To the Manner Born

A new study of twins suggests that nature, as much as nurture, determines our personalities

Twin Brothers(9-12 months old) sitting on blanket Credit: George Marks/Getty

Soon after his birth, Oskar Stöhr was taken by his mother to Germany, where he was raised as a Nazi. His identical twin, Jack Yufe, remained with his father in the Caribbean and was raised as a Jew. In middle age, the two men were reunited for the first time. They looked alike, of course, but the physical resemblance was amplified by their matching epauletted shirts, wire-rimmed glasses and mustaches. Such superficial coincidence soon paled beside a spookier synchronicity. Despite the brothers' strikingly different biographies, their personalities — traits of character, temperament and behavior — resonated in almost every respect, right down to idiosyncrasies like dunking toast in coffee, flushing the toilet before urinating, wearing rubber bands on their wrists and reading magazines from back to front.

Like Oskar and Jack, Tony Milasi and his identical twin, Roger Brooks, were separated shortly after birth. Tony was immediately adopted by a prosperous Italian Catholic family in New York, but Roger spent three years in an orphanage before he was adopted and raised in straitened circumstances by his single Jewish mother in Florida. When the two met at the age of twenty-five, after Tony was mistaken for his brother in a restaurant, they noticed that their mannerisms were uncannily alike — they even held their coffee cups the same way, without touching the handles. As Tony unpacked for their visit, Roger pointed out that they used the same after-shave, hair cream and imported Swedish toothpaste. The circumstances of their lives had been remarkably dissimilar, but the men expressed themselves so similarly that no one could tell them apart on the phone. Tony sums up their mirror-image personalities: "We're so alike it scares me."

Identical twins, formed from the splitting of a single fertilized egg, are equivalent to genetic clones. (Two-thirds of all twins are fraternal, formed by two eggs and two sperm; genetically, they are just like regular siblings.) Because nature has made them identical, they can differ only through nurture — social and environmental influences, such as upbringing, culture and intrauterine conditions. Most psychologists have maintained that nurture pretty much determines personality, shaping it as a sculptor molds a lump of clay. According to this theory, the personalities of identical twins who are raised apart should be quite different.

Like Jack and Oskar and Roger and Tony, however, identical twins who have been raised apart have eerily similar personalities that seem mostly like nature's creation — diamonds in the rough that nurture has only faceted and polished. Among the guests at a gathering of British identical twins in Hertford, England, were two claustrophobic sisters who could only go into the ocean backward and who counted themselves to sleep. One pair of sisters always wore three rings on one hand and four on the other, and another pair giggled incessantly, drank their coffee cold and black and had shown up in matching outfits at their first meeting after years of separation. There was a set of brothers who worked similar jobs, had married teachers in the same month and had two sons and a daughter of the same ages.

All of these twins are participants in an important and controversial ongoing study conducted by a group of scientists at the University of Minnesota's Center for Twin and Adoption Research, in Minneapolis. The team's preliminary data suggest that nature, as much as nurture, determines such traits as leadership, optimism, persistence, selfishness, even the capacity to be swept away by poetry or music — not just in identical twins, but in everyone. After conducting comprehensive physical and mental examinations of their subjects, the research team found that the personalities of forty-four sets of identical twins who were raised apart, most from infancy, were more alike than those of fraternal twins who were raised together.

That much of personality is engraved at conception is partly bad news. It means that right from the outset, not everyone has an equal shot at being a superstar — a very un-American idea. The whiff of hopeless determinism given off by the concept of a genetic personality alarms critics like Northeastern University psychologist Leon Kamin. "The genetic interpretation is getting a much warmer reception now than it would have gotten twenty or thirty years ago," he says, "because it suits the temper of the times. If you decide that social problems like poverty and crime are genetic, you don't have to try to do anything about them — people want to forget that there's an environmental influence on behavior as well."

But the biological perspective on personality is also good news: it takes some of the heat off. If everyone has innate advantages and disadvantages, the aspiration to be everything to everybody is unrealistic. The twin research suggests that if you're the laid-back type, like Ferdinand the bull, in the children's book, it's wise to eschew the corrida and enjoy smelling the flowers. "Each person is unique, and a large part of that uniqueness is endogenous," says Thomas J. Bouchard Jr., a professor of psychology at the University of Minnesota and the director of the twin project. "Therefore, it seems only common sense to play to your strengths and to be careful of your weaknesses."

