The Fascinating World of Twin Studies: Six Twin Experts Tell All


A few weeks ago, I attended a conference hosted by the UCLA Institute for Society and Genetics where six panelists presented their findings in the field of twin development and genetics. The evening was moderated by Abigail Pogrebin, author of One and the Same: My Life as an Identical Twin and What I’ve Learned About Everyone’s Struggle to Be Singular.

Here is a transcript and some highlights of the evening.

Twin Genetics

First up was Dr. Eric Vilain, professor of Human Genetics, Pediatrics and Urology at UCLA, and director of a center for gender based biology. In his words, his lab looks at what makes boys behave like boys, and girls behave like girls. He looks at twins from a molecular genetic point of view and studies their differences rather than their similarities, and what can we conclude from their sexual orientation or gender identity.

From a genetic perspective, Dr. Vilain finds twins to be fascinating subjects, especially when studying sexual orientation. But there’s a difference in the rate of concordance with twins who are identical (MZ) with twins who are fraternal (DZ), he says. For example, the rate of concordance for sexual orientation in identical twins is anywhere between 20 to 50 percent, meaning if one twin is gay then the odds of his cotwin also being gay is less than 50 percent. But if you compare that with fraternal twins where one twin is gay, the rate of concordance is between 0 and 5 percent. The difference in these figures, Dr. Vilain says, shows heritability; there are some genetics factors in homosexuality. But that’s not the whole story, he says, as you now have to look at why the majority of identical twins who have a gay co-twin are not gay themselves. Something else other than genetics is at play here. If being gay were completely genetic then the other twin would indeed be gay too since identical twins share the exact same DNA. Dr. Vilain believes that the answer may lie in the epigenome (the way the genes are arranged in the nucleus of a cell). He believes the epigenome is somehow modified chemically over time by the environment. Although twins have the same genome (DNA), and begin life with similar epigenome, over time their individual epigenome changes. These epigenetic modifications will either “turn on” or “turn off” certain genes (such as the sexual orientation gene). And Dr. Vilain’s hunch is that this change in sexual orientation in the epigenome probably takes place in utero.

The next speaker of the evening was Dr. Thomas Mack, a Professor of Preventive Medicine and Pathology at USC. As an epidemiologist he tries to find the factors that predict and cause illnesses, especially chronic illnesses. In his lab, he compares identical twins that have certain diseases with those who don’t with the hope of untangling the complex relationships which lead to medical problems. He calls himself a medical investigator who considers twins to be great subjects as they’re used to being compared with one another and he uses that to learn something about the problems we all have. Over the years, his lab has studied diabetes, MS, and a variety of cancers including Hodgkin’s, breast cancer, and melanoma.

Like Dr. Vilain, Dr. Mack believes something happens to epigenome that causes diseases in some identical twins but not in others. In his lab, he’s trying to detect if one twin is exposed to certain environmental factors more than the other, will it actually produce some of the epigenetic changes that Dr. Vilain referred to? Yet Dr. Mack admits that some scientists are more skeptical than others. Currently, Dr. Mack’s lab is looking at whether twins who differ in their smoking history and twins who have different diets have an effect on their epigenome. He also talked extensively on Hodgkin’s disease. A number of years ago, he showed that if one identical twin gets Hodgkin’s disease then his cotwin is 14 to 15 times more likely to get Hodgkin’s as well, making it the single most inheritable cancer, far more than breast cancer or colon cancer. Yet only a minority of identical twins who get Hodgkin’s disease both get it. There has to be a reason, he said. He’s concluded that the strongest predictor of Hodgkin’s for singletons is the number of brothers and sisters one has. He believes the reason is if you’re exposed to a lot of different viruses at a very early age brought home from school by your brothers and sisters you may be protected. Our immune system is constructed in such a way, he explained, that the exposure we get in the few years right after birth determines how effectively our bodies fight certain infections. He believes, therefore, certain immune functions must protect us from diseases such as Hodgkin’s. So he began to ask identical twins themselves their differences in everyday behaviors such as, “Which one of you sucked your thumb more?” to find out which twin was more exposed the environment. It looks as though this is also a predictor of Hodgkin’s disease.

