I think this is true. I recently read "The Fates of Nations" by Paul Colinvaux, which was very interesting. One of his main points was that people are very sm

I think this is true. I recently read "The Fates of Nations" by Paul Colinvaux, which was very interesting. One of his main points was that people are very smart about choosing how many kids to have, and they generally choose to have as many kids as they can afford. For richer people, it tends to be less, since they want to give their kids all the best: the best schools, the best daycare, set them up with a good start in life, etc. For poorer people, they don't spend as much on their kids. They don't worry so much about education because it's often out of their reach, so they have more kids since it doesn't cost them as much. (The book isn't so much about demographics as about how demographics have shaped history throughout time.)