Nutrition, pollution, exercise, and education are strong factors. Beyond them, I wish I could ascribe it to low-IQ immigration, but I can't because it seems to be down across the board. I blame it on grifter culture whereby most jobs don't require a demonstration of actual competence, either because they're bs jobs or because they're for bs companies or because they have no enforced obligation to perform reasonably. Grifter jobs lead to a web of grift which will end only when the currency collapses, and can no longer pay such individuals or organizations that don't bring sufficient real value to offset their paycheck.