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What Light Does ‘Three Identical Strangers’ Throw on the Nature/Nurture Debate?

It seems fitting that twins (1) come in two types, (2) are fascinating at two levels and (3) enhance understanding of human development in two ways.

· 10 min read
What Light Does ‘Three Identical Strangers’ Throw on the Nature/Nurture Debate?
Feature photo by Anne Richard / Shutterstock.

The 2018 film, Three Identical Strangers, is gripping. It is both an exposé of a 1950s secret twin study (studies of triplets fall under the general heading of “twin or multiple birth studies”) and a heartbreaking portrayal of identical triplets, Robert, David and Eddy, who were separated at birth, then unknowingly placed under a microscope by psychologists intent upon learning how different family practices might yield different behavioral outcomes. This unthinkable episode in the history of twin research challenges not just established norms, but belief in the sacredness of family, and faith in the integrity of science.

More needs to be researched and revealed about this dark study and I will be writing just such a book over the next year. And as one twin observed, “I wish someone would start a support group. This is all so crazy—no one, not even us twins understand the complexity.” Looking into the twins’ and triplets’ stolen childhoods and its profound effects on their lives will be a key goal. It is also vital to understand why identical twins, especially reared-apart identical twins and triplets, were critical to this inconceivable research enterprise.

It seems fitting that twins (1) come in two types, (2) are fascinating at two levels and (3) enhance understanding of human development in two ways. All three are in full bloom in Three Identical Strangers. However, watching the film is not enough—understanding the science behind twin research and the public’s fascination with the findings shows why the film is so gripping, provocative and popular.

Identical twins, more properly labeled as monozygotic or MZ, result when a single fertilized egg (zygote) divides between the first and 14th day after conception. With some exceptions, these twins share 100 percent of their genes and are always of the same sex. Fraternal twins, also known as dizygotic or DZ, form when a woman simultaneously releases two eggs that are fertilized by two separate sperm. These twins share 50 percent of their genes, on average, just like two non-twin siblings in a family and can be same-sex or opposite-sex.1

Twins, especially identical twins, are of great human interest, as evidenced by the frequent coverage of their births, lives and accomplishments in both professional and popular sources. Twins are also convenient artistic and literary devices, allowing representation and exploration of identity and duality. We see twins in the images of painters and photographers, such as Alberto de Veiga Guignard’s “Léa e Maura”2 and the McLaughlin sisters’ “Twins on Twins.”3 We also find twins in the plays of William Shakespeare, such as Twelfth Night and The Comedy of Errors.4 In fact, Shakespeare was the father of male-female twins, Judith and Hamnet, most likely explaining his interest and curiosity in twinning as a phenomenon. Indeed, his knowledge of twins has been the subject of scholarly inquiry.5

The reasoning behind the classic twin design is simple and elegant. The degree of similarity within identical twin pairs is compared with that of fraternal twin pairs in any measurable trait of interest, such as spatial ability, body weight or running speed. Greater resemblance between identical twins than fraternal twins is consistent with genetic influence on the trait(s) in question. By now, thousands of twin pairs have participated in hundreds of studies around the world, yielding a 49 percent average estimate of genetic effects across characteristics.6 But what does that number really mean? It does not mean that a person’s ability or build can be dissected into genetic and environmental components because these effects are inextricably intertwined within individuals. It does mean that the members of a given population will vary because of both genetic and environmental differences between them: the 49 percent figure means that about half the traits’ variability within a given population can be explained by genetic differences among people within that population, with the other half explained by environmental differences. It is also possible to separate the environmental inputs into the experiences we share with our family members and those we do not.

A provocative finding from twin studies, revealed over the last few decades, is that sharing the same family environment with a biological relative does not make you alike. For example, it turns out that identical twins raised apart are as similar in personality as identical twins raised together. Thus, what makes relatives alike in most traits are their common genes, and the important environmental influences on how we develop are those we experience individually and not those we experience alongside our family members. In behavioral genetics, this is the distinction between the shared and non-shared environment.7

The power of twin research to show the greater resemblance between identical twins than fraternal twins in virtually all behavioral, physical and medical traits explains why it has attracted so many scholar across diverse disciplines. Aside from the more standard psychological and physical measures like intelligence and stature, researchers are using twins to study the roots of political participation, religious interest and financial investment. Some researchers use a variant of the classic twin design, namely co-twin control. This approach involves administering different treatments or training programs to identical co-twins, such as cold medications or sports training regimens, and later comparing the outcomes. Similar results in genetically identical individuals, with the members of identical pairs divided into the treatment and control groups, show that such interventions do not alter health or improve performance, whereas differences between the two groups mean that they did. Alternatively, researchers may explore existing differences between identical twins, relating to personality, disease or experience. They can then try to determine why one twin is more outgoing, less depressed or more cautious than the other twin, and use this information to assist non-twins.

