The July 2007 Psychological Review has Michael Mingroni reviewing an interesting theory he published in Intelligence in 2004, that IQ has increased mainly because of more interracial and cross-cultural mating:

“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.”

Regardless of whether Mingroni’s theory is true, I find it striking that it seems less politically correct than it could be. 

The first response you often hear to genetic explanations of IQ, or even the very idea of IQ, is that such ideas encourage racists, such as Nazis.  But Mingroni’s hybrid vigor theory seems tailor-made to oppose racist and other xenophobic mating policies; instead of killing off “lower” races or preventing interracial mating, Mingroni’s theory suggests one wants to encourage diverse mating and preserve other races as sources of genetic diversity. 

The currently political correct environmental explanations of IQ, in contrast, are quite compatible with racist and other xenophobic mating policies.  That is, one can nearly as easily oppose contact and mating with outsiders for fear of contamination from outsiders’ cultural and other environmental influences, as from outsiders’ genes.  Such arguments were offered in the recent immigration debate, for example.  So why are environmental IQ theories so praised for opposing racism, relative to hybrid vigor? 
http://www.overcomingbias.com/2007/08/is-hybrid-vigor.html