Is the gene king? Studies of twins find that genes shape multiple areas including personality.
By Roderick Angle and Jill Neimark published July 1, 1997 - last reviewed on June 9, 2016
Can genes explain our passions and prejudices, the mates we choose, that mystery we call the self? New research on twins upsets some of our most cherished notions about how we become who we are -- and gives nature and nurture a whole new meaning.
In April of 1997 I went down to West 27th Street in Manhattan to sit in the audience of the Maury Povich show, and meet four sets of identical twins who had been separated at birth and adopted into different families. I wanted to see if the same soul stared out of those matched pairs of eyes, to contemplate the near miracle of DNA -- double helix twisting around itself like twin umbilical-cords ticking out a perfect code for two copies of a human. One pair, a Polish nun and a Michigan housewife, had been filmed at the airport by CNN the week before, reunited for the first time in 51 years and weeping in each other's arms, marveling at their instinctive rapport. Yet how alike were they really, if one spent her days on rescue missions to places like Rwanda, while the other cleaned houses to supplement her husband's income?
Twins are nature's handmade clones, doppelgangers moving in synchrony through circumstances that are often eerily similar, as if they were unwitting dancers choreographed by genes or fate or God, thinking each other's thoughts, wearing each other's clothes, exhibiting the same quirks and odd habits. They leave us to wonder about our own uniqueness and loneliness, and whether it's possible to inhabit another person's being. Twins provoke questions about the moment our passions first ignite -- for they have been seen on sonogram in the womb, kissing, punching, stroking each other. They are living fault lines in the ever shifting geography of the nature/nurture debate, and their peculiar puzzle ultimately impacts politics, crime and its punishment, education, and social policy. It isn't such a short leap from studies of behavioral genetics to books like the infamous The Bell Curve (by Richard Herrnstein and Charles Murray) and a kind of sotto-voce eugenics. And so everything from homosexuality to IQ, religious affiliation, alcoholism, temperament, mania, depression, height, weight, mortality, and schizophrenia has been studied in identical and fraternal twins and their relatives.
Yet the answers -- which these days seem to confirm biology's power -- raise unsettling questions. Twin research is flawed, provocative, and fascinating, and it topples some of our most cherished notions -- the legacies of Freud and Skinner included -- such as our beliefs that parenting style makes an irrevocable difference, that we can mold our children, that we are free agents piecing together our destinies.
Today, we've gone twin-mad. Ninety thousand people gather yearly at the International Twins Day Festival in Twinsburg, Ohio. We're facing a near epidemic of twins. One in 50 babies born this year will have a fraternal or identical double; the number of such births rose 33 percent in 1994 alone, peaking at over 97,000 -- largely due to women delaying childbirth (which skews the odds in favor of twins) and to the fertility industry, which relies on drugs that superovulate would-be mothers. Recently, a stunning scientific feat enabled an ordinary sheep to give up a few cells and produce a delayed identical twin a -- clone named Dolly, who was born with her donor's 6-year-old nucleus in every cell of her body. The international furor this Scottish lamb engendered has at its heart some of the same wonder and fear that every twin birth evokes. Twins are a break, a rift in the customary order, and they call into question our own sense of self. Just how special and unique are we?
The history of twins is rich with stories that seem to reveal them as two halves of the same self -- twins adopted into different families falling down stairs at the same age, marrying and miscarrying in the same year, identical twins inventing secret languages, "telepathic" twins seemingly connected across thousands of miles, "evil" twins committing arson or murder together, conjoined twins sharing a single body, so that when one coughs the other reflexively raises a hand to cover the first one's mouth. And yet the lives of twins are full of just as many instances of discordance, differences, disaffection. Consider the 22-year-old Korean twins, Sunny and Jeen Young Han of San Diego County; Jeen hired two teenagers to murder her sister, hoping to assume her identity.
So what is truly other, what is self? As the living embodiment of that question, twins are not just the mirrors of each other, they are a mirror for us all.
Separated at Birth But Joined at the Hip
The woman seated alone onstage at the opening of the Maury Povich show was already famous in the twin literature: Barbara Herbert, a plump 58-year-old with a broad, pretty face and short, silver hair, found her lost twin, Daphne Goodship, 18 years ago. Both had been adopted as babies into separate British families after their Finnish single mother killed herself.
