What Makes People Intelligent? (page 2)

By — John Wiley & Sons, Inc.
Updated on Dec 31, 2010

Other aspects of the data from twins studies, however, show that the environment quite clearly counts for something. If a child was living in a relatively deprived home and then was adopted into a family with greater means, the child's intelligence increased. This increase might have been due to a richer home environment, better schooling, better nutrition, or higher parental expectations, to name just a few possible factors. Other studies using different methods have also indicated that the environment counts for something. Good preschool intervention programs seem to give a modest boost to intelligence, but the effect of the environment in these studies is usually small—maybe 10 IQ points—compared to the effect of genetics.

That was the story until about twenty years ago. Most researchers seemed to have the sense that the range of intelligence was set mostly by genetics, and that a good or poor environment moved one's intelligence up or down a bit within that range.

A real turning point in this work came during the 1980s with the discovery that over the last half-century IQ scores have shown quite substantial gains. 2 For example, in Holland, scores went up twenty-one points in just thirty years (1952 – 1982), according to scores from tests of Dutch military draftees. This is not an isolated case. The effect has been observed in more than a dozen countries throughout the world, including the United States (Figure 5). Not all countries have data available—very large numbers of people are needed to be sure that we're not looking at a quirky subset—but where the data are available, the effect has been found. The discovery is sufficiently important that it has been named the Flynn effect, after James Flynn, who first described it.

What Makes People Intelligent?

Here's why this evidence is so surprising. If intelligence is largely genetic, we would not expect IQ scores for a whole country to go up or down much over time, because the overall gene pool changes very slowly. But that's not what has happened. There have been huge increases in IQ scores—increases that are much too large to have been caused by changes in genes. Some of the increase may have come from better nutrition and health care. Some of it may have come from the fact that our environment has gotten more complex and people are more often called on to think abstractly and solve unfamiliar problems—the exact sorts of things they're asked to do on IQ tests. Whatever the cause, it must be environmental.

How does this assessment fit with the studies of twins? The twins studies—and there are many of them—consistently show that genetics counts for a lot. But the rapid IQ increase over a short period can't be due to genetic factors. How can this paradox be resolved?

No one is completely sure, but Flynn (along with Bill Dickens, his frequent collaborator) has a pretty good suggestion. He claims that the effect of genetics is actually fairly modest. It looks large because the effect of genetics is to make the person likely to seek out particular environments. Dickens offers the following analogy. Suppose identical twins are separated at birth and adopted into different families. Their genes make them unusually tall at a young age, and they continue to grow. Because each twin is tall, he tends to do well in informal basketball games around the neighborhood (Figure 6). For that reason, each twin asks his parents to put up a net at home. The skills of each twin improve with practice, and each is recruited for his junior high school basketball team. More practice leads to still better skill, and by the end of high school each twin plays quite well—not a future professional, perhaps, but still better than 98 percent of the population, let's say.

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