By contending that the genetic blueprint received at conception has a far stronger impact on personality than previously imagined, the Minnesota researchers have sparked a touchy nature-nurture debate in academia. But they've also raised provocative questions for parents and teachers coping with different sorts of kids, for psychotherapists wondering how much change is possible for their patients, for civic leaders struggling with the causes of and cures for social blights and for people just trying to figure out why they behave the way they do.

Twins have intrigued man since he enshrined Castor and Pollux in the heavens as Gemini. Twins are regarded — and regard themselves — as special, even if they don't achieve the celebrity of twins like Elvis or Montgomery Clift. They particularly fascinate scientists, and until modern technology provided the tools to explore chromosomes and genes directly, studying twins was a cornerstone of genetics. Sir Francis Galton, a cousin of Charles Darwin, did the first rigorous twin research over a century ago. Dr. Josef Mengele, the "Angel of Death" of Auschwitz, also "studied" twins: of some 3000 individuals, 157 survived. According to Dr. Nancy L. Segal, an assistant director at the Minnesota center, the distress of the surviving Auschwitz twins is intensified by their knowledge that the ordeals to which Mengele subjected them were scientifically meaningless: he apparently did not adequately distinguish between identicals and fraternals.

The Minnesota project is the most comprehensive and the best of the modern twin studies. Most of its 350 sets of subjects are fraternal and identical pairs reared together who live in the state of Minnesota. But since 1979, attracted by media coverage, 90 sets of rare identicals reared apart have taken the all-expenses-paid trip to the University of Minnesota. There, they spend six days taking psychological and intelligence tests, undergoing exhaustive physical examinations and answering 15,000 questions on everything from their interests, values and aesthetic principles to their recreational and dietary habits.

The researchers have found that the identicals' IQs, visual acuity, body language, patterns of tooth decay, pulse rates, brain waves — even the types of migraine headaches some of them get — are so similar that evaluations of two identical siblings often look like reports on the same person. Moreover, says Segal, many identicals raised apart have "an instant familiarity" at their first meeting. Bouchard describes the special relationship with a simile: "Identical twins are like a piece of music played by two different pianists — each rendition is unique, but you can always tell it's Tchaikovsky."

The Minnesota team uses the 300-item Multidimensional Personality Questionnaire (MPQ), along with other similar tests, to get a broad picture of the personalities of their subjects. When the researchers compared the personality variations within pairs of identical twins with the variations between pairs of identical twins, they found that genes played a far greater role than they had expected in most of the eleven basic traits measured by the MPQ. Identical twins reared apart make the case for genetic influence on personality because differences between them can only be caused by nurture. Some characteristics, like traditionalism, seemed more genetically determined than others, like social closeness, but Bouchard predicts that when the final data are analyzed, it will be discovered that nature and nurture share equally in the development of personality.

Even the researchers were surprised that of all the traits measured, traditionalism — considered just the sort of thing that parents pound into reluctant children — was among the most heritable. Traditionalism smacks of old-fashioned values and high moral tone, but it's really just a fancy new term for "the authoritarian personality," once associated by some with the rise of Nazism. Along with being religious, proper and unselfish, those who score high in traditionalism can tend to think that one social group (like the Aryans) is better than others, to believe in powerful external forces (like the fatherland) and to favor discipline and punishment (like concentration camps). Studies failed to show, however, that Germans are more authoritarian than other peoples. Bouchard thinks the association of the trait with Nazism springs from the fact that authoritarian types are likely to emerge in conspicuous numbers when the social and political environment is hospitable to their temperament. The racial and economic conditions in the American South that fueled the conflagrations of the Ku Klux Klan are, like those of post-World War I Germany, good examples of the way the cultural climate can affect the inborn inclinations of individuals. Bouchard offers an analogy for the effect of social environment on heritable personality traits: "If you take a genetic hybrid of corn developed for the Minnesota growing season and sow it in Florida, the plant is going to look different when it matures down South, even though it has the same genes."

