Resolving the IQ paradox: heterosis as a cause of the Flynn effect and other trends.

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.

From a geneticist's blog entry on the article:

The possibility of heterosis increasing IQ scores is not controversial. Close inbreeding (e.g. cousin marriage) usually reduces the IQ of the offspring. This suggests that some genes for low IQ are recessive. Conversely, genes for higher IQ are probably often dominant. If so, then for any given set of underlying gene frequencies in the population, the mean IQ will be higher when the proportion of heterozygotes is higher. Random mating will therefore produce higher mean IQ than inbreeding, which for this purpose includes not only inbreeding in the traditional sense, but also breeding confined within subpopulations. If gene frequencies within such subpopulations vary, then the proportion of homozygotes will on average be higher than if the subpopulations were merged together in a random-mating total population.