Mark Kleiman makes a good point, noting that the gold standard of intelligence studies -- twins raised in separate environments -- is unable to separate out that the two subjects spent nine crucial developmental months packed into a crowded womb, dependent on a mother's health and nutrition that varies wildly with class. If, during that period, intellectual potentiality is degraded through poor diet or environmental pollution, the study's end results will look much more depressing than they should necessarily be. And given that we already know that one diet and environment related condition -- prematurity -- has lifelong impacts on cognition, we should be taking the importance of the gestation period much more seriously than we are.