The concordances in their lives send a shiver up the spine: both women grew up in towns outside of London, left school at 14, fell down stairs at 15 and weakened their ankles, went to work in local government, met their future husbands at age 16 at the Town Hall dance, miscarried in the same month, then gave birth to two boys and a girl. Both tinted their hair auburn when young, were squeamish about blood and heights, and drank their coffee cold. When they met, both were wearing cream-colored dresses and brown velvet jackets. Both had the same crooked little fingers, a habit of pushing up their nose with the palm of their hand -- which both had nicknamed "squidging" -- and a way of bursting into laughter that soon had people referring to them as the Giggle Twins. The two have been studied for years now at the University of Minnesota's Center for Twin and Adoption Research, founded by Thomas J. Bouchard, Ph.D. It is the largest, ongoing study of separated twins in the world, with nearly 100 pairs registered, and they are poked, probed, and prodded by psychologists, psychiatrists, cardiologists, dentists, ophthalmologists, pathologists, and geneticists, testing everything from blood pressure to dental caries.
At the center, it was discovered that the two women had the same heart murmurs, thyroid problems, and allergies, as well as IQ's a point apart. The two showed remarkably similar personalities on psychological tests. So do the other sets of twins in the study -- in fact, the genetic influence is pervasive across most domains tested. Another set of twins had been reunited in a hotel room when they were young adults, and as they unpacked found that they used the same brand of shaving lotion (Canoe), hair tonic (Vitalis), and toothpaste (Vademecum). They both smoked Lucky Strikes, and after they met they returned to their separate cities and mailed each other identical birthday presents. Other pairs have discovered they like to read magazines from back to front, store rubber hands on their wrists, or enter the ocean backwards and only up to their knees. Candid photos of every pair of twins in the study show virtually all the identicals posed the same way; while fraternal twins positioned hands and arms differently.
Bouchard a -- big, balding, dynamic Midwesterner who can't help but convey his irrepressible passion about this research -- recalls the time he reunited a pair of twins in their mid-30s at the Minneapolis airport. "I was following them down the ramp to baggage claim and they started talking to each other. One would stop and a nanosecond later the other would start, and when she stopped a nanosecond later the other would start. They never once interrupted each other. I said to myself, `This is incredible, I can't carry on a conversation like that with my wife and we've been married for 36 years. No psychologist would believe this is happening.' When we finally got to baggage claim they turned around and said, `It's like we've known each other all our lives."'
Just Puppets Dancing To Music of the Genes?
I asked Bouchard if the results of his research puncture our myth that we consciously shape who we are.
"You're not a believer in free will, are you?" he laughed, a little too heartily. "What's free will, some magical process in the brain?"
Yet I am a believer (a mystical bent and fierce independence actually run in my family, as if my genes have remote controlled a beguiling but misbegotten sense of freedom and transcendence). I was mesmerized and disturbed by the specificity of the twins' concordances. David Teplica, M.D., a Chicago plastic surgeon who for the last 10 years has been photographing more than 100 pairs of twins, has found the same number of crow's feet at the comers of twins' eyes, the same skin cancer developing behind twins' ears in the same year. Says Teplica, "It's almost beyond comprehension that one egg and one sperm could predict that."
I could imagine, I told Bouchard, that since genes regulate hormones and neurochemicals, and thus impact sexual attraction and behavior, DNA might influence the shaving lotion twins liked or the hue they tinted their hair. But the same esoteric brand of toothpaste? Walking into the sea backwards? This implies an influence so far-reaching it's unnerving.
"Nobody has the vaguest idea how that happens," he admitted, unfazed. "We're studying a set of triplets now, two identical females and a brother, and all three have Tourette's syndrome. How can the genes get so specific? I was talking yesterday in Houston to a bunch of neuroscientists and I said, `This is the kind of thing you guys have to figure out.' There is tons of stuff to work on here, it's all open territory."
