Maybe there is in fact an evolutionary advantage to having people like us in the population. (recognizing that is a pretty broad spectrum in itself) There are lots of examples among other species where the reproductive success of the species depends on cooperation between the actual parents and other related members of a group. Wolves for example. The alpha's reproduce, but the pups are protected, nurtured and taught by non-reproducing members - relatively submissive females AND MALES.
In human societies, often the eldest son inherited the property or leadership stature. And as research has recently shown, younger brothers are more likely to have feminine traits ...the more older siblings the more likely the younger ones are to exhibit feminine traits. How does that help - perhaps, just guessing, that more feminine younger brothers are less likely to usurp and more likely to support their elder brother, again leading to a higher possibility of reproductive success. (We'll discount the example of inbreeding among royal families which messes up the whole reproductive order!)
