Why are families smaller in developed countries?

What do you think about this question?? I think that developed countries' almost parents are likely to spend money for their child. Developed countries are the society which is thought that their school background is important. The parents wish their child to enter to "good" school. Recently, it is popular that children take an entrance examination to a junior high school. It is for that purpose that they sent their child to a cram school. The parents pay money, and if their child passed an examination of a private school, they will pay still more a money. It compares the old days and today, it cost a huge sum of money NOW. So families become smaller in developed country, I think. The parents are spending of money in their child too much.