Interesting to see so many people completely and totally misunderstand this topic.
Shadow1980 and Mr Kahn have answered this quite well. People doing well tend to have less children, which allows them to focus on those they do have and on themselves. People in poverty tend to have more children. It's an evolutionary impulse, kind of like laying a hundred eggs in the hope that some of them make it. That just makes it worse, of course, as more children fall into the cracks and they end up having to fight with one another to survive.
There really isn't much we can actively do beyond raising the standard of living for everyone, which isn't going to happen. The problem isn't the contraction, which would eventually right itself, but rather the rising population in poorer groups.
