[metabagel]

Couple of factors...

People in developing countries tend to have children in order to boost the family's income. As the economy develops, this turns out to no longer be beneficial.

Generally, people choose to have less children as infant mortality goes down. Introduction of better medical care leads to a temporary spike in population, as more people live longer. But, lower infant mortality means that people don't need to have as many children in order to have some survive, and so they typically choose to have fewer children. The adjustment period can be a problem, as we have seen in the developing world.