Twin Aggression

Up next was Dr. Laura Baker, Professor of Psychology at University of Southern California and Director of USC Twin Project, a twin registry consisting mostly of school-age twins from the Los Angeles school district. She’s currently directing a large, longitudinal twin study consisting of 750 sets of twins that focuses on behavior problems with a focus on aggression and anti-social behavior. Dr. Baker wanted to start by explaining the significance of twin studies. It clearly has changed the way in which scientists think of behavior in psychology. For the past 50 years, there’s been a shift away from thinking that the environment was the sole determinant to explain individual differences. Because of twin studies, scientists now know that genetics play a huge role in many normal individual psychological traits such as personality, intelligence, and even “subjective well being,” those people who always seem naturally happier than most. But genetics also plays a huge role in psychological disorders and aggression.

In her lab, Dr. Baker studies psychopathology with a focus on aggressive behavior, and has found a strong genetic influence in deviant behavior as early as age nine (that’s the age of her youngest participants; other scientists have found that deviant behavior can begin earlier). Her studies involve asking the parents, teachers and the twins themselves to rate the twins’ aggressive behavior and then she pulls all this information together, and what she’s found that as much as 90 percent of the variance in aggressive behavior is attributed to genetics. She doesn’t dismiss the environment as a contributing factor, however, but individual differences in twin aggression can clearly be linked to genetics.

Twins Reared Apart

Next, Dr. Nancy Segal, Professor of Psychology at California State University, Fullerton, director of the university’s Twin Study Center, and author of several books on twins including Entwined Lives: Twins and What They Tell Us About Human Behavior, and Someone Else’s Twin: The True Story of Babies Switched at Birth, spoke extensively on twins reared apart. One of the most surprising findings in twin research, Dr. Segal said, is just how similar identical twins raised apart are relative to twins who’ve been raised together all their lives. It’s counterintuitive, the opposite of what we would expect but Dr. Segal stressed that the shared environment of twins is just not that important as we think it is when it comes to personality or intellectual development. She doesn’t dismiss parents and their roles in shaping their children–parents do matter especially when it comes to responding to their children’s preferences, talents, and tastes. According to Dr. Segal, parents aren’t creating their identical twins’ similarities, they’re merely responding to their twins’ similarly expressed behaviors. She also noted that many parents misclassify their twins as fraternal. The reason is that parents are so sensitive to the subtle differences between the children that they automatically think that their kids are fraternal. (She added that nearly one third of identical twins have two placentas making it an inaccurate way of determining zygosity.) What fascinates Dr. Segal the most about her research with identical twins reared apart is when they meet for the first time as they have an instant rapport with one another, something that most fraternal twins don’t express. One twin she interviewed described it as, “It’s as if my brother has been away on a vacation all our lives and he’s simply filling in the details of the trip.” She has also accumulated data on the twins’ feelings and attitudes towards their unrelated siblings (as most twins reared apart have lived in adoptive homes.) Most twins (both fraternal and identical) said that they felt closer to the twin that they just met than with the siblings that they’ve lived with all their lives.

Twin Separation

Rounding out the evening was Dr. Joan Friedman, a psychotherapist in private practice in Santa Monica, California, who specializes in twin issues, and author of Emotionally Healthy Twins: A New Philosophy for Parenting Two Unique Children. She’s also an identical twin and the mother of fraternal twin boys. She spoke of the heartache and guilt that many twins feel within their own twinship. (Author Abby Pogrebin called it the “myth of a perfect intimacy.”) Dr. Friedman related a common scenario she hears often–a 28-year-old identical twin girl has a new boyfriend. But this new relationship is causing an enormous amount of stress and conflict with her twin sister. So much so that the twin in the relationship chose to go visit her sister rather than her boyfriend’s family as she couldn’t handle her sister’s disappointment and pain. Although many identical twins embrace their bond, some don’t. Many missed their opportunity to develop other outside relationships so they’re stuck within their own twinship.

