If we look at species other than human, those that have the sexes behave as equal partners tend to be very identical in traits with very few sex-type differences. Having division of complimentary abilities and preferences also means that losing half of that to predators results in a bigger loss than if both acts as backup for the other. Those species that do have many sex-type differences are usually refereed to as tournament species, with very little partnership in the care of young.

In pair bonding species, sex-type differences are few. In the extreme end, some species can even switch gender depending on the local balance. Some fish even "keep track" of who went female/male last time, as eggs cost more energy to produce. Those that cheat are then shunned.