One of the most interesting things that I learned from Richard Conniff's The Natural History of the Rich is an implication of hypergamy. Hypergamy is the tendency of women to marry up socially, no matter how high they themselves are in the social structure. The highest-status women, therefore, run out of suitable mates, as do the lowest-status men. Therefore, the rich through history have favored sons and disfavored daughters, while the poor have done the reverse.
The new thought that this led me to was this helps explain why poor neighborhoods tend to produce matriarchal clans. It is not simply that the men are not expected to be responsible, but that the mothers favor their daughters as the more likely child to lift the whole family up.
I am still chewing on the implications of this idea.