[travoltaj]

Perhaps - 1) The people in the richer society are exposed to more ideas and options than the people in poorer societies, which makes them evaluate having/not having children vs "have them because everyone does it" 2) The people in richer societies have a social security net/wealth to fall back on in their old age, whereas the poorer societies are always "hand to mouth" without having the luxury of any retirement savings - all their time and energy goes just for basic survival - and they need children to support them in their old age.