The relatively rare subset of reared-apart twins offers a powerful sample for understanding trait development. If identical twins grow up apart then the similarities between them cannot be ascribed to their rearing, education and/or communities—or to influence from their co-twin. Instead, it appears that within their different spheres of existence, each twin is drawn to particular people, places and events that are most compatible with their genetic backgrounds. The triplets in Three Identical Strangers were typical in this regard, each making similar choices. Intuitively, it seems that even genetically identical people living apart should differ more than they do, but the data tell a different story, which is why scientists are surprised and captivated by twins.

Of course, similarity in any trait is not absolute between identical twins regardless of whether they are raised apart or together. Identical twins share nearly identical genetic predispositions, but it takes environmental triggers to activate those tendencies. Sometimes only one identical twin develops diabetes, schizophrenia or gender dysphoria. Perhaps that’s because only the affected co-twin experienced an infection, trauma or some other misfortune, before or after birth, that triggered a chain reaction leaving them in a very different place than the unaffected co-twin. Also important is that extreme environments can overwhelm inherited propensities as I discussed in my recent book, Accidental Brothers (2018). In 1988, the members of two sets of identical Colombian male twins were inadvertently switched at birth, placing one in a culturally rich city and his brother in a remote farming village.8 Despite many behavioral and medical similarities within each pair, some differences in abilities, attitudes and accomplishments reflected the lack of education and other opportunities available to the country-raised twins.

Twins reared apart seem to have cast a spell over people that can’t be fully explained by their similarities in measured quantitative traits. This may be because their resemblance is also striking at an anecdotal and idiosyncratic level, extending to unusual habits such as reading books back to front, placing a pinky finger under a beer can, leaving one square of toast uneaten and scattering love letters around the house.9 The triplets in Three Identical Strangers talked, walked and gestured in matched fashion. These behaviors, once thought of as mere coincidences, assume scientific significance given their duplication across reared-apart identical co-twins. It’s unlikely that there are specific genes for these tendencies, but the twins’ resemblance could be explained by a combination of many genes, each with small effects, linked to personality, anatomy and appetite. Seeing such behavior replicated in identical people who have never met opens new areas of inquiry and thought about how we turn out as we do. 

Podcast #172: Nancy Segal on Lessons in Human Nature Revealed by Identical Twins
Sydney. London. Toronto.

The triplets we meet in Three Identical Strangers are a single case study, not a full-scale investigation, but their similarities and differences can be viewed in the larger context of twin research. Their remarkable resemblance in abilities, interests, food preferences and choice of partners, in addition to their curly locks, sturdy build and outgoing personalities, are typical of the findings reported by five comprehensive studies of separated twins.10 As the triplets admitted, their similarities were unrelated to the kinds of parenting they each received which differed greatly. The film makes this point quite clearly and accurately.

The close relationship that evolved so quickly and effortlessly among the three is also striking, challenging us to decide how these identical strangers, meeting for the first time at the age of 19, could immediately “fall in love with one another.” As Robert observed, when he met Eddy it was just the two of them as the rest of the world fell away. This is also consistent with the published findings.

