Twins: And What They Tell Us About Who We Are Read Online Free PDF Page B

of the observed variation in a population that is caused by differences in heredity. Lush was working with farm animals, and he had the

 

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luxury of doing breeding experiments. It is easy to establish the transmission of traits in plant and animal populations; in fact, it is the basis of selective breeding. In humans, however, matters are more complex. The most common way of measuring heritability in humans is through twin studies. For any trait, the greater the difference in concordance between identical twins and fraternal twins, the greater the heritability. An example is tuberculosis, which is caused by an infectious organism; however, people have differing degrees of susceptibility to the disease. An identical twin has a fifty-six percent chance of getting the disease if his sibling catches it, but a fraternal twin has only a twenty-two percent chance. The impressive difference between these rates demonstrates a genetic factor at work. If a trait is completely heritablesuch as blood type or eye colorthen it will be one hundred percent concordant in identical twins and about fifty percent in fraternal twins and other siblings. * The heritability correlation would be a perfect 1.0. If all the differences between siblings are environmental, the heritability would be 0.0.
But environmental factors can also affect traits that are genetically transmitted. Height, for instance, is a highly heritable trait, and in well-nourished Western populations most of the variation in stature is an expression of the genes. The heritability for height among white European and North American populations is about 0.90. This does not mean that if a man is six feet tall, seven inches are due to his environment and the remaining ninety percent to his genes. If an individual is ten inches taller than the average for his population,
* Heritability depends also on frequencies within a population. For instance, if nearly everyone in a population has brown eyes, then fraternal twins will be nearly as concordant for this trait as identical twins.

 

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however, one could estimate that one of those inches is probably accounted for by environment and the other nine inches by his genes. Moreover, heritability can vary significantly in different populations, because the genes require a supportive environment in order to be expressed in the first place. A population that exists on a starvation diet would have little variation in height because growth would be arrested; there would be no way of telling who had tall genes and who had short ones. If one group within the population enjoyed an abundant diet while the rest were starving, the variation of height would be largely environmental. Children of Japanese immigrants who are born and raised in North America tend to be taller than their parents but shorter than the North American average, a difference that is attributed to changes in nutrition (more meat in the diet, for example). The North American grandchildren of those immigrants are taller still, which must mean that environmental effects gain an even stronger hold in the second generation.
Nowhere is the argument about heritability more heated or more consequential in its implications than over the question of intelligence. For much of this century, the debate was grounded in the work of Sir Cyril Burt, who was in his lifetime Britain's most honored and acclaimed psychologist. His name has become associated with a bizarre controversy that scandalized the academic world and is still furiously contested more than a quarter of a century after his death in 1971. Burt was a crusty, opinionated, dominating figure, whose political views were slightly left of center, and whose personality was characterized even by his friends as neurotic and occasionally paranoid. He began his career in 1913 as a research psychologist in the London County Council, which put him in the position of being in

 

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charge of all mental and scholastic testing