A Review of Deliberately Divided: Inside the Controversial Study of Twins and Triplets Adopted Apart by Nancy L. Segal. Rowman & Littlefield, 520 pages (November, 2021)

When I first heard about the Louise Wise Services-Child Development Center (LWS-CDC) twin study, I was shocked but also skeptical. A doctor had separated monozygotic (MZ, popularly known as “identical”) twins to study them, but he never told the twins or their adoptive parents of the existence of their siblings. It sounded like something from a dystopian science fiction novel. Surely this could not happen in the United States. But as I investigated further, I discovered, to my surprise and horror, that such a study had in fact happened. The details were not quite as straightforward as I had been led to believe, but twins were indeed separated from each other and studied by scientists, who kept the purposes of their work secret.

In 2018, Tim Wardle brought this surprising story to popular attention with his poignant (but somewhat tendentious) documentary, Three Identical Strangers, about the LWS-CDC triplets Edward Galland, David Kellman, and Robert Shafran who discovered one another’s existence by chance aged 19. Now, Dr. Nancy Segal has written Deliberately Divided, the most thorough account of this scandalous episode to date. Segal is an eminent behavioral geneticist who has spent her career wrestling with issues raised by the study of twins, and her new book is by turns moving, infuriating, and enraging. Everybody who thinks about human nature and scientific research should read it.

Philosophers and scientists have long been interested in the sources of human similarities and differences. We know that no two people are exactly alike. Sally might be sullen and serious, whereas Jenny is joyful and ironic. But we also know that some people are more similar to each other than they are to others. Tim and Tom might both love horror films but hate sour food, whereas Douglas might despise horror but love lemons.

There are two plausible causes of this variation in human traits and tendencies: inborn characteristics (nature) and the environment (nurture). The problem is that we are all the products of both, and it is very difficult to disentangle one from the other. How do we know whether the woman next door is gloomy because her life has been filled with tragedy and sorrow or because she was born with a morose disposition? As if in sympathy with the plight of our unresolved curiosity, nature has provided a potential solution to this formidable puzzle: Twins.

Twins are, of course, fascinating and disconcerting, and their eerie similarity remains a source of astonishment to friends and strangers alike. But they are also potentially valuable to researchers because they offer a way to separate and isolate nature from nurture. Monozygotic twins share the same nature (they have almost the same DNA), so if they grow up apart in different environments (different nurture), researchers can determine the proportion of the variation in traits caused by nature and nurture, respectively. The natural traits would be the same, while those fashioned by nurture would be different.

Let us suppose, for instance, that Sally and Sarah are MZ twins reared in separate families in separate states (and who know nothing of each other’s existence). After, say, 20 years, any similarity between them greater than the similarity between random strangers can be attributed to nature (so long as the homes in which they were raised were not more alike than two random homes). The obvious problem is that twins are usually raised by the same parents. But what if they weren’t? What if researchers could separate them and observe them across time? What a wealth of knowledge that could provide.

One’s judgment about the LWS-CDC twin study—one’s level of outrage and disgust—likely depends upon one’s judgement about how (and why) these questions were answered by the researchers. Louise Wise Services was a reputable adoption agency, and responsibility for the twin study remains a matter of uncertainty and dispute. The organization’s head researcher was Dr. Peter B. Neubauer and its chief psychiatric consultant was Dr. Viola Bernard. Bernard was an ambitious, intelligent, and, by many accounts, charming woman influenced by psychoanalytic theories of human development. The decision to separate twins may have been her idea because, Segal tells us, she believed “that twins were better off growing up in separate families as they would not be competing for parental attention with a same-age sibling.” In this version of events, Neubauer “learned that this policy had been implemented and decided to follow the newborn twins’ development over time.”

On the other hand, it is possible that Neubauer “was interested in exploring the relative contributions of nature and nurture during child development” and “persuaded Bernard to propose this policy to LWS.” Three Identical Strangers promotes this version of the story, and never mentions Bernard. Instead, Wardle emphasizes the Frankenstein-like hubris of Neubauer, who is depicted as the clear villain. This was more or less the version of the story I originally heard, but the truth is more complicated.

One of the strengths of Segal’s account is its dispassionate and thorough analysis of the ethical thorns and tangles of the LWS-CDC study. She clearly finds the study abhorrent, describing it as “a great example of how not to do research.” But she does not allow her outrage to overwhelm her prose or cloud her judgment. She notes that she began with the view that the separation policy was initiated by Bernard, but that she has become open to the possibility that Neubauer’s desire to study twins influenced (perhaps even unconsciously) Bernard’s thinking. Bernard’s views about the importance of separating twins to reduce the burden on adoptive parents may have provided a veneer of justification over uglier motivations of scientific arrogance and reckless curiosity that actually motivated the policy.

Whatever the answer to this important question, Neubauer was certainly aware of the monumental decision to separate twins and to keep their existence from each other secret, and he actively participated in the cruel deception by studying the twins in their adoptive homes for many years. This decision had tragic consequences that continue to reverberate. Separated twins eventually found their siblings, sometimes in the most bizarre and haphazard ways. The shock of discovering a real-life doppelgänger is difficult for most of us to imagine, but real for a small handful of humans. The results are not always happy.

