I'm wondering if these statistics adjust for age. Wealthier people are usually older and therefore less fertile. Also, there may be selection bias at play here since whoever is more wealthy is more likely to prioritize their career over family & children.

Comments like these remind me of people who distrust climate models telling me (a meteorologist) what the climate model ignores, which of course it doesn’t ignore because scientists aren’t morons. There is a lot of questionable statistics in the social sciences, but I am 1000% sure that people studying fertility account for age.