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Authors: Lawrence Wright

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

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Page 70
even more questionable, she charged. "Magnify every problem in the IQ datatests, sampling bias, and so fortha hundredfold for the measurement of personality difference," she wrote, charging that the validity of all personality tests is open to question.
Farber found in the twin literature reason to believe that environment played a substantial role in the formation of individual personalities. "Family influences show clearly in attitudes, values, choice of mate, and in presence of nonpsychotic psychiatric symptomatology. The cultural or regional influences transmitted by the family and social network also are present and probably show in general personality traits ranging from emotional expressivity to drinking habits . . . everything in these data points toward the massive and perhaps predominant influence of family and culture on attitudes and psychological traits."
The paradox that Farber discovered, however, was that, as a group, "the more time MZ twins spend with no contact with each other, the more similar they seem to become," which directly counters the environmental argument advanced by Kamin. Early in the Minnesota study, Bouchard also got results indicating that separated twins were sometimes more similar than twins raised together. "You could argue that the twins reared together, because of the presence of each other, forced themselves apart. They differentiate. Whereas, if they were reared apart, they couldn't care less about this," he says. As the number of twins tested increased, however, this idea became harder to support. Bouchard now believes that twins reared apart and twins reared together are "about the same" in most measured tests. Separated twins who were separated later or who have spent more time together have no more difference in their IQs than those who were separated at birth and have practically never met.
 
Page 71
Among the twins who came to Minnesota, there was a British pair with a wide disparity in education and family background. One of the twins was raised by working-class parents and left school at sixteen. She spoke with a distinct Cockney accent. Her twin was raised by a university professor and was educated in private schools. Their difference in IQ, however, was less than the average difference between MZ twins raised together. According to David Lykken, the largest IQ difference among the separated twins in the Minnesota sample was found in a pair of older men. One had been adopted by illiterate parents and had quit school at the age of thirteen; his brother, whose IQ was 29 points higher, was raised by better-educated parents and had gone into the military. Lykken theorizes that it may be easier to lower IQ, by suppressing education and stimulation, than to raise it. He also wonders if the lower-IQ twin may have suffered some kind of brain damage at birth. In any case, it is clear that the overwhelming flow of data about the near-perfect correlation of intelligence, personality, and behavior among identical twins masks vivid distinctions within individual pairs.
Another common critique of the classic comparison of identical versus fraternal twins is that identical twins by definition look more alike; their parents are also more likely to treat them similarly and dress them alike. "There is no great imagination required to see how such a difference between MZs and DZs might produce the reported difference in IQ correlations," Kamin wrote with two other prominent and persistent critics, Steven Rose, a neurobiologist at the Open University in England, and Richard C. Lewontin, an evolutionary geneticist at Harvard. Their 1984 book,
Not in Our Genes: Biology, Ideology, and Human Nature
, assailed the entire field of behavior genetics. Kamin
 
Page 72
points to a study in which photographs of MZ twin pairs were sorted by judges on the basis of which twins looked more nearly identical; those who were most similar in looks were also somewhat more alike in IQ.
*
It is likely, says Kamin, that identical twins spend more time studying together, and that they are often treated as a single unit by their teachers, so it is unsurprising that they would receive similar grades and similar scores on intelligence tests. ''When I first got interested in this, the following thought occurred," Kamin recalls. "Try to imagine a family in which there is a pair of twins plus a third kid. Suppose you try to get IQ correlations between the third kid and either one of the twinsyou'd expect a correlation of 0.50. But the correlation is amazingly small." The point, says Kamin, is that "twins form a closed society. The third kid has a much lower resemblance to them than he would have if either of the others were not twins."
**
These arguments don't address the situation of reared-apart twins, but it is certainly possible to imagine that reared-apart identical twins would be treated somewhat alike by their adoptive families because of their physical similarity.
Other kinds of family studies have found lower genetic correlations for intelligence: between 0.40 and 0.70. At the lower end of that scale, one could expect the environment to play a much more decisive role, especially as one grows older. All the factors that go into cre-
*
Most studies that have examined the degree of physical resemblance in identical twins have found little or no correlation to similarities in personality or ability. Indeed, parents rated MZ twins who were most frequently confused for each other as least alike in behavior.
**
In fact, the Louisville Twin Study tested Kamin's thought experiment in 1978, matching the cognitive performance of twins with the performance of non-twin siblings at the same age. The Louisville team found that fraternal twins were more like each other than their non-twin sibling at age three, but these differences had disappeared by the age of six.
 
Page 73
ating what we term the "environment"one's family, culture, level of education, occupation, nutrition, health, exposure to stress, and so onnaturally increase with the storehouse of life experiences. A team of Swedish, British, and American scientists under the leadership of Dr. Gerald E. McClearn recently concluded a two-year study of 240 elderly Swedish twin pairs, all born before the First World War. They were given a range of tests for mental ability. Identical twins were much more alike than fraternal twins of the same sexno surprisebut what seems so confounding is that the elderly identical twins were more alike in mental functioning than younger identical twins. The role of heredity in determining intelligence only seems to grow with age,
*
and yet a lifetime of living different lives in different environments apparently had served only to make the identical twins more similar, whereas the fraternal twins became less alike over time.
The most recent and possibly the most substantive criticism of twin studies was advanced in a 1997 letter to the British journal
Nature
by Bernie Devlin, a psychiatrist at the University of Pittsburgh School of Medicine, and two statisticians at Carnegie Mellon University. Behavioral geneticists had discovered a puzzling inconsistency in the data on intelligence derived from twins reared apart and twins reared together. When one calculated the heritability of intelligence based on separated twins, it was about 0.70; but when one calculated the same data on twins who grew up in the same environment, the heritability of intelligence dropped to about 0.50. The behavioral geneticists assumed that the difference was accounted for by the fact that nearly all
*
The McClearn team found that heritability of general cognitive ability increased from infancy (about twenty percent) to childhood (forty percent) to adolescence (fifty percent) to adulthood (sixty percent).
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