Identical twins rarely fail to fascinate.
There is, after all, a compelling oddity in the stories of twins who speak their own languages, twins who experience each other`s physical pain or twins who adopt each other`s habits even though reared apart from early childhood.
And among scientists, few know the value of capitalizing on that fascination more than Thomas Bouchard.
As director of the University of Minnesota`s Center for Twin and Adoption Research, Bouchard has never shied from enthusiastically telling newspaper, magazine or television journalists about the results of his research on twins, particularly the studies of more than 55 pairs of identical twins separated at birth and reared apart.
In Bouchard`s opinion, such publicity-seeking, normally antithetical to the scientific temperament, serves a noble purpose: to recruit more reared-apart twins for research into the role genetics plays in influencing behavior or personality.
”We admit we`ve had to rely on the general media perhaps more than other scientists,” Bouchard said. ”But that`s because the twins find out about us by reading the newspaper. . . . These are very rare individuals. They`re hard to find.”
Within the last three years, however, Bouchard has been criticized for attending more to informing the general press than to publishing his team`s results in peer-reviewed journals.
This summer, however, Bouchard had a paper published in the Journal of Personality and Social Psychology, the first peer-reviewed article on the team`s flashiest conclusions about the genetic basis of personality traits such as leadership ability or deference to authority.
The paper quelled criticism somewhat but not completely. At a time when biologists have begun finding specific genes related to mental conditions, such as manic-depressive disorder, scientists are increasingly skeptical of embracing the results of Bouchard`s research, as well as other twins studies, as a way of understanding the genetic basis of behavior.
Among the problems, they say, are that the sample of twins is often too small, too biased and too uncontrolled for meaningful results.
”It`s not whether the data has been published but rather, So what?”
said Richard Rose, a psychologist at Indiana University in Bloomington.
”These kinds of data are always so inherently selective that one has to be very cautious about drawing conclusions.”
The idea of using identical twins to study inherited traits is nothing new. In 1875, British anthropologist Francis Galton compared a small sample of identical and fraternal twins and concluded in his published study that
”nature prevails enormously over nurture” for conditions in which the environment is the same.
Galton realized that at least in theory identical twins might be to behavioral geneticists what volcanoes are to geologists: an ideal natural laboratory. Unlike regular siblings-or even fraternal twins, which come from two eggs-identical twins share every gene. Such genetic copies, particularly if put in different environments, appear to be a gold mine for discovering where nature and nurture reign.
Since Galton`s study, psychologists have used statistical analysis of data on twins to discern the scope of genetic influence in such areas as intelligence, personality, mental illness or even political attitudes.
And within the last few years, most of those twins studies have suggested that statistically, at least, genes account for about 50 percent of the range of behavioral traits, and environment takes care of the rest in ways that can`t be predicted.
In 1976, for instance, psychologists John C. Loehlin and Robert C. Nichols at the University of Texas at Austin assessed personality data collected from 514 pairs of identical twins and 336 pairs of fraternal twins who had taken the 1962 National Merit Scholarship Qualifying Test.
The study found that personality traits in identical twins correlated about 50 percent, those in fraternal twins only about half as much. According to the mathematics, the higher the correlation, the more likely the trait is linked to genetics.
What`s difficult in such a study, however, is discriminating which traits are more likely to be shaped by genetics and which are more likely products of environment. Consequently, researchers, Bouchard`s team included, have turned to a better twins model to answer that question: genetically identical twins who had been separated early in life and reared apart.
Since 1979, when Bouchard began his effort, the Minnesota group has become one of the more publicized and one of the larger studies of twins reared apart. Twins are brought to the center for one week of intensive screening, in which they are tested for personality and also for such attributes as pulmonary function, eyesight, motor skills, IQ and a host of other factors.
The results are filed for use by researchers affiliated with the center. The group plans to publish a book on their project within a few years.
At the very least, the project has produced interesting anecdotes.
Two female twins reared apart in Britain, who met for the first time at the center, each showed up wearing seven rings. Oskar Stohr and Jack Yufe, separated at birth and reared under different religions in different countries, arrived at the center wearing the same type of blue shirt, mustaches and wire-rimmed glasses. And in 1986, Mark Newman and Gerald Levey, previously separated twins who lived unknown to each other in the same region of New Jersey, discovered they were both volunteer firefighters whose jobs were to install household alarm systems and whose tastes included Budweiser beer, Chinese food late at night and women with long legs.
