[jyl]

As countries become more developed, people naturally have fewer children. Cost of education, opportunity cost to parents' careers, expensive housing, etc. Every developed country sees a steep decline in fertility, from 4-5 children per woman in the developing years to below 2 by the time the country becomes well developed.

I believe a few developed countries in Europe have been able to raise their fertility rate from below-replacement rate of 1.4 to replacement rate of 2.0. I haven't studied exactly how this was done, but I think it involved lots of financial incentives to have and raise children - very liberal parental leave, free child care, free and good education, etc. I don't really see China going in that direction, but I'm not sure.

In the US, our large and relatively cheap houses help maintain larger family sizes. Not speaking of SF or NYC of course, but of much of the country. US fertility rate is about 2.