Twins: And What They Tell Us About Who We Are (11 page)

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Authors: Lawrence Wright

Tags: #Family & Relationships, #Siblings, #Science, #Life Sciences, #Genetics & Genomics, #test

BOOK: Twins: And What They Tell Us About Who We Are
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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.
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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
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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, while the contribution of environmental effects decreases from 0.31 to 0.05 during the same period of timea complete turnaround. On the other hand, a more recent study of more than 2,500 Australian twins found a heritability correlation for adolescent misconduct of 0.71 and no environmental effect. It seems this issue is very much unresolved.
 
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neurosis or alpine skiing or traditional values? Nothing in molecular biology indicates anything of the sort. The assumption of the twin model is that if one controls for the genes, by comparing identical versus fraternal twins, then the differences must be environmental; and further, if one controls for the environment, by comparing reared-apart identicals to reared-together identicals, then what is the same must be genetic. The logic seems to be unassailable, but it leads to unanswerable riddles. "Events have no DNA," the authors of the Swedish study conceded; "therefore, genetic factors cannot affect events per se. However, life events are defined as events that happen to peopletheir experiences. Genetic influence on experiences must be due to genetically influenced characteristics of individuals, not of the environments."
 
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5
The Critics Respond
In the early 1950s, James Shields, a British researcher, went on BBC television and made a public appeal for separated twins to step forward to be tested. More than forty separated sets of identical and fraternal twins responded. These were, of course, twins who knew of their kinship. Shields concluded in his own account of the project that family environments can vary significantly without affecting the profound similarity of separated identical twin pairs; on the other hand, identical twins brought up together can vary quite widely. Unlike the Minnesota team, Shields provided the raw data in the form of case studies of each of his twins, and it was clear that many of the so-called "separated twins" Shields studied had been raised by branches of the same family and had been companions during childhood.
Leon Kamin, the psychologist who led the attack on Burt, has become perhaps the most outspoken critic of twin studies. He charges that most separated identical twins haven't really been apart anywhere near the amount of time that Bouchard and his colleagues have advertised, and if they had, presumably they would be far less similar than they seem to be. For instance, when Kamin examined the twins cited in Shields's
 
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sample, he found that the IQ correlation for those who had been brought up in related families was 0.83, while for those brought up in unrelated families it was 0.51. Shields himself, however, recalculated his own data based on the similarity of environments. His correlation for twins who grew up in the most similar environments was 0.87about the same as Kamin's figurebut for twins who grew up in the least similar environments the correlation was 0.84. Environmental differences, in other words, essentially made no difference in the intelligence of identical twins.
Twins have been separated for a variety of reasons, such as financial hardship, the death of the mother, and illegitimacy. Sometimes twins were divided among family members; in those cases, it was usually the maternal grandmother who received one of the twins and the birth mother who kept the other. In one case that Shields studied, the biological father, a bankrupt Scandinavian ship's carpenter, sold one of his twins to a wealthy South American doctor to settle his debts. Given the oddity of their circumstances, it's difficult to know to what extent separated twins represent the general population. And despite the intensive research and interest in identical twins reared apart, fewer than 300 pairs have been identified, so the entire sample remains small.
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But in any case, if the environment shapes intelligence, then presumably the age of separation of the twins, and the amount of time that they spent together during their formative years, would be critical factors. "Bouchard reports that there is no correlation between
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There may be many more to be discovered in Japan. According to Professor Juko Ando, a twins researcher in the Department of Education of Keio University, twins born in rural areas were often separated because of the stigma attached to multiple births. This practice has died out in recent decades.
 
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the age of separation and the similarity of IQ, but I can't do the analysis," Kamin complains. "Nit-picking critics like me don't have the opportunity to go over their data." The Minnesota team no longer publishes case studies as it did in the early days of the program, and Bouchard refuses to release raw research material, citing privacy concerns on the part of the twins. "I wouldn't let Leon Kamin anywhere near it," he says. "If he has a legitimate question, we could answer it."
Susan Farber, a clinical psychologist now in private practice in Boise, Idaho, reviewed all known case studies of separated twins (excluding the Burt data) in her 1981 book,
Identical Twins Reared Apart
, and found only three examples out of 121 cases cited worldwide in which the twins truly were separated shortly after birth, reunited as adults with no intervening contact, and studied at their first reunion. (Farber did not deal with the Minnesota project, which was only getting started when her book appeared, nor was she aware of the Neubauer study. According to Bouchard, the average age of the twins who have come to Minnesota is forty; they have spent thirty years apart, so they have had a decade or so of contact before he has a chance to test them. In several instances, however, he has personally reunited twins who have never met before. The average age of the twins at the time of separation was five months.) Farber's survey left her unconvinced by the claim that IQ is largely heritable. "To say that these data close the case on environmental effects on IQ is a scientific farce," she concluded. "Those who persist in maintaining that an accurate heritability estimate can be obtained from these data and who extend the estimate to discuss racial differences in IQ . . . should question their own motivation and commitment to a dispassionate search for full understanding." The data for personality development was

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