The discovery that both achievement and social potency, or leadership, can be inherited casts new light on families like the Kennedys. Old Joe talked up winning and current events at the dinner table, but he may have been preaching to the genetically predisposed — many of his offspring seemed born with the take-charge charisma of the natural leader and the persistence and drive of the innate achiever. In fact, says David T. Lykken, a professor of psychology and psychiatry at the University of Minnesota and a member of the twin team, trying to separate nature's impact from nurture's in such cases is a pointless exercise, because forceful, dynamic people who enjoy attention create spotlight situations in which they can be forceful and dynamic — which reinforce those same tendencies. "It's an endless circle," he says. "Your genes determine who you are, which in many respects determines the kind of environment you'll have, which determines who you are." Lykken points out that the circular process starts in the earliest days of life, when the naturally extroverted, socially potent baby gives off winner vibrations and elicits approving smiles, even from strangers, and the quiet, introspective baby produces a chilling effect on others that reinforces his or her shyness.

The genes of low scorers in harm avoidance seem to reinforce the risk taker's preference for (a) being out on a sailboat during a great storm at sea over (b) having to stay home every night for two weeks with a sick relative. Lykken's favorite example of this type is Chuck Yeager, who seems to have had the right stuff right from the start. Thanks to nature, says Lykken, Yeager is a "great mesomorph" with a walk-on-the-wild-side temperament, terrific eyesight and coordination and a stunning ability to "habituate," or turn off unpleasant stimuli, which enables him to regard a flaming fuselage as lesser men do a flat tire. According to Lykken, these innate traits determined Yeager's adventurous boyhood, which prepared him for the life of a fighter pilot, which in turn positioned him to be America's first unofficial astronaut. It's often said that the cop and the crook are really the same guy working different angles, and Lykken points out that the same go-for-it qualities that made Yeager a hero — like the desire to take risks and a negligible fear of punishment — could have made him a felon had his nurture been malign. "This type requires a lot of socialization," he says.

Auke Tellegen, the psychologist on the twin team who devised the MPQ, seems proudest of isolating "absorption," a spacey trait that isn't measured by other personality tests. His thumbnail sketch of a high scorer sounds a bit like an LSD tripper: he or she is able to relinquish reality, experience synesthesia (the sensation that sounds have colors, colors have emotions, and so on), merge with music or poetry and think in images. As unworldly as absorption sounds, it's a practical tool of the trade for actors, artists and even advertising executives, because high scorers are able to lose themselves in creative work for long stretches. Bouchard says that top people at one ad agency excelled in an exercise that required them to imagine themselves as historical figures in order to open themselves up to new points of view.

In addition to measuring eleven basic personality traits, the MPQ measures significant clusters of these traits; Tellegen has labeled these clusters "constraint," "positive emotionality" and "negative emotionality." Had one of those ad execs who scored high in absorption imagined himself in turn as Lyndon Johnson, Hubert Humphrey, Richard Nixon and Jimmy Carter, he would have experienced very different perspectives indeed. Unlike the pragmatic, wily LBJ, who was an exceptionally effective legislator, the high-minded and moralistic Jimmy Carter, who closely monitored every area of his administration, would probably score too high in constraint — a convergence of high scores in traditionalism, harm avoidance and control. "Leaders must have the ability to take risks," says Tellegen. "You can see that Churchill had it even as a boy. Once he was chased by some other children to the edge of a high bridge, and the only way to elude them was to jump — so he jumped."

Hubert Humphrey epitomized positive emotionality, a sunny conjunction of high scores in well-being, social potency, achievement and social closeness. His old opponent Richard Nixon exemplifies negative emotionality, an unhappy confluence of high scores in aggression, stress reaction and alienation. The longtime rivals operated in the same ferocious political environment, but the Happy Warrior, who thrived on kissing babies and rubbing elbows, was liked and admired even by his adversaries, while Tricky Dick, who once shook his own wife's hand at a reception, was the butt of used-car-salesman jokes and the keeper of an enemies list. Surprisingly, Tellegen says, positive and negative emotionality are not mutually exclusive: "LBJ was a very complex mix of both."

The concept of genetically influenced positive or negative emotionality may hold important clues to the origins of mental health. It's been known for some time that an identical whose twin has a serious psychiatric disorder like manic depression or schizophrenia is at high risk of developing the illness as well (seventy-percent and fifty-percent risk, respectively). Bouchard thinks that neurotic tendencies, too, may sometimes have a genetic base, as they seemed to in the set of identical twins raised apart who were both somewhat phobic and compulsive. "Milder mental illnesses may be caused by a concatenation of several personality traits," he says. "A high scorer in alienation and stress reaction might be more inclined to suffer from mild depression than the cheery, optimistic type, like our set of gigglers — raised-apart identicals who breeze through stressful situations and steer clear of conflict. Both kinds of temperament could be endogenously determined."

