Twins: And What They Tell Us About Who We Are (19 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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8
The Emotional Life
One of the most provocative questions in the nature-versus-nurture debate concerns the influence of genes on criminal behavior. Twin studies in Nazi Germany confidently placed the heritability of criminality at 60 or 70 percent. Although more recent American studies tell us that there is a genetic connection to the antisocial behavior of very young children, they also point out that from adolescence on the environment plays an increasingly important role. In Sweden, where all adoptions are recorded for national statistics, a 1982 study found that the rate of criminality in adopted children was 2.9 percent when neither their biological nor adoptive parents had committed a crime; the figure rose to 6.7 percent if their adoptive parents were criminal and 12.1 percent if their biological parents were criminal. That would seem to be a nice demonstration of the relative weight of environment and genetics on antisocial behavior.
If both
sets of parents were criminal, however, the chances of the child being criminal as well were 40 percent. Together, genes and environment appear to be several times more compelling than either force acting alone.
Cancer also shows the effects of genes and environment acting in concert. For ten years Thomas Mack, a
 
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professor of preventive medicine at the University of Southern California, has been building a registry of adult twins with chronic diseases, and he has now identified more than 17,000 twins with cancer, Hodgkin's disease, multiple sclerosis, and autoimmune disease. According to Mack's data, a twin whose identical cotwin has Hodgkin's is one hundred times more likely to get the disease than a fraternal twin whose sibling has the disease, which demonstrates how powerfully genetic Hodgkin's is. On the other hand, out of the 500 twins he studied in which melanomas had occurred, there were only ten instances of both twins having the disease, indicating that despite the genetic contribution of inherited traits such as light skin, a powerful environmental influence is at work. If you are an identical twin with breast cancer, which is known to be a familial disease, the chances that your co-twin will also get the disease is about five times as high as the average; still, not all co-twins get the disease. The likelihood of getting breast cancer is affected by the age of first menstruation: the younger a woman is when she has her first period, the higher her chances of developing breast cancer in the future. Breast cancer rates also rise with a woman's age at first pregnancy and with her age at menopause. In each case, environmental factors create differences between identical twins, and those differences affect the health of women who are equally susceptible to disease.
Perhaps the most puzzling feature of identical twins is their love life. They do have much in common. David Lykken and his colleague Matt McGue examined 1,500 sets of twins from the Minnesota Twin Registry, finding that if one identical twin had been divorced, there was a forty-five percent chance that the other twin would also have been divorced, a rate that is twenty-five percent
 
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above the average for Minnesotans. Lykken postulates that genes influence divorce rates through personality characteristics that contribute to or detract from marital harmony. Identical twins tend to have their first date at about the same time and to date with equal frequency. They begin sexual activity at about the same time, and the intensity of their sexual drive, as well as their sexual dysfunctions, tends to be very similar. They marry and begin having children at roughly the same points in their lives, although there doesn't seem to be much correlation between the number of children each twin has. The extraordinary difference between identical twins lies in whom they choose to marry.
When Louis Keith was in medical school in Chicago, he had a friend named Phyllis Markuson. They dated a couple of times but were never more than pals. On New Year's Eve of 1960, Phyllis bumped into Louis's identical twin, Donald, at the ballet. Louis and Donald were as physically alike as identical twins can be, both of them dark-eyed, long-lashed, full-lipped, Latin-lover types. Donald, who was just home from the army on leave, was wearing his dress blues. ''There was just something different about him, like a spark," Phyllis recalls. "They were both very good-looking. I think Louis has softer features, especially around the mouth. Donald is more angular. His jaw is more set and refined, which reflects his personality. But who knows what one sees in another personphysically, emotionally, or intellectuallythat makes him different from everybody else." Phyllis was instantly smitten, and so was Donald.
Immediately after the meeting, he called his brother and asked if he had any romantic intentions where Phyllis was concerned. "Most twins know this area is a big, fat, hot potato," says Donald. Louis told him he liked Phyllis, but he wasn't romantically interested.
 
