There are natural longitudinal experiments in the form of twin studies. They show pretty conclusively that the largest systematic influence of intelligence is D

There are natural longitudinal experiments in the form of twin studies. They show pretty conclusively that the largest systematic influence of intelligence is DNA at about 50%. Almost everything else about intelligence is due to random variation in the environment (and not due to all of the things you would think make a difference, like parenting, schooling, class size, economic status, etc.). The person I was responding to could also just go talk to a bunch of teachers.