Twin Psychological Development

And finally, Dr. Eileen Pearlman, a psychotherapist in private practice in Santa Monica, California, director of TwInsight, offering psychotherapy, workshops and counseling for families with twins and twins themselves, spoke on twin separation—the stage in which twins “break” from one another and move towards separation and individuation. Dr. Pearlman began by noting that the psychological development of twins is sometimes different from singletons beginning in utero where they hear not only their mother’s heartbeat but their co-twin’s as well, but also continues during the early mother-child bonding experience. Additionally, the separation and individuation process is different and sometimes difficult for twins. Separation means: “I am separate from you. This is me. That’s you. So what’s me and what’s not me?” Individuation refers to: “What do I like? What are my dislikes? Who am I? What’s my self concept?” All of this helps us to become separate, independent, and unique individuals. The separation and individuation process begins early in life, and for some twins it takes longer than others as not only do twins need to learn to separate and individuate from their mothers but they also have to learn to separate and individuate from each other. This process is ongoing and fluid throughout their lives, too. Sometimes their twinship is close and other times they seem distant but it’s all a journey, their way of establishing their individuation. Most twins begin to part company during adolescence when one will mature faster than the other, and then again at age 18 but some twins don’t separate until they’re adults. It can be painful for some twins to separate emotionally especially if they think of themselves as a “we.”

Then moderator Abigail Pogrebin asked the panelists their opinions surrounding the research on intra-uterine behavior as some believe that the twin bond begins in utero. Psychologist Alessandra Piontelli and others, for instance, have documented twins actively interacting in the womb with one another. So Pogrebin wanted to know if any of the panelists held any belief that the intra-twin bond begins in utero.

Dr. Vilain answered: “I’m a scientist which makes me a skeptic. I would like to see some differences between twins who haven’t bumped into each other in the womb versus those who have. Can I actually measure these differences? And there might be. I’m not discrediting it on principle but can I actually measure these differences in environment? There are only a small number of examples in which you can actually measure the affect of environment on the epigenome, which is a great measure. It exemplifies how you can see how the environment can influence our genetic makeup. Maternal behavior, for instance, is a great example where you can actually measure that. It’s been shown in animal models.”

Dr. Segal answered: “I’m reluctant to accept that the idea of intra-uterine interaction between the twins is going to be meaningful linked to what comes out later. I know Piontelli’s films and I’ve seen some of them, and they’re beautiful. They’re fantastic to watch these twins interacting; sometimes they suck each other’s thumbs. It’s quite fascinating actually but what those behaviors mean for the twins’ ultimate relationship with one another is a great big mystery. And how we’d tease that out is an even more difficult thing to imagine. We also need to think about the reared-apart twins who had the same interaction intra-uterinally as do the twins raised together but yet they were raised apart from one another and yet they never had any sense of being a twin. When we bring them together they often had no idea that they were born with someone else.”

Dr. Segal was also quick to warn that with all this talk of twins’ difficulties in sometimes individuating, they were still no more likely to be represented in psychological clinics and psychiatric institutions than any other group. The audience laughed but Dr. Segal stressed that it was an important point to make as sometimes society sees twins as a bit pathological. Although they may face unique developmental issues and parents have unique issues in trying to raise them but it doesn’t mean that twins will have any more difficulty in life than any other individual. It’s just a different experience.

The evening ended with questions from the audience.

Q: “I have 15-year-old identical twin girls and I’m wondering at this point if I were the mother of those 28-year-old twins, what would I have liked to have known now.”

Dr. Friedman answered: “If I had you in my office, I would ask you if they had any separate experiences. Have they ever been in different classes? Have they ever been away at different summer camps? Have they had different play dates? Do they have separate groups of friends? How do you see them now at adolescence? That adolescence developmental time is when you really see children looking, searching, and needing an identity. A 15-year-old girl is going to want to differentiate from her sister and be known for whom she is, not just noticed because they’re twins.”

Dr. Friedman went on to say that parents need to provide opportunities for their twins to experience life on their own, sans their cotwins. Parents often lack knowledge or experience in knowing that their twins need some time on their own individually, and parents shouldn’t be concerned or upset by it. If one twin wants to go off on her own without her co-twin, Dr. Friedman says this is a good thing as it’s the beginning of this child’s quest for autonomy.

