Is IQ a "Fundamental Cause" of Health? Cognitive Ability, Gender, and Survival

Long-term studies of cognitive ability and mortality have documented a robust relationship between those two variables. Such studies have, for the most part, been remarkably silent on explanations for that relationship. Published explanations range from suggestions that raw intelligence may be a "fundamental cause" of mortality, that it "enhances individuals' care of their own health because it represents learning, reasoning, and problem-solving skills useful in preventing chronic disease and accidental injury," to findings that the association between measured IQ and mortality largely reflect its correlation with educational and economic success. In this analysis of data from the Wisconsin Longitudinal Study, we look at the association between measured IQ and survival across the life course in light of the relationship between measured IQ and grades in secondary school. Briefly, we find that, net of social and economic origins, high school grades have a much larger effect than measured IQ, and the association between measured IQ and survival turns negative once high school grades are controlled. That is, the association between IQ and health is fully explained by its correlation with evaluations of academic performance in secondary school. Moreover, the fact that girls are similar in IQ to boys, but earn higher grades in school, partly explains the gender differential in survival. This finding suggests that survival is largely explained by normative behaviors, that is, doing the right thing in the right way at the right time, and such behaviors are well established by late adolescence.