Studies of identical twins have challenged our most entrenched views of behavioral development. What do these studies reveal about human nature?
A set of identical-twin girls were surrendered to an adoption agency in New York City in the nineteen-sixties. The twins, who are known in psychological literature as Amy and Beth, might have gone through life in obscurity had they not come to the attention of Dr. Peter Neubauer, a prominent psychiatrist at New York University’s Psychiatric Institute.
All in all, the research team characterized Amy’s family as a well-knit threesome—mother, father, and son—plus an alienated Amy. It was a family that placed a high value on academic success, simplicity, tradition, and emotional restraint. Beth’s family, on the other hand, was sophisticated and full of energy—“frenetic” at times—and it tended to put more emphasis on material things than on education. Clearly, Beth was more in the center of her home than Amy was in hers.
The separated-twins story is a chestnut of American journalism—one that is guaranteed to gain national exposure, along with stories of pets that have trekked across the country to find their masters. The appeal of the separated-twins story is the implicit suggestion that it could happen to anyone.
All this comes after several decades of heightened political struggle between those who believe that people are largely the same, with differences imposed upon them by their environment, and those who conclude that people differ mainly because of their genes, and that their environments are largely of their own making. Obviously, the roots of liberal and conservative views are buried in such contrary presumptions about human nature.
A survey of Australian twins in the early nineteen-eighties also found a surprisingly significant genetic component for attitudes toward such wide-ranging political and social issues as apartheid, the death penalty, divorce, working mothers, and some forty other subjects. Only on a motley assortment of topics—coeducation, the use of straitjackets, and pajama parties—was there no meaningful genetic influence on individual attitudes.
One reason for the dearth of such knowledge is that research involving the genetic underpinnings of behavior has often been discouraged. When Dutch scientists announced, two years ago, that they had found a connection between a genetic defect and a form of familial aggression, they were denounced for even considering a genetic basis for violent behavior.
The morning the tests were to begin, Bouchard took the Jims to breakfast. He intended to brief them on the particulars of the study, but it was the first time he had ever worked with twins, and he found himself obsessing over little things about them—the way each one had bitten his nails, for example. Each of the Jims had a peculiar whorl in his eyebrow, and Bouchard started absently counting the number of hairs in their brows. “You’re staring at us,” one told him. Bouchard apologized.
Bouchard was standing with Jack at the Minneapolis airport when Oskar got off the plane. “I remember Jack pulling in his breath, because Oskar walked exactly the same way he did,” Bouchard says. “They have a kind of swagger to their bodies.” Each sported rectangular wire-rimmed glasses, a short, clipped mustache, and a blue two-pocket shirt with epaulets. They shook hands but did not embrace. Bouchard thought that they would be an ideal pair for detecting environmental influence.
Considering all that we’ve learned about human nature from studying twins, it’s astonishing to realize how little we know about the twinning process itself. In spite of the burst of twin-based scholarship in recent years, much that is commonly believed to be true about twinning is either wrong or in dispute. It is not clear, for instance, whether twinning is a kind of birth defect or, contrarily, whether birth defects are caused by twinning .
Twins are far more susceptible to the birth defects, spontaneous mutations, and vascular problems that threaten early life. Simply being a twin is stressful and raises the odds against survival. Because all twins battle in the womb for space and nutrition, the experiences of twins before birth may be considerably different from those of singletons. “Twins compete physically,” says Dr. Louis Keith, of Northwestern. Several years ago, one of the doctors on his staff observed twin fetuses fighting.
Charles Boklage cites another interesting phenomenon, which, though it has rarely been detected, may not be at all uncommon. “Possibly some of us are twins who are walking around in a single body,” Boklage says. Such a creature is called a chimera, after the mythological Greek monster that had the head of a lion, the body of a goat, and the tail of a serpent.
“If you took a normal girl and looked at a thousand cells, you’d expect five hundred of those cells to be using the mother’s X and five hundred to be using the father’s X—just like tossing coins,” Burn reasoned. “Now and again, very rarely, a girl will toss heads in every cell, and not tails, and switch off all her good copies.” As it turned out, Burn was right: the afflicted twin was using only the mother’s X, whereas her unaffected sister was using only the father’s X.
Twins of both kinds have a higher rate of left-handedness. Some scientists, like Luigi Gedda, the director of the Gregor Mendel Institute, in Rome, have suggested that all left-handed singletons may be survivors of a vanished-twin pair. Like twins, lefties are a puzzling minority whose origins have never been satisfactorily explained.
Cancer also shows the effects of genes and environment acting in concert. For more than ten years, Dr. Thomas Mack, an epidemiologist at the University of California in Los Angeles, has been amassing a registry of adult twins with chronic diseases, and he has now identified more than fifteen thousand sets of twins with cancer, multiple sclerosis, and Lou Gehrig’s disease, among others.
“We are left with a curious and disquieting conclusion,” Lykken and Tellegen wrote. “Although most human choice behavior . . . reflects the characteristics of the chooser and of the choice, the most important choice of all—that of a mate—seems to be an exception. Although we do tend to choose from among people like ourselves, another person who is remarkably like ourselves is not likely to be drawn to the same choice we make.
“O.K., Senator, we’ll have lunch and talk about it,” David Lykken said as he hung up the phone. The state legislator on the other end of the line wanted to talk about Lykken’s proposal to require the licensing of parents. “We already have criteria for parents who adopt a child,” Lykken told me as he sat in his office in Elliott Hall amid piles of books and research papers.
Early in her career, Scarr began studying why so many black children did poorly on tests and in school achievement. She wondered whether it was a result of sociocultural disadvantage or genetically based racial differences. This was a forbidden question in 1967, when Scarr first started studying the records of black and white twins in the Philadelphia public schools. Two years later, the Berkeley psychologist Arthur Jensen stirred up a nasty debate by airing his theory, based on I.Q.
Scarr found that children in the same family who were genetically unrelated were alike in their early years but grew to be different over time. They became more like their biological parents, whom they didn’t know, than like the adoptive parents who raised them—not only in social attitudes, vocational interests, and certain personality features, such as prejudice and rigidity of belief, but also in I.Q.
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