Dirt poor societies all over the world have large families.
They have large families in order to insure their economic well-being when they get too old to work. Also, a lot of those kids in dirt poor societies tend to die.
Yes, that's true but how is it easier for a dirt poor person to raise 2,3,4+ kids but the middle class and above can not do it. They just don't want a lot of kids. It's not the money issue in the sense that they can not afford them it's a money issue in that they can but would rather have 2 cars, a jet ski, a big house, a plasma screen tv, etc. Why struggle with 4 kids when you can be comfortable and happy with 1 or 2 and still have the lifestyle you want. People do not have an obligation to humanity to have 2.1 children so the world population is sustainable.