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Friday, November 23, 2007

Intelligence: How much is heredity and how much environment?

Double helix DNA pioneer James Watson had to resign his job recently over remarks about race and intelligence. Recently, I learned about a finding on intelligence that might prompt people to rethink their approach to it - the Flynn effect. As Richard Restak explains it in American Scholar,
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.

Well, that’s basketball. What about IQ?
“Genetic advantages that may have been quite modest at birth have a huge effect on eventual basketball skills by getting matched with better environments,” Flynn writes. He suggests a similar environmental influence on genetic inheritance in regard to IQ: Twins with even a slight genetic IQ advantage are more likely to be drawn toward learning, perform better during their primary and secondary education, and thereby gain acceptance into top-tier universities. In the process, their IQ levels are likely to increase even further.

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.

In other words, the focus on genetic inheritance is misplaced if it encourages people to look for smart genes, at the expense of looking for environments in which they may be expressed.

While we’re here, one thing has always bugged me about the stories of “identical twins raised apart” who turned out to be similar: While they do provide strong evidence for genetic inheritance of some characteristics, it may not be as strong as some think. When social workers must find homes for identical twins, they don’t send one to live with a drug dealer down on the waterfront and the other to live with a mad prophet up on a lonely mountain. They place children with conventional people who can’t have kids. So the people who get to adopt babies are not a random sample of humanity. They are people whom social workers expect to be responsible parents. They are also people willing to undergo the hassle of home studies and the endless waits for a baby (and also unwilling or unable to just raise $50,000 cash and hit the black or grey market).

I am not knocking identical twin studies. I just think that we shouldn’t discount the “who do social workers think would make a good parent?” effect in determining outcomes. And remember, it was usually the same social workers making the decision for both of the twins ...

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