can afford it, or higher rates of women dying in child birth. The former matches the higher bidding price women have with polygamy, explaining why multiple men need to go in to afford a single wife. The latter case is really speculative, just spit balling at why a society might have more men than women. If multiple spouses are allowed, unmarried women are very likely to be soaked up as they bid down the price of a wife. Someone is going to want spare concubines, so something has to keep the number of single women low. Junior wives that are basically servants is one possibility, but then that gets back to polygamy.