What Makes People Intelligent? (page 2)
I emphasized the importance of practice and hard work to expertise in cognitive tasks. Perhaps people who are intelligent are those who have had a lot of practice doing the sorts of tasks that are used to define intelligence; for whatever reason, they have been exposed to lots of complex ideas (and explanations of these ideas), have had many opportunities to reason in a supportive environment, and so on.
The other view is that intelligence is a matter not of work and practice but rather of carefully selecting one's parents. In other words, intelligence is mostly genetic. Some people are born smart and although they might further develop this ability through practice, they will be pretty smart even if they do little or nothing to develop their intelligence (Figure 3).
I've proposed two answers to the question Where does intelligence come from? and both answers are rather extreme: all nature (that is, genetics) or all nurture (that is, experience). Whenever the question Is it nature or is it nurture? is asked, the answer is almost always both, and it's almost always difficult to specify how genes and experiences interact. The same answer applies to the question about intelligence, but there has been a significant shift in researchers' points of view in the last twenty years, from thinking that the answer is "both, but probably mostly genetic" to thinking it's "both, but probably mostly environmental." Let me describe the evidence on both sides. Once we better understand why people are intelligent, we'll better understand how to help students who seem to lack intelligence.
I've just said that intelligence is very likely a product of genetic and environmental factors combining in complex ways. So how can we untangle them? The most common strategy is to examine whether pairs of people are similarly intelligent. For example, identical twins share 100 percent of their genes, and fraternal twins (like all siblings) share 50 percent of their genes. So, testing whether identical twins are close to each other in intelligence more often than fraternal twins are will help us determine the importance of genes (Figure 4). In addition, we can examine whether the intelligence of siblings raised in the same household is more similar than the intelligence of siblings who were raised in different households—that is, siblings who were separated at birth and adopted by different families. Siblings who were raised in the same household didn't have identical environments but they had the same parents, had similar exposure to literature, television, and other sources of culture, likely went to the same school, and so forth.
Table 1 compares several types of relationships and tells us a lot about the relative importance of genetics and how we are raised.
The results of these studies are startling. Genetics seems to play a huge role in general intelligence; that is, our genes seem to be responsible for something like 50 percent of our smarts. The 50 percent figure is actually an average, because the percentage changes as we age. For young children, it's more like 20 percent, then it goes up to 40 percent for older children, and it's 60 percent or even higher later in life. This increase is the opposite of what you might expect. You might think that genetics would be most important in small children, because even if their environments are different, they haven't been exposed to them for very long, whereas older adults have lived in their environments for decades, so those environments ought to have had more impact. The data don't fit the pattern, however, making us even more likely to suspect that the environment doesn't affect intelligence much.
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.
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.