Genetic and environmental influences work together to shape our behavioral and physical traits, and in the case of intelligence we can say that about 50 to 75 percent of the variation is associated with genetics and 25 to 50 percent is associated with the environment. It is impossible to divide the intellectual makeup of a single person into genetic and environmental components because they are completely intertwined within an individual.12
I have been pleased with my students’ proposals, but my favorite is a study of sets of same-sex triplets comprising an identical twin pair and a fraternal cotriplet. That trio poses a great natural experiment because all three triplets share the same environment, but only two share all their genes. If the identical twins are more temperamentally alike and socially closer to each other than to the fraternal member of the pack, this tells us that genetic factors influence our personality traits and friendship choices. Research using twins, who are far more plentiful in the population than triplets, shows exactly that.13
This triplet experiment can be extended even further. Let’s say that one identical twin was raised by the biological family, while the other identical twin and the fraternal cotriplet were raised together in an adoptive home. If researchers found that the separated identical twins were more alike than the reared-together nonidentical pair, this would show even more convincingly the genetic influence on the traits being measured. Researchers would need to study many of these rare sets to find definitive answers to nature-nurture questions, but even one case or a few can provide important insights that can be further explored in large-scale studies. When it comes to triplets’ social activities and relationships, identical partners generally align more closely with each other than they do with their fraternal ones. For example, my colleague’s triplet children, Mike and Mark love to indulge in overeating, whereas Matt is proud to not be a member of that culinary club.
None of my students’ ideas, or my earlier ones for that matter, came close to the exceptional research possibilities offered by the switched Colombian brothers. I began thinking of the four twins as Transformers, the popular toy that assumes different forms and features by manipulating its parts. The potential experimental contrasts the Colombian twins generated also reminded me of matryoshka dolls, the painted Russian figurines that fit inside one another in increasingly smaller sizes. I grabbed some paper from my printer and started mapping out the myriad relationships and comparisons that were possible.
Reared-Apart Twins, Virtual Twins—and the New Replicas
In mathematics a combination is a selection of a specific number of items (e.g., two people) from a larger number of items (e.g., four people) without attention to who comes first or second with respect to order. Higher-order multiple births—triplets, quadruplets, and more—are fun to work with in this regard. When thirty-three-year-old Nadya “Octomom” Suleman of southern California gave birth to octuplets in January 2009, I wrote a paper showing that her eight babies yielded twenty-eight unique twin pairs when they were organized into groups of two.14 The situation is even more amazing when we consider that Suleman had had six fertilized eggs implanted in her uterus and two of those eggs divided, producing two identical twin sets in the bunch.
I performed similar calculations for the four Colombian brothers and came up with three results, two that were not surprising and one that was completely unexpected and exciting. By pairing the brothers every way I could, I ended up with six distinct sets:
• Two identical reared-apart twin pairs, Jorge and William, and Carlos and Wilber. These pairs share their genes but did not share their environment.
• Two unrelated, or virtual twin, pairs: Jorge and Carlos, and William and Wilber. These pairs shared their environment but do not share their genes.
And for the first time in any study:
• Two replicas: Jorge and Wilber, and Carlos and William. These pairs share neither their genes nor their environment, but they replicate the unrelated, or virtual twin, pairs, who share only their environment.
* * *
Reared-apart twins are easy to define—they are twins separated at or soon after birth and raised by two different families. They are genetically identical, actually clones by definition, but because they do not share an environment, their behavioral and physical similarities are the result of their common genes. All of us can use this information to understand why we are the way we are and how we got that way. In addition, reared-apart identical twins give us direct estimates of how much genes matter, making these twins a cleaner, neater twin research design than twins reared together, whose shared experiences could enhance their similarities. No wonder reared-apart identical twin pairs have high status in the hierarchy of twin research methods.
