The Influence of Heredity

The isolation studies discussed above may seem to suggest that inheritance can be dismissed as a factor in the social development of humans and animals. However, the interplay between heredi­tary and environmental factors is evident in two fascinating studies that began to produce results in 1987—one involving pairs of twins reared apart and the second examining the inheritability of a form of mental illness.

Inheritability and Traits of Twins The case of Oscar Stohr and Jack Yufe, discussed at the be­ginning of the chapter, is a dramatic example of two genetically identical persons who neverthe­less developed quite different personalities and political values—presumably because of their dif­fering socialization experiences. By contrast, an­other pair of identical twins reared apart, Jim Springer and Jim Lewis, both chain-smoke the same brand of cigarettes, chew their fingernails, drive the same model blue Chevrolet, and have owned dogs named Toy (Leo, 1987).

Researchers at the Minnesota Center for Twin and Adoption Research are studying pairs of identical twins reared apart to determine what similarities, if any, they show in personality traits, behavior, and intelligence. Thus far, the prelimi­nary results from the available twin studies indi­cate that both genetic factors and socialization experiences are influential in human develop­ment. Certain characteristics, such as twins' tem­peraments, voice patterns, and nervous habits, appear to be strikingly similar, thereby suggest­ing that they may be linked to hereditary causes. However, there are far greater differences be­tween identical twins reared apart in terms of atti­tudes, values, types of mates chosen, and even drinking habits. In examining clusters of person­ality traits among such twins, the Minnesota stud­ies have found marked similarities in their ten­dency toward leadership or dominance, but significant differences in their need for intimacy, comfort, and assistance.

Researchers have also been impressed with the similar scores of twin pairs on intelligence tests. Most of the identical twins register scores even closer than we would expect if the same person were to take a test twice. At the same time, how­ever, identical twins brought up in dramatically different social environments score quite differ­ently on intelligence tests—a finding that sup­ports the impact of socialization on development (S. Begley et al., 1987; Holden, 1987b; Tellegan et al., 1988).

Inheritability and Mental Illness Lancaster County, Pennsylvania, is hardly a likely site for scientific breakthroughs. Often the destination of tourists, this area is best known for its Amish communities—whose distinctive subculture was popularized in a 1985 Hollywood film, "Witness." For 10 years, a team of psychiatrists and biologists (Egeland et al., 1987) studied the occurrence of manic-depressive behavior in three generations of Amish families. Victims of manic depression (or bipolar affective disorders) shift between the ex­treme emotional states of euphoria and depres­sion. In studying this illness, scientists found that the Amish served as an excellent sample. Their communities keep accurate genealogical records; moreover, many environmental factors which contribute to manic depression, such as alcohol­ism, drug abuse, unemployment, and divorce, are extremely rare within the Amish subculture.

The findings of the Pennsylvania study suggest a certain hereditary basis for manic-depressive behavior, which is apparently linked to genes in the specific region of a chromosome. The re­searchers emphasize that this genetic characteris­tic neither guarantees nor precludes manic de­pression; they can only report that persons with the characteristic show a predisposition to manic-depressive behavior. It should be noted that this degree of specificity was not present in the twin studies discussed earlier. There was no sugges­tion, for example, that a single gene could be linked to timidity (Goleman, 1986; Wallis, 1987; Woller, 1987).

In reviewing the studies of twin pairs and the Amish, one should proceed with some degree of caution. Janice Egeland, head researcher for the Amish study, notes an encouraging aspect of findings which point to the importance of hered­ity in human development. "Too often," she ar­gues, "personal embarrassment and social stigma are associated with an illness whose cause is be­yond the control of the individual." By contrast, Princeton University psychologist Leon Kamin fears that overgeneralizing from the Minnesota twin results—and granting too much importance to the impact of heredity—may be used to blame the poor and downtrodden for their unfortunate condition. As the debate over nature versus nur­ture continues, we can certainly anticipate nu­merous replications of these fascinating investiga­tions to clarify the interplay between hereditary and environmental factors in human develop­ment (Leo, 1987; Wallis, 1987:67).