My analysis of twin relationships shows that identical reared-apart twins experience greater social closeness and familiarity upon meeting and subsequently, relative to fraternal reared-apart twins, mirroring what we see among reared-together twins. More provocative is the finding that both types of twins report closer relations with the twin they have just met than with the adoptive siblings with whom they were raised since childhood. This counterintuitive finding encourages hard thinking about what attracts people to one another and sustains relationships. I suspect that the perception of similarities functions as a social glue, drawing people together and keeping them close. This process may be akin to experiences we have when we meet people from our high school, summer camp or country of birth when we are far from home—feelings of familiarity, warmth and comfort are triggered, at least in that moment. However, such similarities are not pervasive, the way that they are in identical twins, so it is unlikely that such familiar strangers will segue into life-long friends. Importantly, it is the behavioral, not the physical matches that feature significantly in forging close relationships, evidenced by studies of spouses, friends and unrelated look-alikes.11

Three Identical Strangers also shows Robert and David discussing the loss of their identical brother Eddy to suicide, so difficult to witness as their extreme pain is palpable. Eddy was in his mid-30s and left behind a wife, Brenda, and a young daughter, Jamie. What caused this tragic event is speculated upon by those interviewed in the film, as well as by behavioral science professionals and the public at large. Some viewers may be left with the impression that the suicide was linked mostly to nurture, but the situation is more complex. It is true that Eddy’s rearing father was a strict disciplinarian, and Eddy never felt a comfortable fit in his adoptive home. In the film, Elliott Galland was tormented by the thought that he failed to teach his son more about how to live. It is, therefore, not surprising that Eddy’s wife believed that Eddy gained more than the other two from being with his brothers. In contrast, the triplets thrived from the warmth and welcome of David’s father who boasted that he had gained “two more sons.” Perhaps David’s father’s passing and Robert’s leaving Triplets, the New York restaurant that the three brothers jointly owned and ran, initiated a chain of events that eventually culminated in Eddy ending his life.

Clearly, when it comes to behavior the environment does not act alone—it activates genetic predispositions. Everyone loses a parent and many people dissolve working family relationships, but most do not end their lives as a result. We learn that all three triplets had experienced mental health issues and behavioral difficulties during their teenage years. It also appears that their biological mother had mental health problems, but the degree of their severity is unclear. In fact, Robert asks, “Why him and not me?” Robert and David’s more supportive families may have buffered them against adversity. However, any number of factors, before and after birth, may have increased Eddy’s likely inherited vulnerability to behavioral battles. Twin studies point to genetic influences on suicide, although estimates of their contribution vary widely.12

Three Identical Strangers is worth seeing more than once because of the behavioral, societal and ethical issues it raises. The film is also powerful testimony to our universal fascination with twins. Perhaps twins are intriguing because they challenge cherished beliefs and expectations about each person’s uniqueness in appearance and behavior. Moreover, most identical twins enjoy an intimacy, trust and acceptance that many people truly envy. No two twins are exactly alike, nor is their individuality sacrificed by their shared heritage—but the power of two, or three, is a force to be reckoned with.


1 Segal, N.L. (2017). Twin Mythconceptions: False Beliefs, Fables, and Facts About Twins. San Diego, CA: Elsevier.
2 See Segal, N.L. (2000). Entwined Lives: Twins and What They Tell Us About Human Behavior. N.Y.: Plume.
3 Szekely, J., Abbe, K.M., & Gill, F.M. (1981). Twins on Twins. New York: Clarkson Potter.
4 Segal, N.L. (2007). Indivisible by Two: Lives of Extraordinary Twins. Cambridge, MA: Harvard University Press.
5 Walsh, G., & Pool, R.M. (1940). Shakespeare’s knowledge of twins and twinning. Southern Medicine and Surgery102(April), 173-176.
6 Polderman, T. J., Benyamin, B., De Leeuw, C. A., Sullivan, P. F., Van Bochoven, A., Visscher, P. M., & Posthuma, D. (2015). Meta-analysis of the heritability of human traits based on fifty years of twin studies. Nature Genetics47(7), 702–709.
7 Segal, N.L. (2012). Born Together-Reared Apart: The Landmark Minnesota Twin Study. Cambridge, MA: Harvard University Press.
8 Segal, N.L., & Montoya, Y.S. (2018). Accidental Brothers: The Story of Twins Exchanged at Birth and the Power of Nature and Nurture. New York: St. Martin’s Press.
9 Segal (2012).
10 See Segal (2012).
11 Segal, N.L., Hernandez, B., Graham, J.L., & Ettinger, U. (2018). Pairs of genetically unrelated look-alikes: Further tests of personality resemblance and social affiliation.  Human Nature, 29(4), 402-417.
12 Pedersen, N. L., & Fiske, A. (2010). Genetic influences on suicide and nonfatal suicidal behavior: twin study findings. European Psychiatry25(5), 264-267.

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