I would say that high density cities have a high cost if living, and high cost of living impacts the birth rate. It's also the case that young urban population

I would say that high density cities have a high cost if living, and high cost of living impacts the birth rate. It's also the case that young urban populations don't get the discount rate on children right. A lot of young urban people see children as a very high cost, and see no benefit. The benefit of children is all in the long tail. Many people who say 'kids are expensive and too much effort' have failed to factor in what happens when they get old and lonely. Anyone pondering this question should consider how much your parents like getting a phone call, then project forwards in their life where this possibility of joy is eliminated, forever. You don't even have to see adult children to get joy from them.