IQtests and scores are not in fact crucial to our thesis, but they are useful. Intellectual accomplishment is all that really matters: If people routinely won Nobel Prizes in physics with low IQscores, or, for that matter, routinely aced calculus exams but flunked the IQ test, we'd junk the IQ tests. But that doesn't happen: IQis an imperfect but useful measure of intelligence.
You'll frequently hear that we don't really know what intelligence is, that we don't know how to measure it, that IQtests are biased, and that IQ scores don't predict anything, or that they don't predict anything outside of school. Often these complaints are salted with personal anecdotes about some acquaintance that had a high IQscore but was lazy, crazy, or suffered from unforgivable personal hygiene. And in recent years, other forms of intelligence have become all the rage. Daniel Gole-man has written of "emotional intelligence" and "social intelligence," pointing out how they can help to predict job success and personal happiness. And other forms of intelligence have been proposed. In his 1993 book, Howard Gardner suggested that there are many types.28 But the data hardly support these attempts to complexify cognitive testing. The supposed special kinds of intelligence don't predict anything useful or, when they do, predict only to the extent that they are correlated with general intelligence.
Yet IQ tests work in the sense that they predict performance. They were originally developed in order to predict how well children would do in school, and they do an excellent job of that. They also have moderate to high predictive power on many other questions, such as job performance, health, risk of accidental death, income, and other characteristics that may be less obvious, such as susceptibility to Alzheimer's disease. To make our position perfectly clear, we'd like to emphasize that saying IQscores have some predictive power is not the same thing as saying that they determine everything.
Of course, exceptions don't make trends disappear. Muggsy Bogues may have played in the NBA while being 5 feet 3 inches tall, and there may be numerous individuals who are 6 feet 8 inches but were so clumsy on their high-school basketball teams that they sat on the bench the entire season. But in general, height still matters in basketball. It's not the only thing that matters, it doesn't absolutely determine success, but on average it makes a lot of difference. The same can be said of IQ^For most life events it's not as important as height is in basketball, but it's fairly important. Nor are IQtests biased: They predict academic performance with the same accuracy in different ethnic groups.29
Moreover, IQis highly heritable. What this means is that an individual's IQis partially determined by genetic factors, so that it tends to be more similar to that of his or her parents and siblings than a randomly chosen person's IQwould be. Siblings with the same biological parents have similar IQs even when they are raised separately, whereas adopted siblings don't, even when raised together.
The same is true of height: Tall people tend to have taller-than-average children. In fact, IQin adulthood is about as heritable as height. IQin childhood, on the other hand, is less heritable and more susceptible to environmental influences. These effects of environment on the measured IQs of children, which disappear at or after puberty, are the basis for claims that IQcan be improved by interventions like Head Start.
Nongenetic factors also influence IQ,but for the most part, the ones that matter are not the ones people thought would matter. Prenatal care, breastfeeding, nutrition, access to early education, Mozart in the womb, and oat bran all have little or no effect. Surprisingly, the way in which a family raises children seems to have no effect on adult IQ. This argues against some popular environmental explanations for high intelligence among the Ashkenazi Jews—in particular, the notion that Jewish mothers have a special way of rearing children that boosts IQ.
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