He paused to marvel over the tremendous shift in our understanding of human behavior. "When we began studying twins at the university in 1979, there was great debate on the power of genetics. I remember arguing in one graduate school class that the major psychoses were largely genetic in origin. Everyone in the classroom just clobbered me. It was the era of the domination of behaviorism, and although there's nothing wrong with Skinner's work, it had been generalized to explain everything under the sun. Nothing explains everything. Even genetics influences us, on the average, about 50 percent."
Yet that 50 percent seems omnipresent. It impacts everything from extroversion to IQ to religious and social attitudes -- and drops only in the influence on homosexuality and death. Though some researchers have criticized Minnesota's twin sample for being too small and perhaps self-selected (how many separated twins out there don't participate or don't even know they're twins?), it generally confirms the results of larger studies of twins reared together -- studies that have taken place around the world.
Twin studies allow us to double blind our nature/nurture research in a unique way. Identical twins share 100 percent of their genes, while fraternals share 50 percent. But usually they grow up together, sharing a similar environment in the womb and the world. When separated, they give us a clue about the strength of genetic influence in the face of sometimes radically different environments. Soon Bouchard and his colleagues will study siblings in families that have adopted a twin, thus testing environmental influences when no genes are shared. Like a prism yielding different bands of light, twin studies are rich and multifaceted. Here are some of the major findings on nature and nurture thus far:
* Political and social attitudes, ranging from divorce to the death penalty, were found to have a strong genetic influence in one Australian study. A Swedish study found genes significantly influenced two of the so-called "big five" personality traits -- "openness to experience" and "conscientiousness" -- while environment had little impact. In contrast, environment influenced "agreeableness" more than genes did. (The two other traits are "neuroticism" and "extroversion.") Another study, at the University of Texas at Austin, found that personality in identicals correlated 50 percent, in fraternals about 25 percent.
* Body fat is under genetic influence. Identical twins reared together will have the same amount of body fat 75 percent of the time; for those reared apart it's 61 percent, showing a heavy genetic and mild environmental influence, according to a 1991 study.
* Both optimism and pessimism are heavily influenced by genes, but shared environment influences only optimism, not pessimism, according to a study of 522 pairs of middle-aged identical and fraternal twins. Thus family life and genes can be equal contributors to an optimistic outlook, which influences both mental and physical health. But pessimism seems largely controlled by genes.
* Religiosity is influenced by genes. Identical and fraternal twins, raised together and apart, demonstrate that 50 percent of religiosity (demonstrated by religious conviction and church attendance) can be attributed to genes.
* Sexual orientation is under genetic influence, though not solely, according to studies by Michael Bailey, Ph.D., associate professor of psychology at Northwestern University. In one study he found that if one identical twin is gay, the other is also gay 5O percent of the time. However, when Bailey analyzed a sample of 5,000 twins from the Australian twin registry, the genetic impact was less. In identical male twins, if one was gay the likelihood of his twin being gay was 20 percent; in fraternal twins the likelihood was almost zero. In women, there was little evidence of heritability for homosexuality.
* When substance abuse was studied in 295 identical and fraternal twin pairs, year of birth was the most powerful predictor of drug use. Younger twins were most likely to have abused drugs, reflecting widespread drug use in the culture at large. Alcoholism, however, has a significant genetic component, according to Andrew Heath, Ph.D., at the Virginia Institute for Psychiatric and Behavioral Genetics at Virginia Commonwealth University School of Medicine.
* Attention deficit disorder may be influenced by genes 70 percent of the time, according to Lindon Eaves, M.D., director of the Virginia Institute for Psychiatric and Behavioral Genetics. Eaves and colleagues studied 1,400 families of twins and found genetic influence on "all the juvenile behavior disorders," usually in the range of 30 to 50 percent.
* Twins tend to start dating, to marry, and to start having children at about the same time. David Lykken, Ph.D., and Matthew McGue, Ph.D., at the University of Minnesota, found that if an identical twin had divorced, there was a 45 percent chance the other had also. For fraternals, the chance was 30 percent. The researchers think this is due to inherited personality traits.
* Schizophrenia occurs more often in identical twins, and if one twin suffers from the disorder, the children of the healthy identical sibling are also at greater risk, according to psychiatrist Irving Gottesman, M.D., of the University of Virginia. The risk is about twice as high for the children of a twin whose identical counterpart is ill, as it is for the children of a twin whose fraternal counterpart is ill.
