Bell Curve

108 The Emergence of a Cognitive Elite Steeper Ladders, Narrower Gates 109tability of IQ, owing to the varying models being used for estimationand to the varying sets of data. Some people seem eager to throw uptheir hands and declare, "No one knows (or can know) how her~tahleIQ is." But that reaction is as unwarranted as it is hasty, if one is content,as we are, to accept a range of uncertainty about the heritabilitythat specialists may find nerve-racking. We are content, in other words,to say that the heritability of IQ falls somewhere within a broad rangeand that, for purposes of our discussion, a value of .6 f .2 does no vio-lence to any of the competent and responsible recent estimates. Therange of .4 to .8 includes virtually all recent (since 1980) estimatescompetent,responsible, or other~ise."~'Recent studies have uncovered other salient facts about the way IQscores depend on genes. They have found, for example, that the moregeneral the measure of intelligence-the closer it is to g-the higher isthe heritabili~.'~ Also, the evidence seems to say that the heritabilityof IQ rises as one ages, all the way from early childhood to late adulthood.'%ismeans that the variation in IQ among, say, youths ages 18to 22 is less dependent on genes than that among people ages 40 to 44.l"'Most of the traditional estimates of heritability have been based onyoungsters, which means that they are likely to underestimate the roleof genes later in life.Finally, and most surprisingly, the evidence is growing that whatevervariation is left over for the environment to explain (i.e., 40 percent ofthe total variation, if the heritability of IQ is taken to be .6), relat~velylittle can be traced to the shared environments created by families." Itis, rather, a set of environmental influences, mostly unknown at present,that are experienced by individuals as individuals. The fact that familymembers resemble each other in intelligence in adulthood as much asthey do is very largely explained by the genes they share rather than thefamily environment they shared as children. These findings suggest deeproots indeed for the cognitive stratification of society.The SyllMsm in PracticeThe heritability of PQ is substantial. In Chapters 2 and 3, we presentedevidence that the relationship of cognitive ability to success in life is farfrom trivial. Inasmuch as the syllogism's premises cannot be dismissedout of hand, neither can its conclusion that success in life will be basedto some extent on inherited differences amongFurthermore, a variety of other scientific findings leads us to concludethat the heritability of success is going to increase rather than diminish.Begin with the limits that heritability puts on the ability to manipulateintelligence, by imagining a United States that has magically made goodon the contemporary ideal of equality. Every child in this imaginaryAmerica experiences exactly the same environmental effects, for goodor ill, on his or her intelligence. How much intellectual variation wouldremain? If the heritability of 1Q is .6, the standard deviation of IQ inour magical world of identical environments would be 11.6 instead of15 (see the note for how this calculation is done)-smaller, but still leavinga great deal of variation in intellectual talent that could not be reducedfurther by mere equali~ation.~~" As we noted earlier, when asociety makes good on the ideal of letting every youngster have equalaccess to the things that allow latent cognitive ability to develop, it isin effect driving the environmental component of 1Q variation closerand closer to nil.The United States is still very far from this state of affairs at the extremes.If one thinks of babies growing up in slums with crack-addictedmothers, at one extreme, compared to children growing up in affluent,culturally rich homes with parents dedicated to squeezing every lastIQ point out of them, then even a heritability of .6 leaves room forconsiderable change if the changes in environment are commensurablylarge. We take up the evidence on that issue in detail inChapter 17, when we consider the many educational and social interventionsthat have attempted to raise IQ. But those are, by definition,the extremes, the two tails of the distribution of environments.Moving a child from an environment that is the very worst to the verybest may make a big difference. In reality, what most interventionsaccomplish is to move children from awful environments to ones thatare merely below average, and such changes are limited in their potentialconsequences when heritahility so constrains the limits of environmentaleffects.14"So while we can look forward to a future in which science discovershow to foster intelligence environmentally and how to use the sciencehumanely, inherited cognitive ability is now extremely important. Inthis sense, luck continues to matter in life's outcomes, hut now it is morea matter of the IQ handed out in life's lottery than anything else aboutcircumstances. High cognitive ability as of the 1990s means, more thaneven before, that the chances of success in life are good and getting het.