Intelligence isn't generally considered to be part of personality, nor does it correlate with personality traits; someone can be very smart yet very violent, impulsive, prudish or shy. IQ is, however, approximately sixty percent genetically determined: a set of identical twins' test scores are almost interchangeable, and intellectual differences between them usually reflect educational differences, especially in those raised apart. "In the London of Dickens, identicals raised apart would differ much more in IQ than those in our study, because the environment of nineteenth-century England was far less uniform," says Lykken. "In our society, sixty to seventy percent of the variation in IQ is probably genetic, because we provide schooling for everyone. If we uniformly supplied the best possible education for all kids, then the variation would all be genetic."

The concept of genetic influences on intelligence can really raise hackles, especially when twisted into an ethnic slur, as it was by the Japanese premier last year when he remarked that America's intellectual level was limited by the purportedly lower intelligence of the nation's black and Hispanic citizens. Bouchard, an expert on intelligence, thinks that some of the hostility provoked by the idea of genetically influenced IQ is caused by the widespread tendency to think that if a trait is genetic, you can't do anything to overcome it "That's not true," he says. "A hundred years ago, only mathematical geniuses could understand calculus, but it's now taught to a substantial proportion of our population."

Nothing infuriates many men and women more than being told they're getting more like their fathers and mothers every day. To say, however, that personality traits are powerfully influenced by genes is not to validate all the Borscht Belt jokes about how a man's wife inexorably turns into his mother-in-law. In fact, the chances of getting genetically stuck with one parent's personality, much less Aunt Phoebe's, are almost nonexistent.

In order to secure the adaptability of the species, nature has given man a kind of uniqueness insurance. At what Lykken calls "the great lottery of conception," each person gets a genome — thousands of genes arrayed on his or her forty-six chromosomes, half of them contributed by the mother, half by the father — that provides a rough sketch of what he or she will be like. Half of a genome is held in common by all people, and this half differentiates them from chimps and bats or whatever: the other half varies from person to person and — except in the case of identical twins — guarantees that each person will be one of a kind. This great human genetic diversity ensures that the species will have enough biological variability to adjust to different environments.

Family members can share certain traits — as many of the Kennedys appear to share social potency — but researchers find that their overall personalities are quite distinct. The personalities of siblings, who are about fifty percent genetically similar on average by descent, are not particularly alike. This variety even within the nuclear family has several causes. One is that although a simple physiological trait like colorblindness is determined by a single gene, a personality trait like leadership is controlled by the combined action of numerous genes; because a whole personality depends on so many of these complex trait configurations, duplication is impossible.

Not all the traits determined by genes run in families. Some do — like the so-called metrical traits, ones that can be measured on a sliding scale, such as height, IQ and the traits on the MPQ, as opposed to absolute characteristics, like blue eyes. For example, identicals correlate ninety-four percent in height and eighty percent in IQ, while fraternals correlate fifty percent in both categories. Similarly, traditionalism is shared by half as many fraternals as identicals, which indicates that it runs in families. The tendency to be easily upset and woebegone also runs in families, yet the inclination to be cheerful and upbeat appears not to. And impulsiveness has no correlation in fraternals but a very high one in identicals — an example of a trait that reveals its genetic basis most clearly in studies of twins.

George Bernard Shaw showed an intuitive grasp of the complexities of genetics when he declined a famous belle's invitation to impregnate her, on the grounds that the child would probably end up with her brains and his beauty. Shaw's wit and the lady's comeliness are examples of what Lykken calls an "emergenic" trait, the result of an idiosyncratic configuration of genes that doesn't run in families. Emergenesis explains why the genius of Ben Franklin, Mozart or even Secretariat, whose 400-odd foals have been also-rans, can't be transmitted to their offspring. It's also the reason that the idea of a master race composed of beautiful, virtuous intellectuals is absurd: emergenic traits are unpredictable and can appear in any line at anytime.