HIDDEN DIFFERENCES BETWEEN TWINS
A few fascinating kinks in the biology of twin research have recently turned up, weaving an even more complex pattern for us to study and learn from. It turns out that not all identical twins are truly identical, or share all their genetic traits. In one tragic instance, one twin was healthy and a gymnast, while the other suffered from severe muscular dystrophy, a genetic disorder, and was dead by age 16. Yet the twins were identical.
One way twins can differ is in the sex chromosomes that turn them into a male or female, and which contain other genes as well, such as those that code for muscular dystrophy or color blindness. All girls inherit two X chromosomes, one from each parent, while boys inherit an X and a Y. Girls automatically shut off one X in every cell -- sometimes some of the mother's and some of the father's, in other cases all the mother's or all the father's. A girl may not shut off her extra set of X chromosomes in the same pattern as her identical twin does.
Identical twins may not be exposed to the same world in the womb, either. It depends on the time their mother's fertilized egg splits--and that timing may explain why some identical twins seem more eerily alike than others. At Lutheran University, researchers have looked at the placentas of some 10,000 twin births. They've found that an egg that separates in the first four days of pregnancy develops not only into separate twins, but results in separate placentas, chorionic casings, and amniotic sacs. These twins are like two singletons in the womb and have the best chance of survival. Twins who separate between the fifth and eighth days share a single placenta and chorion, but still have the benefit of two amniotic sacs. Here, one twin can have a distinct advantage over the other. The umbilical cord may be positioned centrally on one sac, while the other is on the margin, receiving fewer nutrients. Studies of these twins show that with a nurturing environment, the weaker twin will catch up in the first few years of life. However, it's possible that viruses may penetrate separate sacs at different rates or in different ways -- perhaps increasing the risk for schizophrenia or other illnesses later in life.
Twins who split between the eighth and 12th days share their amniotic sac, and often their cords get entangled. One cord may be squeezed until no blood flows through it, and that twin dies. Finally, twins who split after the 12th day become conjoined -- and even though they share organs and limbs, anecdotal evidence suggests that they often have distinctly different temperaments, habits, and food cravings.
In one hotly debated hypothesis, pediatrician and geneticist Judith Hall, of the University of British Columbia in Vancouver, speculates that twinning occurs because of genetic differences within in an embryo. Perhaps mutations occur at a very early stage in some cells, which then are sensed as different, and expelled from the embryo. Those cells may survive and grow into a twin. Hall suggests this could account for the higher incidence of birth defects among twins.
While identical twins can be more distinct than we imagine, fraternal twins might come from the same egg, according to behavioral geneticist Charles Boklage, M.D., of the East Carolina University School of Medicine. Boklage proposes that occasionally an older egg may actually split before it is fertilized by two of the father's sperm. With advances in gene mapping and blood testing, he says, we may find that one-egg fraternal twins occur as often as do two-egg fraternals. We may be mistaking some same sex fraternal twins for identical twins.
TWINS WHO VANISH, TWINS WHO MERGE
Whatever the cause of twinning, once it begins, mysterious and unsettling events can occur. Some twins disappear or even merge together into one person. Ultrasound equipment has revealed twin pregnancies that later turn into singletons. One of the twins is absorbed into the body, absorbed by the other twin, or shed and noticed by the mother only as some extra vaginal bleeding.
"Only one in 80 twin conceptions makes it to term as two living people," notes Boklage. "For every one that results in a twin birth, about 12 make it to term as a sole survivor. And those people never know they were twins." Because twins tend to be left-handed more often than singletons, Boklage speculates that many left-handers could be the survivors of a twin pregnancy. And a few of those twin pregnancies may lead to what Boklage terms a "chimera," based on the Greek monster with the tail of a serpent, body of a goat, and head of lion -- a mosaic of separate beings. "We find people entirely by accident who have two different blood types or several different versions of a single gene. Those people look perfectly normal, but I believe they come from two different cell lines."
It's as if fantastical, primitive acts of love, death, merging, and emerging occur from the very moment life ignites, even as the first strands of DNA knit themselves into the human beings we will later become -- carrying on those same acts in the world at large, acts that define us, and that we still are not certain we can call our own.
