11-4 What is heritability, and how does it relate to individuals and groups?

So our biology helps form our personality. Yet asking whether our personality is more a product of our genes or our environment is like asking whether a flat-screen TV’s size is more the result of its length or its width. We could, however, ask whether the different TV sizes are more the result of differences in their length or differences in their width. Similarly, we can ask whether person-to-person personality differences are influenced more by nature or by nurture.

Using twin and adoption studies, behavior geneticists can mathematically estimate the heritability of a trait—the extent to which variation among individuals can be attributed to their differing genes. By one estimate, the heritability of general intelligence is 66 percent (Haworth et al., 2010). This does not mean that your intelligence is 66 percent genetic. (The heritability of height is 90 percent, but this does not mean that a 60-inch-tall woman can credit her genes for 54 inches and her environment for the other 6 inches.) Rather, it means that genetic influence explains about 66 percent of the observed variation among people. This point is so often misunderstood that we repeat: We can never say what percentage of an individual’s personality or intelligence is inherited. It makes no sense to say that your personality is due x percent to your heredity and y percent to your environment. Heritability refers instead to the extent to which differences among people are due to genes.

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Even this conclusion must be qualified, because heritability can vary from study to study. Consider humorist Mark Twain’s (1835–1910) fictional idea of raising boys in barrels to age 12, feeding them through a hole. If we were to follow his suggestion, the boys would all emerge with lower-than-normal intelligence scores at age 12. Yet, given their equal environments, their test score differences could be explained only by their heredity. With the same environment, heritability—differences due to genes—would be near 100 percent.

As environments become more similar, heredity becomes the primary source of differences. If all schools were of uniform quality, all families equally loving, and all neighborhoods equally healthy, then heritability would increase (because differences due to environment would decrease). But consider the other extreme: If all people had similar heredities but were raised in drastically different environments (some in barrels, some in luxury homes), heritability would be much lower.

If genetic influences help explain variations in traits among individuals in a group, can the same be said of trait differences between groups? Not necessarily. As we have seen, height is 90 percent heritable, yet nutrition (an environmental factor) rather than genetic influences explains why, as a group, today’s adults are taller than those of a century ago. More available food has caused Americans to grow to greater heights (Floud et al., 2011). In 1850, the average American male stood 5 feet 7 inches; in the 1980s, his counterpart stood three inches taller. The two groups differ, but not because human genes have changed in a mere century’s eyeblink of time. And today’s South Koreans, with their better diets, average six inches taller than today’s North Koreans, who come from the same genetic stock (Johnson et al., 2009). Genes matter, but so does environment.

As with height and weight, so with personality and intelligence scores: Heritable individual differences need not imply heritable group differences. If some individuals are genetically disposed to be more aggressive than others, that needn’t explain why some groups are more aggressive than others. Putting people in a new social context can change their aggressiveness. Today’s peaceful Scandinavians carry many genes inherited from their Viking warrior ancestors.

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