However, despite some critics’ claims that most identical twins are alike because they are raised together and are treated alike, it would be incorrect to assume that they are alike because they were raised together. It would also be incorrect to suppose that increased contact between reunited reared-apart twins makes them alike. This has been shown repeatedly, but was revealed most dramatically by the first-ever personality study of four groups of twins that my Minnesota colleagues and I conducted in 1988. This project included identical twins reared apart, fraternal twins reared apart, identical twins reared together, and fraternal twins reared together. As expected, we found that the identical pairs were more alike than the fraternal pairs. The surprise was finding that identical reared-apart twins are as alike in personality as identical reared-together twins.15
This counterintuitive finding means that personality similarity between family members in traits like traditionalism and aggression come from their shared genes, not their shared environment. There is no other way to explain why separated twins like Lily and Gillian, who were scared of Santa Claus and clowns, dressed up as ballerinas at Halloween, why Samantha and Anaïs chew on their hair when they get nervous, why Bridget and Dorothy wear more rings and bracelets than anyone they know, or why Todd and Josh both identified as female and independently underwent surgical transformation to have their biological sex and gender identity align.16
Todd and Josh are the only known pair of separated identical twins who have experienced gender dysphoria (the persistent discontent with one’s birth or assigned gender and identification with the opposite gender) and have undergone surgery to change their sex. After their birth their forty-year-old mother was diagnosed with facial cancer so, following advice from various hospital staffers who believed that caring for more than one newborn would be too taxing, she placed one twin up for adoption. By the time the twins reunited at age fifteen, both felt uncomfortable living as males, had engaged in cross-dressing, and were apprehensive about developing secondary sex characteristics in adolescence. These twins make a powerful statement about genetic influences on transgenderism (the transient or continued identification with the gender that is different from one’s birth gender) and transsexuality (the social or surgical transition from male to female or female to male). Genetic effects on gender dysphoria are also shown by the finding that one-third of transsexual identical male twins have a transsexual twin brother, whereas no transsexual fraternal male twins have a twin who is also transsexual.17 Of course, environmental influences before and after birth also play a role because identical twins’ similarity is less than 100 percent.
It seems reasonable to attribute parent-child or brother-sister similarities to shared experience, but researchers cannot disentangle genes and environment when biological relatives live together. The only way to do this is to compare resemblance in kinships that are genetically and environmentally revealing, and the best pairs are twins raised together and apart. Upon doing this we find that environment matters nearly as much as genes when it comes to personality, but it is mostly the individual experiences that we do not share with our relatives, such as being mentored by an inspiring professor, taking an exotic vacation, losing assets in a Ponzi scheme, or being attacked by an unknown assailant, that
influence our personality traits. However, nonshared experiences work in unpredictable ways—after being attacked, one person might be energized to improve public safety, whereas another might be too fearful to leave home.
By now scores of twin studies have shown that genes affect just about every human trait that anyone has ever measured. A curious exception is love styles—it seems that how quickly or slowly we fall in love with someone is mostly experiential in origin.18 However, the only twin study to look specifically at love styles was conducted nearly twenty-five years ago, and recent findings about the brains and personalities of people in love could make for great twin research. The anthropologist Helen Fisher, who is an identical twin, and her colleagues found that certain neural mechanisms are associated with mate attraction and mate choice, and that, based upon a personality questionnaire, people might be classified as one of four love types—negotiator, director, builder, or explorer—although the types overlap.19 Comparing the similarities and differences between identical and fraternal twins could reveal genetic influences on brain processes and personality traits that affect our love lives.
The wealth of genetic findings does not surprise me, perhaps because I shared my home environment with a fraternal twin sister who was, and still is, quite different from me in most respects. As children our preferred cereals were Rice Krispies and Kix, our favorite ice cream flavors were chocolate and coffee, and we rarely, if ever, sampled our sister’s choices. Our differences in eye color, height, and running speed were apparent from an early age, and I had fun seeing the surprised looks on people’s faces when I told them we were twins. I witnessed genetic effects before I knew what genes were.
* * *
Researchers have found other, more surprising results. Since the mid-1980s studies of adult twins, raised apart and together, have shown that genetic factors influence political perspectives, social attitudes, divorce tendencies, financial decision-making, and religious involvement, behaviors previously thought to reflect how our parents raised us.20 Earlier twin studies did not detect genetic effects on these behaviors because they studied children still living at home, under the thumbs of their families. But as children approach adolescence and adulthood, their actions, tendencies, and choices speak more clearly of their natural preferences. For this reason it is paradoxical, but true, that the older we get the more important our genes become. Most people are surprised to hear this because their logical mind says that as people age they accumulate a wealth of different experiences that should affect their behavior. But logic and reason do not replace what the data tell us: we become more selective about where we go and what we do as we get older, reflecting genetically based choices, some of which may be kicking in for the first time. Consistent with these findings is that adopted siblings who grew up together become less alike in general intelligence over time as genetic effects grow stronger in the environments they individually seek. That is because as people move from childhood to adolescence and beyond they gain freedom and choice over what they do and who they do it with, allowing their genetic predispositions greater expression.21
* * *
Two sets of unrelated, or virtual, twins are among the different pairs generated by the Bogotá brothers. I like the term virtual twins because it’s clever, timely, and fits the novel, twin-like pairing I first came across in 1990.22 I wish I had thought of this term myself, but it came from a smart mother who offered two such children for study. Virtual twins (the same-age, biologically unrelated individuals raised together soon after birth) should not be confused with so-called Irish twins, children born nine or ten months apart to the same parents. Virtual twins are much closer in age, and some are even born on the same day. Furthermore, they have no common genes, but Irish twins do.