WHEN TWINS DIE, KILL, HATE, AND BURN
Though it doesn't happen often, occasionally in history a set of mythic twins seem to burst into our awareness, more wedded and bonded than any couple, even darkly so. Some twins live with a passion the rest of us experience only in the almost unbearably intense first flush of romantic love. England's Gibbons twins are one such pair.
Jennifer and June Gibbons were born 35 years ago, the youngest children of Aubrey Gibbons, a West Indian technician for the British Royal Air Force. The girls communicated with each other in a self-made dialect and were elective mutes with the rest of the world. By the time they were 11, they refused to sit in the same room with their parents or siblings. Their mother delivered their meals on a tray and slipped mail under the door. They taught themselves to read, and eventually locked themselves in their bedroom, writing literally millions of words in diaries.
Later they lost their virginity to the same boy within a week of each other, triggering jealous rage. Jennifer tried to strangle June with a cord, and June tried to crown Jennifer in a river. When publishers rejected their work, they went on a spree of arson and theft, and were committed to Broadmoor, England's most notorious institution for the criminally insane.
"Nobody suffers the way I do," June wrote in her diary. "This sister of mine, a dark shadow robbing me of sunlight, is my one and only torment." In another passage, Jennifer described June lying in the bunk bed above her: "Her perception was sharper than steel, it sliced through to my own perception...I read her mind, I knew all about her mood...My perception. Her perception...clashing, knowing, cunning, sly."
After more than a decade of confinement, they were set free. That same afternoon, Jennifer was rushed to the hospital with viral myocarditis, an inflammation of the heart, and that night she died. The pathologist who saw her heart seemed to be speaking poetically of their lethal passion when he described Jennifer's illness as "a fulminating, roaring inflammation with the heart muscle completely destroyed." June, the survivor, has said that she was "born in captivity, trapped in twinship." Eventually, June claims, they began to accept that one must die so the other could be free. Today, June lives in Wales.
Another set of twins, 22-year-old Jeen Young Han (nicknamed Gina) and her sister Sunny, have been dubbed the "evil" and "good" twins by the media, after one tried to murder the other. Although the twins were both valedictorians at their small country high school in San Diego County and got along well, after they graduated they began to battle one another. Both sisters were involved in petty crime, but when Gina stole Sunny's BMW and credit cards, Sunny had her jailed. She escaped, but in November 1996 Sunny and her roommate were attacked and Gina was arrested for conspiracy to commit murder. She'd planned to have Sunny killed at her Irvine condominium, and then assume her identity.
For twin researcher and obstetrician Louis Keith, M.D., of Northwestern University Medical School, the idea of killing a twin is practically unthinkable. "I'm an identical twin, and yesterday I attended the funeral of another identical twin. I kept trying to imagine what my life would be like without my twin. My brother and I have had telepathic experiences. I was in East Germany, being driven on a secluded highway with evening snow falling, and suddenly felt intense heat over the entire front of my body and knew it could only mean one thing, that my brother was sending intense signals to me to call him. When one of the Communist telephone operators agreed to put the call through, I found out that my aunt had died and my twin wanted me to come to the funeral. The twin bond is greater than the spousal bond, absolutely."
Raymond Brandt, publisher of Twins World magazine, agrees. "I'm 67, and my identical twin died when we were 20. I love my wife and sons in a very special way, but my twin was one half of me, he was my first love. Living without my twin for 47 years has been a hell of an existence."
These remarkable stories seem to indicate an extra dimension to the twin bond, as if they truly shared a common, noncorporeal soul. What little study has been done on paranormal phenomena and twins, however, indicates that -- once again -- genes may be responsible. A study by British para-psychologist Susan Blackmore found that when twins were separated in different rooms and asked to draw whatever came into their minds, they often drew the same things. When one was asked to draw an object and transmit that to the other twin, who then was asked to draw what she telepathically received, the results were disappointing. Blackmore concluded that when twins seem to be clairvoyant, it's simply because their thought patterns are so similar.
IS THERE NO NURTURE?