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"Are you sure?" asked Donald. "I don't want you to say ten years from now that I stole a girl you were interested in." Donald and Phyllis went out every night for forty-four nights in a row. ''I had a thirty-day leave and I got a fifteen-day extension," says Donald. They have now been married for thirty-two years. "It's lucky we don't have the same taste in women," says Donald. "It would be very difficult going through life wanting the same spouse."
Although there have been many instances of twins competing for the same potential partner, the relationship between Phyllis and the Keith twins is more typical. In 1993 Lykken and Auge Tellegen, also a psychologist at the University of Minnesota, reported on a study of about 1,000 middle-aged twins and their spouses, which examines various widely held assumptions about mate selection. One is that we tend to choose mates who are similar to ourselves. When Lykken and Tellegen compared sets of twins with the people they had chosen to marry, they discovered that, in fact, traditionalism, physical attractiveness, education, and height were strongly correlated, but in other respects spouses had too little in common to explain their selection of each other. A second assumption is that we are all looking for a special someone who has certain qualities that we admire. Presumably identical twins who have been raised together will have similar criteria; after all, they do tend to make very similar choices about clothes and furniture and holidays. But, when Lykken and Tellegen compared the spouses of identical twins, they resembled each other even less than they resembled the twin they were married to; they were no more alike than people who were married to unrelated individuals. Another surprise was the fact that when the twins were asked to evaluate their twin's spouse, about as many
 
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disliked the spouse as not. The spouses, for their part, returned the favor, although one would expect that the spouses would be at least somewhat attracted to the identical twin. And, while nearly twice as many husbands of identical twins approved of the other twin as did the wives of identical twins, even among the husbands a quarter were not attracted at all to the identical twin and only thirteen percent agreed that they "could have fallen for her myself." Among wives, the figure was only seven percent.
"We are left with a curious and disquieting conclusion," Lykken and Tellegen wrote.
Although most human choice behavior lawfully 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 (our MZ twin) is not likely to be drawn to the same choice we make. Having made a choice, when we are then confronted with a second mate candidate who is remarkably like the person we have chosen, we are not also strongly attracted to that person.
The authors concluded that human pairing is inherently random: "Romantic infatuation, we suggest, like imprinting, forms an initial bond almost adventitiously and then sustains it long enough, in most instances, for an enduring bond to be forged by the slower processes of learning and adaptation that result in compassionate love."
Several other twin studies have begun to look at what makes people happy. The findings so far point to two independent components of happiness, one called extraversion, or positive emotionality, and the other
 
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called neuroticism, or negative emotionality. Curiously, the two traits appear to operate independently of each other, and not, as might be expected, like a seesaw, where a rise of one marks a decline of the other. A person who is free of negative moods is not necessarily happy, just as a person who never experiences positive moods is not necessarily unhappy. Some researchers have proposed that both extroversion and neuroticism arise out of particular situations, with positive experiences leading to positive emotionality, and negative experiences to negative emotionality. This is the basic environmentalist stance: that an accumulation of pleasant experiences will endow a person with a happy nature, while unpleasant experiences have the opposite effect. Other researchers assert that both extroversion and neuroticism are genetically disposed.
To test these theories, Laura Baker, a professor of psychology at the University of Southern California who heads the Southern California Twin Project, recruited some fifty sets of twins from the Los Angeles area, who ranged in age from sixteen to seventy-two. The twin study was done in conjunction with a family study comprising three generations of 220 families. What the study found was that the more closely family members were related, the more similar they were in experiencing negative emotionality; that is, levels of this trait were more alike among fraternal twins and siblings, who share, on average, fifty percent of their genes, than among grandparents and grandchildren, who have about a quarter of their genes in common. This finding suggests the presence of a marked genetic effect where neuroticism is concerned. Predictably, MZ twins were much more alike in this respect than DZ twins. But a different conclusion emerged when positive emotionality was measured. The differences between MZ twins
 
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and DZ twins were not nearly so great; suggesting that shared experiences, instead of genetics, may influence levels of positive emotionality.
Unhappy natures, as we might as well call the negative element of happiness, seemed to pass through the generations in genes of descendants. Yet happiness itself seems to be largely a gift of the environmentin particular, the family environment. Although countless studies have now documented the paucity of the effect that ordinary families have on children, Baker's study suggests that the one thing a good family can do is to make a child happy.
 
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9
The Environment We Make
Should couples be required to obtain a license before they are allowed to become parents? This notion has been proposed by David Lykken. "We already have criteria for parents who adopt a child," Lykken said as he sat in his office at the University of Minnesota amid piles of books and research papers. "Typically, the parents have to be mature." There has to be a father and a mother, and they have to have means of support, and they can't be actively psychotic or seriously criminal. I say that our problems are not going to be mitigated until we establish similar criteria for those who would produce children biologically.
"I think if you put children with a sociopathic, immature, irresponsible, drug-addicted teenage mother, there's a high-percentage chance that the child is also going to be a sociopath," he continued. "We know that mothers of illegitimate children are [on average] ten points lower in IQ than mothers of legitimate children. So one of the interesting consequences, if we do something to stem the tide of illegitimacy, would be to reduce the number of low IQ children." In Lykken's scheme, an "unlicensed woman" who got pregnant, and who wanted to carry her baby to term, would be placed

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