There is a paradox in IQ studies that has so far not yet been satisfactorily resolved. The paradox is that IQ has been shown to be determined largely genetically. Yet, over the past century or so, average IQs in developed countries have grown 3-7 IQ points per decade (known as the Flynn effect), which is too fast for genetic selection. Therefore IQ appears at once to be determined predominantly environmentally (the global trend) as well as predominantly genetically (other studies). This appears to be a paradox; both influences cannot simultaneously dominate.

Michael Mingroni argues convincingly that the cause for the Flynn effect in IQ is heterosis. Quoting Michael:
Briefly, heterosis is a genetic effect that will cause populationwide changes in a trait whenever three conditions are met. The first condition is that the population in question must initially have a mating pattern that is less than completely random prior to the occurrence of the trend. Such a deviation from panmixia, or random mating, creates an excess of homozygotes in the population and a deficit of heterozygotes. Second, the population must undergo a demographic change toward a closer approximation to random-mating conditions. This causes the frequency of homozygotes to decline and that of heterozygotes to increase. Of course, this second condition presupposes that the first condition is already met, as a trend toward more random mating cannot occur in a population already mating randomly. Third, the trait in question must display directional dominance, with more of the genes that influence the trait in one direction being dominant and more of those that influence the trait in the opposite direction being recessive. Given such nonadditive gene action, any increase in the ratio of heterozygotes to homozygotes will cause the distribution of the trait to shift over time in the dominant direction. Heterosis has been mentioned as a potential cause of the IQ trend by a number of researchers over the years (Anderson, 1982; Flynn, 1998; Jensen, 1998; Kane & Oakland, 2000; Mingroni, 2004; see also Dahlberg, 1942, chap. 10). Few would dispute that heterosis could be responsible for at least some part of the trend; what is mainly at issue is whether it could be a major cause.
Mingroni discusses other models that have so far been suggested to explain the Flynn effect, which all have significant shortcomings. He then argues compellingly that heterosis can explain not only the Flynn effect, but also other unresolved paradoxes of genetics vs. environment in trends such as height or myopia. Conversely, the presence of these other trends explainable by heterosis supports his proposition that heterosis is occuring in the first place, because if it's occuring, it would be expected to cause all these other trends as well, not just the IQ Flynn effect. Mingroni's arguments are surprising and compelling, consistent in a wider view of things than just intelligence alone.

It would be great to see Mingroni's paper followed up by research to test this promising hypothesis.