Zeraph

Keep in mind that having children, is a very long term investment. You wouldn't want to do it in times of stress because it takes a long time to "pay off" the initial investment. They would also be more likely to die before they "paid off" their debt, thereby reducing their economic value even more.

The time of stress I was more referring to, is when there literally arn't enough resources to go around. Infertile land, droughts, over hunting, etc. Having more mouths to feed, because of the heavy cost of the initial investment (and then half of them leaving before they "pay off" it) would not help in such a time. But another adult, that can produce much more, and only has to feed him/herself is a lot more useful.

This is part of the same mechanism that brought us together in large alliances or groups. Simply having more kids doesn't usually help (not to mention who is watching/protecting all these extra kids?) but having more productive adults, does.