More wealth seems to mean fewer children. Look at a graph of fertility in the U.S from 1800-1990. The birth line plunges. Consider India, China, South Korea, Singapore. As per capita GDP rises, births plummet. See this gapminder graph for rich countries and poor countries. The number of children per woman is much less in more affluent nations.
1977 was a turning point in the U.S. Before, a majority of Americans told Gallup pollsters that they preferred 3 or more children. After, the number declines to 2. Gallup also tells us that currently, families earning more than $75,000 annually are more likely to want smaller families than lower income households. Here, you can see the Gallup information.
The Economic Lesson
An economist would call a good, “normal,” if your demand for it increases when your income rises. By contrast, for “inferior” goods, our demand decreases when we earn more. Dinners at nice restaurants are normal goods. Used cars are “inferior” goods.