It has less to do with economics than with biology: because it gives the children a better chance. Reproduction isn’t just a matter of siring or bearing lots of children – they need to thrive and survive to breeding age themselves, and they have the best chance of doing so if the parents stay together.

Because human children have a relatively long infancy and childhood compared with other mammals, during which time they need the mother’s attention and when both mother and child are vulnerable, the father’s protection and support is vital to their survival.

From the female point of view, monogamy is the way of making sure daddy’s around to provide dinner instead of doing so for the female next door. This is why human females are always sexually receptive (as opposed to 1 day a month during which time their bottoms turn blue or something similarly obvious.)

From the male point of view, the fact that female estrus is covert and she’s always receptive to sex means he never knows when she’s fertile, so it’s to his benefit to make sure he stays around; that way he knows the children he’s helping to raise are his, not those of the male next door.

Coincidentally – or probably not – the age of seven, which is when children can (just about) survive on their own if they had to, is also when many marriages begin to break up.