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

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.

 

Page 66
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."

 

Page 67
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

 

Page 68
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. *
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