King Solomon and Ziddania, contrary to our belief that the cream rises to the top evolutionally speaking, people at the lower end of the income scale have more kids. As long as they don't die, they are ahead of the game on the procreation curve. Maybe not Witness young couples. They are supposed to go out and peddle magazines. But I am willing to bet that the social pressures are stronger and that these struggling couples will have children sooner rather than later.
Now, evolutionally speaking, as a survival as a species on a planet with limited resources, we may need to dramatically slow our birth rate. There are good indications this can be done by raising income levels and narrowing income disparity.
For a great interactive graph (click play), check out Hans Rosling's "Smaller Families and Longer Lives"