To continue on the discussion of genetics and intelligence a bit, one of the more important trends in IQ studies is the so-called "Flynn effect," which describes the rise in IQ numbers over the past century. This would, to some, seem like definitive proof that something's very wrong with the way we test IQ -- if the scores are changing, that wrecks the metric's claim to be an objective measure of natural capacity. Not so, say hereditists, you can look at studies of twins where they end up with similar IQs. Well James Flynn himself just wrote a book called "What Is Intelligence," in which he argues:
Flynn's most intriguing and controversial claim concerns the preponderant influence of the environment over genetic inheritance in determining intelligence. The direct effect of genes on IQ accounts for only 36 percent of IQ variance, Flynn tells us, with 64 percent resulting from the indirect effect of genes plus environmental differences uncorrelated with genes. Yet this cheeky claim would seem to be contradicted by the fact that identical twins separated at birth and raised apart end up with very similar IQs, presumably because of their identical genes. Not so, says Flynn, who buttresses his argument by drawing on an analogy from basketball.
If on the basis of their genetic inheritance, separated-twin pairs are tall, quick, and athletically inclined, both members are likely to be interested in basketball, practice assiduously, play better, and eventually attract the attention of basketball coaches capable of transforming them into world-class competitors. Other twin pairs, in contrast, endowed with shared genes that predispose them to be shorter and stodgier than average will display little aptitude or enthusiasm for playing basketball and will end up as spectators rather than as players. [...]
According to Flynn, the environment will always be the principal determinant of whether or not a particular genetic predisposition gets to be fully expressed. “There is a strong tendency for a genetic advantage or disadvantage to get more and more matched to a corresponding environment,” he writes.
Obviously, the implications of that are rather serious, and they suggest, again, that it's essentially impossible to tell what is and is not genetic. The interaction of genes and proclivities with environment is simply too complex.
Also interesting is Flynn's case study of Chinese-Americans:
