Twins: And What They Tell Us About Who We Are by Lawrence Wright Page A

Galton a century before.
Bouchard's studies began at a time when twin research was struggling to regain scientific respectability; now twin studies dominate much of the research in personality. Autism, for instance, was believed by the behaviorists to have been caused by "refrigerator mothers" who raised their children without enough affection; similarly, schizophrenia was supposed to have been generated by mothers who repeatedly placed their children

 

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in emotional binds. Down syndrome was said to be the result of physical and psychological traumas the mother may have suffered during pregnancy. The guilt felt by parents who stood accused of damaging their children is easy to imagine. Twin studies have now established a strong genetic component to many forms of mental illness, including not only autism and schizophrenia (Down syndrome has been shown to be a chromosomal anomaly), but also phobias and neuroses, which were previously presumed to be caused almost exclusively by traumatic emotional events. The Minnesota studies attribute forty-six percent of the personality variables they have measured to genetic factors, and practically none to family environment. Many physical ills, from acne to heart disease, are highly heritable, and even infectious diseases, such as German measles and chickenpox, appear to have a genetic basis, probably because of an inherited vulnerability in the immune system. On the other hand, twin studies have demonstrated that respiratory diseases and many cancers are largely environmental in origin.
Even more controversial and confounding have been twin studies of behavior. A Virginia study of 1,000 female twin pairs concluded that genetic factors account for about half the risk of developing problems with alcohol. Behaviors as diverse as smoking, insomnia, choice of careers or hobbies, use of contraceptives, consumption of coffee (but not, oddly enough, consumption of tea), menstrual symptoms, and suicide have all been shown to have far higher rates of concordance for identical than for fraternal twins, suggesting that they are more influenced by genes than previously suspected. German studies during the Nazi era implicated criminality as a heritable trait (this finding was used to justify the widespread sterilization practices during that period), but the reared-

 

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apart studies have shown little evidence to support the thesis that criminality is genetic and much to suggest that the environment is largely to blame. *
A 1986 survey of Australian twins posed fifty questions about social attitudes and found a significant genetic component for forty-seven of them, including such diverse items as socialism, the authority of the church, the death penalty, chastity, and birth control. Only three motley subjectscoeducation, the use of straitjackets, and pajama partiesshowed no meaningful genetic influence. This seemed particularly strange because one would expect the family environment to play an almost overwhelming role in determining social attitudes; and yet, that simply wasn't found. An especially interesting Swedish study of elderly twins, led by Gerald E. McClearn, a behavioral geneticist at Pennsylvania State University, looked at life events such as retirement, the death of a child, the mental illness of a spouse, and financial reverses, many of which might seem, almost by definition, to be accidents of the environment. The researchers concluded that forty percent of the variance of their total life-events score was attributable to genetic effects. Adding to the puzzle was that fact that twins who had been reared apart were somewhat more alike in terms of their major life events than twins who had been reared together.
Underlying these momentous assertions is an insistent unanswered question: how? Is there a gene for
* According to a 1995 study by M. J. Lyons et al., the heritability of antisocial behavior has been shown to increase from 0.07 in adolescence to 0.43 in adulthood,