Twin studies estimate heritability by gauging whether identical twins, who share 100 percent of their genes, are more similar to each other than are fraternal twins, who share only 50 percent of their genes. Twin studies, and especially studies of twins reared apart, have received tremendous media attention. The Jim twins, described at the beginning of this chapter , are identical twins given up for adoption at birth. Because they were adopted into dif ferent families, they were unaware that they had a twin. When they met for the first time, to everyone s astonishment, these men shared many behavioral habits—having the same favorite TV shows, using the same brand of toothpaste, owning a Jack Russell terrier dog, and so on. They also shared many personality traits, such as being highly conscientious and emotionally stable, as measured by valid personality scales. Is this coincidence? Perhaps, but these coincidences seemed to happen with unusual regularity in the course of studying twins, even those who have been reared apart by different sets of parents (Segal, 1999). Of course, these single examples prove nothing about heritability . It is always possible to find similarities even between tw randomly chosen individuals if you look hard enough (e.g., "they both hate broccoli"). Only by using the logic of the twin method can firmer conclusions be drawn
Twin studies take advantage of a fascinating quirk of nature. Nearly all individuals come from a single fertilized egg, and humans—as contrasted with some other mammals, such as mice—typically give birth to a single child at a time. Occasionally, however, twins are born, occurring only once in 83 births (Plomin et al., 1990). But twins come in two distinct types—identical and fraternal.
Identical twins, technically called monozygotic (MZ) twins, come from a single fertilized egg (or zygote—hence, monozygotic), which divides into two at some point during gestation. No one knows why fertilized eggs occasionally divide. They just do. Identical twins are remarkable in that they are genetically identical, like clones, coming from the same single source. They share literally 100 percent of their genes. In contrast, the odds of being genetically identical to someone else if you are not a twin are about one in several billion.
The other type of twin is not genetically identical to the co-twin; instead, such twins share only 50 percent of their genes. They are called fraternal twins, or dizygotic (DZ) twins, because they come from two eggs that were separately fertilized ( di means "two," so dizygotic means "coming from two fertilized eggs"). Fraternal twins can be same sex or opposite sex. In contrast, identical twins are always the same sex because they are genetically identical. Dizygotic twins are no more alike than regular siblings, at least in terms of genetic overlap. They just happen to share the same womb at the same time and have the same birthday; otherwise, they are no more similar than are ordinary brothers and sisters. Of all the twins born, two-thirds are fraternal, or dizygotic, and one-third are identical, or monozygotic.
The twin method capitalizes on the fact that some twins are genetically identical, sharing 100 percent of their genes, whereas other twins share only 50 percent of their genes. If fraternal twins are just as similar to each other as identical twins are, in terms of a particular personality characteristic, then we can infer that the characteristic under consideration is not heritable: the greater genetic similarity of identical
twins, in this case, is not causing them to be more similar in personality . Conversely, if identical twins are substantially more similar to each other than are fraternal twins on a given characteristic, then this provides evidence that is compatible with a heri-tability interpretation. In fact, studies have shown that identical twins are more similar than fraternal twins in dominance, height, and the ridge count on their fingertip (Plomin et al., 1990), suggesting that heritability plays a causal role in influencin these individual dif ferences. For dominance, identical twins are correlated +.57, whereas fraternal twins are correlated only +. 12 (Loehlin & Nichols, 1976). For height, identical twins are correlated + .93, whereas fraternal twins are correlated only + .48 (Mittler, 1971).
There are several formulas for calculating heritability from twin data, each with its own problems and limitations. One simple method, however , is to double the difference between the MZ correlation and DZ correlation:
In this formula, rmz refers to the correlation coef ficient computed between pairs o monozygotic twins, and rdz refers to the correlation between the dizygotic twins. Plugging in the correlations for height, for example, leads to the following heritability estimate: heritability of height = 2(.93 - .48) = .90. Thus, according to this formula, height is 90 percent heritable (and 10 percent environmental, as the total has to add up to 100 percent). The basic logic of this method can be applied to any phenotypic characteristic—personality traits, attitudes, religious beliefs, sexual orientation, drug use habits, and so on. We must first note two important assumptions of the twi method. If either of these assumptions is not met, then the results from twin studies might be called into question.
The first assumption is known as the equal envir onments assumption. The twin method assumes that the environments experienced by identical twins are no more similar to each other than are the environments experienced by fraternal twins. If they are more similar , then the greater similarity of the identical twins could plausibly be due to the fact that they experience more similar environments, rather than the fact that they have more genes in common. If identical twins are treated by their parents as more similar than fraternal twins are treated by their parents—for example, if the parents of identical twins dress them in more similar clothing than do the parents of fraternal twins—then the resulting greater similarity of the identical twins might be due to this more similar treatment.
Behavioral geneticists have been worried about the validity of the equal environments assumption and, so, have designed studies to test it. One approach is to examine twins who have been misdiagnosed as identical or fraternal (Scarr , 1968; Scarr & Carter -Saltzman, 1979). That is, some twins who were believed to be identical by their parents were really just fraternal. And some twins whose parents believed them to be fraternal turned out to be identical. These mistakes in labeling allowed the researchers to examine whether fraternal twins who were believed to be identical were, infact, more similar to each other than accurately labeled fraternal twins. Similarly , it allowed the researchers to examine whether the identical twins, believed to be fraternal, were, in fact, less similar to each other than identical twins correctly labeled as identical. The findings on a variety of cognitive and personality tests supported th validity of the equal environments assumption. The parents' beliefs and labeling of the twins did not af fect their actual similarity on the personality and cognitive measures. This means that, however twins are labeled, the environments experienced by identical twins do not seem to be functionally more similar to each other than the environments experienced by fraternal twins.
Studies such as this one cannot definitively rule out other possible confounds For example, parents may treat identical twins more similarly than they treat fraternal twins because they look more alike, regardless of the parents' beliefs about their twin status. Nonetheless, additional studies over the years have continued to support the equal environments assumption (e.g., Loehlin & Nichols, 1976; L ytton, Martin, & Eaves, 1977). Although it is true that identical twins do tend to dress more alike than fraternal twins, spend more time together , and have more friends in common, there is no evidence that these environmental similarities cause them to be any more similar in their personalities than they are to begin with (Plomin et al., 1990).
A second potential problem with twin studies is the possibility that twins are not representative of the general population from which they come. As a rule, twins tend to be born a few weeks prematurely and tend to weigh less than nontwins (MacGillivray, Nylander, & Corney, 1975). If twins are not representative of the general population, then this could limit generalizations about heritability based on twin studies. Most behavioral genetic researchers have concluded, however , that twins are reasonably representative of the general populations from which they come. One way to overcome some of the potential biases of the twin method is to use the adoption method—the final behavioral genetic method—to which we now turn
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