Poor people need to have more children to increase the odds that one of those children will get them out of poverty...
Of course consciously, the poor people having many children don't know they are doing it to increase their chances of survival...
A rich child has a very high chance of surviving, and the parents don't need the help...Therefore, rich people don't think that they need that many children...
I think it's a tricky control mechanism Nature implanted in humans to balance our population...
Think about this...
With other mammals, when resources are plentiful, reproduction increases exponentially...However, with humans (especially Westerners) with more resources there is less reproduction...
So Nature ingeniously made it such that the poorest humans will have a rapid rate of reproduction, so that we don't die off as a species...
This is just my opinion...No science to it...