I just read an micro-economic analysis which maximize our utilities whether we should have two children or, say, four.

http://econlog.econlib.org/archives/2005/01/the_selfish_rea.html

Have the number of children that maximizes average utility over your whole lifespan. When you are 30, you might feel like two children is plenty. But once you are 60, you are more likely to prefer ten sons and daughters to keep you company and keep the grandkids coming.

I was so surprised how different people think about the number of children between in US and in Japan. Writing blog articles in Japanese, I found many people in Japan, especially those who write their blogs, do not like the idea of having children. Some of them want none. It seems having children is a burden both economically and psychologically for young generations in Japan.

While people in the U.S. wonder to have their fourth children, in recent article, there is a discussion if Japanese people would have their second children or not.

http://nv-club.nikkeibp.co.jp/members/COLUMN/20050111/105697/
(in Japanese)

I hope someone explain why.