What Makes People Intelligent?
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.
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