> Define "as the economy has gotten better".

People who live in huts and shit in the forest have more children than people who live in shacks and shit in the fields.

This is not a discussion about the top 10% of the population of the planet, although as a part of the conversation, they predictably have the fewest children. The fact is that the more people have to worry about supporting themselves, the higher infant mortality, the closer they live to each other, and the less access they have to education and birth control, the more kids they have.

If anything the top 10% are bucking the trend by having fewer children as they get poorer, as you say. It probably has a lot to do with the fact that they're alienated from their families and live alone, can barely afford to take care of themselves with precarious jobs (or non-job piecework), have access to basic education and extensive birth control, having children will cost them $20K each just for the birth, and they don't have very good (or often any) insurance. They're more like domesticated farm animals than the typical poverty stricken person. Domesticated farm animals reproduce when the farmer wants them to.