Q: “I have six-month-old fraternal twins and a four-year-old singleton. Are there some things that I should be aware when raising both singletons and fraternal twins? What do I need to be conscientious of?”

Dr. Pearlman answered: “I can answer that in a very unique way. My sister and I were born on my brother’s fourth birthday. (The audience exploded into laughter.) What happened was that my sister and I were raised together so my brother was always left out. It wasn’t until I was married and that my sister was home alone with my brother for he wasn’t married that the two of them got a chance to be together and form a relationship.”

Dr. Pearlman recommended that this mom encourage each of her twins as they get older to each spend some quality alone with their older, singleton sister.

Q: “I have 16-year-old triplets and two of them are identical and one is a fraternal. We’ve tried everything but the identical girls just hate each other. We’ve seen therapists. We had to separate them in summer camp. I’m just baffled as we’ve done everything. We tried to separate them as much as we could since they were little. They were in separate middle schools. They’re both close to the fraternal boy but they just hate each other. It’s mostly a verbal abusiveness. They’re extremely competitive, every minute of every day. They basically wish that the other one didn’t exist. (The audience openly sighed.) They’re also really great kids!” (The audience erupted in laughter.)

Dr. Friedman answered: “I’m sure that they are. And you’re a really great mom to take all this in and talk about it. My gut reaction is that if there weren’t a third, none of this wouldn’t have happen. Not that that is any answer at all. But that’s really what it is—that tension in that triad.”

Dr. Segal added: “I will say that your situation is unique because I’ve studied many sets of triplets composed of an identical pair and one co-triplet that’s fraternal, and in most cases it’s the identical set who get along and the fraternal who doesn’t.”

Dr. Baker said: “I want to just come back to the point of individual differences, and what I’m hearing here that no pair of twins is like another pair of twins. Just as individual singletons are different from one another, twin pairs differ from one another too. Some can be close, some can be less close. Some can hate each other. There’s a wide range. I want to also suggest, and there’s evidence of this, that some of the individual differences and how twins relate to one another may stem in part from their genetic make up. Our genes do influence our personalities—how warm and close we are with other people. To some extent twins’ genes determine how they relate to one another. There’s not a one-size-fits all here. Twins as pairs differ from one another.”

Dr. Pearlman closed with: “You say your children are 16-years-old and that’s the adolescent time when they’re each trying to go through their separation and individuation from each other. So this is a very crucial time in their lives. And they’re going to be at a stage where they’ll probably want to be who they are, and not like their co-triplets. So they’re going to be in conflict with each other. This is a stage where there will be conflict. And it’s not always going to be that way. So it may help to say, ‘Ok, this is the way they are now but it may not be that way later on.’”

Q: “My question is about aggression. I have four-and-a-half-year-old boy-girl twins. They manifest aggression very differently. You talked about how it’s genetically indicated, and I was wondering if you could elaborate on that a bit.”

Dr. Baker said: “There is one idea that’s been put out there that there’s this girl form of aggression and a boy form of aggression. Boys tend to be more physical and girls tend to gossip and tell stories about each other, and that they try to manipulate each other through that form of aggression. But in my research I find that that’s not necessarily the case. Boys gossip and push equally, and girls do the same. And both of those forms of aggression have some genetic influence. There are some studies that show both forms of aggression have a genetic basis as young as age four. Having said that, aggression is a normal thing at that age. As kids get into school and become more socialized, they begin to suppress their aggressive tendencies towards one another. To some extent, it’s within the normal range.”

Q: “I have 11-month-old fraternal twin daughters. In my twins club, out of ten moms I think I’m the only one who had my twins naturally. I’m curious now that the technology has been around for so long, is there any research that suggests that children conceived through IVF differ genetically from children conceived naturally.”

Dr. Vilain answered: “There’s a lot of research on that. People are always worried that there could be changes. People are not afraid of changes on the genetic level but they worry about the epigenetic level that we’ve been talking about. There seems to be a little bit of a measurable higher risk for epigenetic changes but it’s small. It’s quite difficult to measure. It’s certainly not a large effect. And there are so many other confounding factors like environment. It’s a worry but not a huge worry.”

A copy of the book Double Duty.