"Higher-income" is not exclusively millionaires and billionaires, the article specifically states this: "Gray matter volume in kids from middle-income families

"Higher-income" is not exclusively millionaires and billionaires, the article specifically states this: "Gray matter volume in kids from middle-income families did not statistically significantly differ from that of their richer peers, suggesting a poverty-specific effect." As far as I can tell from skimming the article, there doesn't seem to be much causal inference going on: "It's not clear what, specifically, about poverty takes a toll on brain development." Seems like a great observational study to me--it doesn't jump to conclusions and opens a lot of questions that need to be answered.