The stories of the vicissitudes of these reunited twins trying to build relationships is the emotional heart of Deliberately Divided. Understandably, many of the twins (there were 11 in all, four pairs and one set of triplets) have chosen to remain more or less private and refrained from talking openly with Segal. But a few—most prominently the MZ twins Paula Bernstein and Elyse Schein and the triplets Edward Galland, David Kellman, and Robert Shafran—frankly discuss their experiences. What is especially heartrending about these stories is the sense of loss and betrayal that many of the twins felt knowing that they had been robbed of a potentially powerful sibling bond. One of the twins, Melanie Mertzel expressed this succinctly, noting that “The person I should have been closest to I did not know.”

I was surprised to discover that some of the twins became estranged from each other after forming quick and powerful bonds. As Segal notes, this is likely for reasons most of us take for granted: Years of growing up with our siblings teaches us to deal with emotional ups and downs and to look past momentary anger and upset. Family love is as close to unconditional love as humans will experience, because of genetic similarity and necessity. People who live together as dependents have to develop certain skills that other acquaintances do not. Roommates who get into a fistfight over a lover can move away from each other; young siblings cannot.

Furthermore, twins who are reunited after years of separation must wrestle with the existential shock of significantly revising their personal story. As Segal writes, “Learning that one is a twin is an extraordinary event, information that significantly revises personal history and identity.” A counterfactual world inevitably haunts the imagination of a divided twin: what if we had been raised together? This can stress and strain a relationship that should otherwise be strong and supportive.

Readers might want to know about the science, even if they strongly disapprove of the study. Did this unethical study actually yield anything important about human nature and nurture? The answer is not entirely clear because most of the data are still locked away, but it is highly unlikely for a couple of reasons. First, the sample size (n = 11) was too small to allow for meaningful extrapolation even if the results had been published.

Second, we now have good evidence about the heritability (or contribution of nature) of human variation from a variety of studies, including those of twins who were reared apart (but not purposefully separated for scientific reasons). Part of the appeal of the LWS-CDC study is that it followed twins closely during their childhood, which set it apart from other twin studies. But I’m skeptical that this would reveal anything important that would cause us to revise our views about the relative contributions of nature and nurture. The future of the study of the sources of human variation will almost certainly belong not to the study of twins, but to advances in the study of actual DNA.

Although we don’t have the data from the LWS-CDC study, we can look at the MZ twins who have gone public. Like other MZ twins reared apart, they appear similar in appearance and psychological traits (the similarity of their mannerisms is especially striking), suggesting that nature plays a strong role in making us who we are. However, they are not identical, and their differences are often as compelling as their similarities. Like all of us, then, twins are singular and unique. Although twins show that we are not wholly plastic, they also show that we are not rigidly determined by our genomes. We are the result of some as yet unspecified and in practice inextricable combination of nature and nurture. In this, twin studies have succeeded in refuting extremists on both sides of the debate while more or less vindicating common sense.

Ultimately, humans are more important than the science. The LWS-CDC study was unconscionable, and it provides a valuable reminder of the dangers of unchecked scientific hubris. However, it’s important not to exaggerate. Bob Shafran, the triplet, called the study “Nazi shit,” and critics have compared Neubauer to Josef Mengele, an infamous Nazi officer and physician who conducted ghastly experiments on twins in concentration camps. But as Segal judiciously writes, “It is easy to label the LWS-CDC study ‘Nazi science,’ but given the important differences I would argue otherwise, as would several ethicists I interviewed.” It is easy to understand the anger, pain, and outrage of the twins in this study, but inflammatory comparisons to Hitler, Mengele, and the Holocaust distort more than they illuminate. We can express our justifiable horror without resorting to hyperbole.

This case also illustrates the ethical importance of motivations. If the LWS policy of separating twins was actually impelled by Bernard’s earnestly held and carefully considered beliefs, then many people would judge it differently than if it was impelled by Neubauer’s scientific ambition. The problem is that there simply is not enough information now to make a confident judgment. But even if Bernard’s sincere beliefs were the initial cause, Neubauer was complicit after the fact and tried to get other adoption agencies (Catholic Charities in New York) to separate twins. They refused. Furthermore, whatever Bernard and Neubauer’s original motives, Segal notes that they were “both continually blinded by scientific ambition and eventually consumed by potential lawsuits. They bypassed the twins’ best interests by keeping them together in foster care, then placing them apart.”

Deliberately Divided is a well-researched and careful book about a shameful episode in the history of US science. Segal is scrupulously fair and has refused to fill her book with caricatured villains and blemishless heroes. Drs. Neubauer and Bernard are presented as three-dimensional humans who are likable, intelligent, and even admirable, but also severely flawed. It is to Segal’s credit that she allows the reader to see this while also unequivocally denouncing all involved for their behavior. And that is one of the most important lessons we can draw from this tragic affair: Decent people, misled by ambition and ideology, are capable of otherwise unthinkable wickedness.