Over a century ago, in 1875, British anthropologist Francis Galton first compared a small group of identical and fraternal twins and concluded that "nature prevails enormously over nurture." Time and research seem to have proved him right. "It's no accident that we are what we are," contends Nancy Segal, Ph.D., professor of developmental psychology at California State University at Fullerton and director of the Twin Studies Center there. "We are born with biological propensities that steer us in one direction or another."
Yet critics of twin studies scoff. Richard Rose, Ph.D., professor of psychology and medical genetics at Indiana University in Bloomington, has studied personality in more than 7,000 pairs of identical twins and concluded that environment, both shared and unshared, has nearly twice the influence of genes.
However, both the nature and nurture camps may be looking at the same data and interpreting it differently. According to Lindon Eaves, unshared environment may actually be "chosen", by the genes, selected because of biological preferences. Scientists dub this the "nature of nurture." Genetically influenced personality traits in a child may cause parents to respond in specific ways. So how can we ever tease out the truth? Nature and nurture interact in a never-ending Mobius strip that can't be traced hack to a single starting point.
Yet if genes are a powerful and apriori given, they nonetheless have a range of activity that is calibrated in the womb by nutrition and later in life by the world. "Remember," says Eaves, "only 50 percent of who you are is influenced by genes. The other 50 percent includes the slings and arrows of outrageous fortune, accidents of development, sheer chaos, small and cumulative changes both within and without."
Environment, it turns out, may be most powerful when it limits -- through trauma, deprivation, malnutrition. Studies by Sandra Scarr, Ph.D., professor of psychology at the University of Virginia, show that IQ scores for white twins at the bottom of the socioeconomic ladder, and for all black twins, are heavily influenced by environment. Social and economic deprivation keep scores artificially lower than twins' genetic potential.
Otherwise, Scarr postulates, genes bias you in a certain direction, causing you to select what you are already genetically programmed to enjoy. Children may be tiny gene powerhouses, shaping their parents' behavior as much as parents shape their children.
BEYOND NATURE AND NURTURE: TWINS AND QUANTUM PHYSICS
I've been interested in identical twins ever since I was old enough to realize I am one. When my brother and I were young we were close but nonetheless epitomized the struggle of twins to achieve individual identities. Now in our 50s' we have both noticed a real convergence of our intellectual, spiritual and philosophical views.
Are the strikingly similar thoughts and behaviors of twins, even those reared apart, due to nature or nurture--or to a third factor? What if what I call the "nonlocal" nature of the mind is involved?
Nonlocal mind is a term I introduced in 1989 to account for some of the ways consciousness manifests, ways suggesting that it is not completely confined or localized to specific points in space or time. Nobel physicist Erwin Schrodinger believed that mind by its very nature is singular and one, that consciousness is not confined to separate, individual brains, that it is ultimately a unified field. David Chalmers, a mathematician and cognitive scientist from the University of California at Santa Cruz, has suggested that consciousness is fundamental in the universe, perhaps on a par with matter and energy, and that it is not derived from, nor reducible to, anything else. Nobel physicist Brian Josephson, of Cambridge University's Cavendish Laboratory, has proposed that nonlocal events at the subatomic level--for example, the fact that there are correlations between the spin of subatomic particles, even after they are separated--can be amplified and may emerge in our everyday experience.
In other words, the macrocosm reflects the microcosm. Systems theorist Erwin Laszlo has suggested that nonlocal mind may mediate events such as intercessory prayer, telepathy, precognition, and clairvoyance.
If consciousness is unbounded and unitary, strikingly similar thoughts and behaviors of identical twins' even separated twins, would not be surprising. Genes do determine how individual brains function, how we each process information, and nonlocal mind could be easier to access if two brains were almost identical in their functioning. Indeed, some people see analogies between the behavior of separated, identical twins and separated' identical subatomic particles.
According to the late Irish physicist John S. Bell, if two subatomic particles once in contact are separated to some arbitrary distance, a change in one is correlated with a change in the other-instantly and to the same degree. There is no travel time For any known form of energy to flow between them. Yet experiments have shown these changes do occur, instantaneously. Neither can these nonlocal effects be blocked or shielded--one of the hallmarks of nonlocality. Perhaps distant twins are mysteriously linked, like distant particles--or, to quote Ecclesiastes, "All things go in pairs, one the counterpart of the other."
Larry Dossey, M.D.