The press has picked up such anecdotes, and the Minnesota group has become savvy about how to market their twins for the widest exposure. Recently Bouchard`s group denied a request for interviews with reared-apart twins from Texas, saying the twins had a ”network television exclusive” that would air in two months.
Yet throughout, only a few results have been published in peer-reviewed journals, and until recently virtually nothing had been published about personality correlations. A few articles by the Minnesota group appeared discussing similarities in twins` lung capacity, heart conditions and eyesight. One article on intelligence similarities, suggesting a very high correlation, was submitted to Science magazine but rejected.
Another controversial article by the group was published two years ago in the British Journal of Psychiatry. The study, based on an analysis of six pairs of twins, concluded that homosexuality may be genetically determined in males but not so in females.
The Minnesota group was harshly criticized for relying on too small a sample in the homosexuality study. After that flap, Bouchard said, he found himself in a difficult position.
”The problem is simple,” he said. ”If we publish without a larger sample size, we get criticized for publishing too early. . . . But if we don`t publish, we get criticized too. We`re in the process of putting our results together in a peer-reviewed study.”
Indeed, the recently published study on personality answers critics to some extent. Based on 44 pairs of reared-apart identical twins and 27 pairs of reared-apart fraternal twins, and compared with 217 identical and 114 fraternal twins who were reared together, the results confirm earlier studies by showing the same high correlations among identical twins.
The study measured 11 personality traits, including the ability to control impulses, the tendency to revere authority and ”social potency,”
which is related to leadership ability. Overall, identical twins who were reared apart showed correlations of about 50 percent for those traits, compared with no more than a 30 percent correlation in reared-apart fraternal twins.
”We really didn`t discover anything new, but we confirmed earlier results using a different population,” Bouchard said. ”One of the reasons we can continue to be confident (about our method) is that these results of twins reared apart confirm studies of twins reared together.”
Loehlin at UT-Austin, who conducted one of the original twins studies, agrees. ”I would be a little skeptical if this were the only data there were,” he said. ”But the fact that it confirms the earlier twins studies makes it quite interesting.”
Critics, however, question not only the sample size. Leon Kamin, a psychologist at Northeastern University in Boston, pointed out problems with the sample itself, which isn`t randomly selected and is therefore
scientifically questionable.
Theoretically, to eliminate bias in twins studies, researchers would have to separate identical twins at birth and assign them to randomly selected environments, a practice that would be unthinkable in all but totalitarian societies.
As a result, the Minnesota study has appealed to twins through publicity. Kamin asserts that this appeal selects certain kinds of participants.
”Try to imagine the kinds of kooks who volunteer when a newspaper says they`re looking for identical twins separated at birth,” Kamin said. ”All kinds of pressure is put on these twins. They can become talk-show
personalities and have all the scientists in the world say they`re fantastic if they exaggerate the truth a bit.”
Jonathan Beckwith, a geneticist at Harvard University, said he also is skeptical of the twins studies because the twins sometimes have contact through childhood, despite their separate homes. Also, he said, it`s often difficult to tell how different the birth families were from the adopted families, which raises questions about whether traits are really inherited or learned.
”I know Tom Bouchard is aware of these issues,” Beckwith said. ”But you have to rely on their word that they`ve done it (checked out those factors) carefully. . . . I would like to see these things addressed someplace.”
Bouchard said he is aware of the problem with sample size and is trying to increase the number of twins in his study. ”It (sample size) has never been enough,” Bouchard said. ”I`ll be the first to admit that.”
Psychologist David Lykken, who works closely with Bouchard, said concerns about a biased sample aren`t warranted. Nearly half of the twins were contacted by the researchers, not lured by publicity, he said.
The researchers also have tracked the twins to see if the amount of contact they had before the study has any correlation with similarities discovered between them, Lykken said. There was no correlation, he said.
But Lykken admits that the University of Minnesota team has been
”derelict” about publishing such findings.
”There are just so many things to publish,” he said.
Meanwhile, Harvard`s Beckwith said twins research, fraught as it may be with potential problems, may not be as effective a research tack as newer ways of investigating the link between genetics and behavior.
”In this day and age when the techniques of molecular biology and DNA analysis are proceeding so rapidly, and one can get yes-or-no answers to these questions, this (twins studies) is just an old-fashioned and dated way of doing research.”