Most virtual twin pairs are made up of two adoptees, but about 25 percent of the more than 160 such sets in my files include one adoptee and one biological child—many seemingly infertile women say that once the adoption process was under way, they relaxed and became pregnant. I have even known couples who adopted a child just before or just after delivering triplets conceived by assisted reproductive methods, generating three pairs of virtual twins in one family. Some virtual twin pairs came into being through even more exotic means, such as adoption plus surrogacy, adoption plus embryo donation, and marriage of same-sex partners whose children had different dads.
Virtual twins are closely matched in age and time of arrival in their family, but they have no genetic connection. These defining features make them an ideal contrast to identical twins raised together and apart. In fact, they are reared-apart identical twins in reverse because virtual twins share their environments but not their genes, whereas reared-apart identical twins share their genes but not their environment. Most adoption researchers study ordinary adoptive siblings who differ in age and time of entry in their new home. They do so largely because they believe that these pairs are more plentiful and easier to find than virtual twins, and perhaps they are, but I am still finding virtual twins. They are excellent subjects for determining the degree to which living together could make people display similar, as well as dissimilar, habits and behaviors, and they outperform ordinary adoptive siblings on these measures because their ages and arrival times match so closely.
Virtual twins are much less alike in general intelligence and in their strengths and weaknesses in specific verbal and spatial skills than are identical and fraternal twins, even those raised apart. The modest intellectual resemblance that virtual twins show in early childhood probably reflects their common rearing, but it fades as they approach adolescence when new genetic effects and unique experiences kick in. One dad told me he expected to see some behavioral differences between his two daughters, Judith, who was adopted, and Sally, who was biological, but the extreme contrasts in the girls’ abilities and personalities astonished him.
Virtual twins come into being one other way, and that is why the Colombian brothers are so important: the switching of Carlos and William that separated the identical twins simultaneously crafted two virtual twin pairs: Jorge and Carlos in Bogotá, and Wilber and William in La Paz. But there is more to this story: when twins are inadvertently switched, an exclusive and exceptional class of virtual twins emerges because its members believe they are fraternal twins, as do their parents and other relatives. After all, their mothers gave birth and took two babies home from the hospital. The math adds up and no one asks questions. The brothers of Bogotá and the brothers of La Paz became unwitting members of this unusual group.
True, these virtual twins and their families were puzzled by the striking physical differences between these siblings. Friends of the Bogotá twins would sometimes ask, “How could you be twins?” or joke that “you’re too handsome to be in your family!” Even more prescient, relatives in La Paz told Ana, “Maybe one of your kids was switched at birth? Ha ha!” She would sometimes say, “My poor little son [William], it seems he has been exchanged.” And when her boys threw fists at each other, as siblings sometimes do, Wilber would yell, “You’re not my brother!” or “You were picked up on the street, and my mom and dad fed you for free!” But everyone knows that fraternal twins can be quite different, so the jokes and threats were neither made in earnest nor taken seriously.
I think of these exclusive virtual twins as the most exceptional of unrelated siblings, whose research status parallels that of reared-apart identical twins, who are the most exceptional among multiples. If these particular virtual twins, who always believed they were fraternal twins, are more alike in taking tests or running races than ordinary virtual twins (who always knew they were unrelated), perhaps belief or investment in being a twin enhances similarity. I would not expect the two types of virtual twin pairs—those who believed they were fraternal twins and those who knew they were not—to differ in how much they resemble one another for traits significantly affected by genes. Parents who think that their identical twins are fraternal still find similarities in their children’s beh
aviors, so their misbelief about twin type does not affect their perceptions of their children or the children’s outcomes.23 Still, the degree of resemblance across characteristics of the ordinary and extraordinary virtual twins is an empirical question that researchers could and should address. However, the very small number of such extraordinary virtual twin pairs precludes a proper test of this notion—the newly found Colombian twins increased that number from seven to just nine.
* * *
The multiplicity of relationships generated by the Colombian twins continued to expand and included two sets, William and Carlos, and Jorge and Wilber, who have never been studied before because they were unknown to researchers until now. We have no word to describe them, so I chose replicas, because they truly are replicas, not originals. If you think of two people standing together in front of a mirror, the reflection in the glass is a re-creation of the original; it preserves the important elements, but it is still a copy. Replicas are an entirely new kinship, created accidentally and for the first time in Colombia because of the switching of two twins from two identical pairs.
Accidental Brothers Page 17