Yet when we look at a predictor that has to be more powerful than the best regression equation—a person’s identical twin, who shares her genome, family, neighborhood, schooling, and culture—we see that the correlation between the two twins’ traits, while way higher than chance, is way lower than 1, typically around .6.36 That leaves a lot of human differences mysteriously unexplained: despite near-identical causes, the effects are nowhere near identical.
