What Makes People Intelligent?

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.

Now, notice what has happened. These are identical twins, raised apart. So if a researcher tracked down each twin and administered a test of basketball skills, she would find that both are quite good, and because they were raised apart, the researcher would conclude that this was a genetic effect, that skill in basketball is largely determined by one's genes. But the researcher would be mistaken. What actually happened was that their genes made them tall, and being tall nudged them toward environments that included a lot of basketball practice. Practice—an environmental effect—made them good at basketball, not their genes. Genetic effects can make you seek out or select different environments.

Now think of how that perspective might apply to intelligence. Maybe genetics has had some small effect on your intelligence. Maybe it has made you a little quicker to understand things, or made your memory a little bit better, or made you more persistent on cognitive tasks, or simply made you more curious. Your parents noticed this and encouraged your interest. They may not even have been aware that they were encouraging you. They might have talked to you about more sophisticated subjects and used a broader vocabulary than they otherwise would have. As you got older, you saw yourself more and more as one of the "smart kids." You made friends with other smart kids, and entered into friendly but quite real competition for the highest grades. Then too, maybe genetics subtly pushed you away from other endeavors. You may be quicker cognitively, but you're a little slower and clumsier physically than others. That has made you avoid situations that might develop your athletic skills (such as pickup basketball games) and instead stay inside and read.

The key idea here is that genetics and the environment interact. Small differences in genetic inheritance can steer people to seek different experiences in their environments, and it is differences in these environmental experiences, especially over the long term, that have large cognitive consequences. For that reason, we shouldn't assume that twins have experienced different environments even though they were raised in different households. The fact that their genes are the same may well have encouraged them to seek out similar environments.

Now, why did I take you through this long story about intelligence? Because what we will consider doing for students who seem unintelligent differs depending on the nature of intelligence. If intelligence were all a matter of one's genetic inheritance, then there wouldn't be much point in trying to make kids smarter. Instead, we'd try to get students to do the best they could given the genetically determined intelligence they have. We'd also think seriously about trying to steer the not-so-smart kids toward intellectually undemanding tracks in schools, figuring that they are destined for low-level jobs anyway. But that's not the way things are. Intelligence is malleable. It can be improved.

Great! So how do we improve intelligence? The first step is to convince our students that intelligence can be improved.