IQ test scores have risen steadily across the industrialized world ever since such tests were first widely administered, a phenomenon known as the Flynn effect. Although the effect was documented more than 2 decades ago, there is currently no generally agreed-on explanation for it. The author argues that the phenomenon heterosis represents the most likely cause. Heterosis, often referred to as hybrid vigor, is a genetic effect that results from matings between members of genetically distinct subpopulations, such as has been occurring in human populations through the breakup of small, relatively isolated communities owing to urbanization and greater population mobility. In Part 1 of the article, empirical findings are listed that are consistent with a heterosis hypothesis but render other hypotheses either implausible or very difficult to test. In Part 2, a formal model of the process of heterosis is presented. The goal of the modeling is to develop a quantitatively rigorous method for estimating the potential contribution of heterosis in the Flynn effect, as well as trends observed in other heritable traits and conditions.

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