The genetic roots of the coincidental behaviors that sometimes occur in families and are common in identicals are even harder to trace than those of wit and beauty. Charles Darwin used to raise his arm in his sleep and let it fall, frequently cutting his nose on the buttons of his nightshirt sleeve. His wife mentioned this quirk to her mother-in-law, who remarked that her husband did the same thing. "How do genes produce this kind of behavior?" says Lykken. "The short answer is we don't know."

Jack and Oskar's odd habit of flushing the toilet before urinating reminds Bouchard of an old army trick played on a sleeping soldier by his bunkmates: they'd slip the slumberer's hand into a jar of warm water, and he'd immediately wet his bed. Bouchard thinks that because of their genetic wiring, Jack and Oskar might require similar cues to "turn on" urination — like the sound of running water.

Harder still to demystify are the ESP-like phenomena that many identicals experience. Their descriptions of their weird affinities have a Catherine Linton-esque I-am-Heathcliff ring. Lu Klutz and her identical twin sister, Jane Vinson, were adopted separately when they were six months old. They finally met at age thirteen, because their parents feared that an accidental meeting would be traumatic "When I saw Jane, I knew that part of me that had been missing was found," says Lu. "I sometimes get these scary vibes that something's wrong, and I'll call Jane, and there's some trouble at her house. She does the same thing with me."

Eva Kor describes having the same sort of sororal telepathy with her twin, Miriam Czaigher. When the girls were nine years old, they were two of Mengele's twins, and although they lived in separate quarters, each seemed to know when the other was in special need. Mengele once ordered Eva to "the house of the living dead" to be injected with toxins for one of his experiments. If one twin died, the other would be killed immediately for simultaneous autopsy. No food was wasted on the human guinea pigs, and Eva's only sustenance was the bits of bread that Miriam stole and sneaked to her via a friend. Eva hovered between life and death for two weeks, suffering terribly: "The thought that I had to live or my sister, too, would die," she says, "was all that kept me going, kept me human."

When discussing the clairvoyant aspects of the relationship between identicals, Lykken points to photos of them that decorate his office. Their subjects look alike, but they also have the same posture, arrange their hands and feet in the same way, smile the same smile. "What makes you stand a certain way to have your picture taken?" says Lykken. "A whole bunch of things, both physiological and psychological, and identical twins share them all."

Based on its study of twins, the Minnesota team believes that genes may be the beginning of personality, but they're certainly not the end of it. "The genetic hand we're all dealt at conception makes us very rich in potential that we won't realize by the time we die," says Lykken, "because it's unlikely we'll encounter all the circumstances that will make us really reach into our genetic bag of tricks."

Timothy Leary, who's reached down into the bag deeper than most people, is fascinated by the twin data and thinks that the influence of genetics on personality has been underestimated. Before he dropped out, this low scorer in constraint was a Harvard psychology professor, the author of Interpersonal Diagnosis of Personality and a bright light in the field of personality development whose ideas on the subject are still influential in academe. In fact, one of his recent enterprises is Mind Mirror, a software program that helps users analyze ideas — and personalities. "People have great potential for change, even though most of them are wired up to be normal, dull humans who are most comfortable in the middle of the road," says Leary. "They've also been saturated with this static, Newtonian idea that personality is permanent — the way it's represented in astrology." He doubts that psychotherapy, which he describes as "hopelessly primitive," is the best vehicle for change; he endorses a more environmental — and controversial — approach: "Change your setting and the people you hang out with, or use drugs."

Trying to change personality might sometimes be advisable, but the twin-study scientists think that most people should lighten up on attempts to alter their basic natures — or their kids'. As Segal says, "You can reward the shy person for taking a risk, but you'll never make him bold."

Such insights should ease the psychic burden of parents by stressing that the nurture they provide isn't the sole determinant of whether their kid turns out to be a winner. Everyone knows about the child born with the silver spoon in his mouth who grows up to be a bum, but Bouchard says, "There are also kids in horrible situations where everything is falling apart around them who are cheerful and do well in school, perhaps because of genetically determined stress-resilient personalities." There's also a strong message inherent in all this that parents shouldn't try to treat each of their kids the same way. Tellegen lays down the ideal parental philosophy: "Your children are guests in your home who happen to have half your genes. Treat them as if they were adopted."

In another sense, the child is the father of the man, and Tellegen's advice equally pertains to the soul-searching adult who examines his persona for clues to his destiny. He says, "See who your child is, adjust to that, and then help him cultivate himself."