The identical twins who discovered their secret sibling

athy Seckler was 16 years old when she made an unexpected discovery that changed her life completely – she had an identical twin sister. It was 4 September 1977 – she recalls with utmost clarity, her voice wobbling only slightly – when a friend told her that she resembled a girl she knew called Lori Pritzl, and asked if she was adopted. Seckler's birthday was the same date as Pritzl's and the two girls looked exactly the same. Seckler had known she was adopted since a young age, enjoying a happy and loved upbringing, but she then learned that Pritzl had also been adopted from the same agency as her.

The girls immediately spoke on the phone and realised their friend's suspicions must have been true – that they were twins. Seckler recalls breaking down in tears when she met her twin sister for the first time. "I saw Lori crossing the street… a big smile on her face," she says. "Then we hugged. It was quite an experience… I felt less alone. Being an adopted child, I always felt different… I felt like, 'Wow, I have a comrade there'."

They were both smokers, had similar artistic interests like dancing and drawing, and both liked music. "It was surreal," says Pritzl. "I felt like I was staring at myself in the mirror."

They could have found out earlier – their similarity to each other had been pointed out previously by acquaintances who knew both families. Pritzl had shrugged it off – doesn't everyone occasionally hear that they look like someone else? However, the girls lived about 15 miles (24km) from each other and they had family friends in common. Unbeknownst to both girls, their parents had known about the other twin for about a decade, but had been told to keep it a secret.

What emerged a few years later was that Seckler and Pritzl were part of a controversial study. In the 1960s, a then well-respected adoption agency – Louise Wise Services in New York, deliberately split up at least 10 sets of infant twins or triplets and placed them in separate families. Seckler and Pritzl were among six sets of newborn identical multiples separated between 1960 and 1969, including one set of triplets.

The agency had partnered with a group of psychiatrists and psychologists in an attempt to tease out what makes us who we are. They wanted to know how much of our identities are defined by our nature, and our nurture – but at what cost? For a BBC documentary about the study, I spoke with both identical and fraternal twin participants, as well as one of the original researchers involved, to explore why the twins today are still seeking answers about their unwitting involvement in this intrusive experiment.

"We were really deprived of being sisters, let alone twins. And I think it was just horrible what they did," Seckler told me in an interview for the film. "It was challenging enough being an adopted child… to deprive me of being a twin and having a sister and twinship was just horrible." (Watch the video below to learn more about Kathy Seckler and Lori Pritzl's story and you can see all three parts of the documentary series Split at Birth on BBC Reel.)

"Any Louise Wise adoptee from the 60s has every right to think that perhaps they have a twin," says Nancy Segal, a geneticist, twin expert and author of Deliberately Divided. She has spent several years tracking down many of the original subjects involved in New York's Child Development Center Twin Study as well as anyone she could find who was involved with the study.

The story of the twins – and one set of triplets – deliberately split up, first came to light publicly in 1980 when three young men discovered by chance at the age of 19 that they were identical triplets. Their reunion made headlines around the world. Soon after, it became clear there were other multiples who had also been split up, both identical and fraternal twins.

Stories of twins have long captured the human imagination. Strangers stop twins in the street – and regularly ask questions about that special bond they are reported to have – questions Seckler still gets asked today if she mentions that she has a twin.

For researchers, twins provide a unique insight into the complex interplay between our genetics and the environments we live in. Identical twins who grow up apart in different families share only their genes, not their environment. Any commonalities discovered can therefore be largely attributed to their genes, although in recent years the relationship between nature and nurture has been found to be a great deal more complicated than this. Traits like intelligence, height and weight, for instance, have all been found to have important genetic influences. Findings like these come from years of data collected from retrospective studies of twins raised apart.

"What we are finding is that many more behaviours than we ever would have thought do have a genetic component to them," says Segal. "Genetics is not everything, but it does explain a great deal of why we differ one person to another."

Though it rarely occurs, identical twins that have been raised apart usually only discover this years later. Any insights are therefore gained retrospectively. Because of their rarity, there are limited cases for scientists to study.

One young twin pair, for instance, both loved ketchup, to the delight of one adoptive mother and the frustration of the other

The researchers working with the Louise Wise Services agency believed they had found a way around that. They realised that they could study identical multiples from birth, capturing their development in real time – which is exactly what they set out to do. The psychiatric advisor to the adoption agency, Viola Bernard, justified splitting up twins as she proposed that it would help them develop their own identity, rather than competing with each other in the same household for their parents' affection. She claimed that this was backed by scientific studies of the time. "I can tell you honestly, there is no such child development literature in existence. They never named studies," says Segal.

Never before in documented history had twins been separated as part of policy. Bernard worked with a researcher called Peter Neubauer, then at the Child Development Center at the Jewish Board of Guardians in New York, who had long sought to study twins raised apart.

The adoptive parents were not informed that their child had a twin or triplet, only that they were taking part in a child development study. "And it was very obvious that if they did not accept the study and having the researchers come to their home periodically, they probably would not get this child," says Segal.

The twins were given numerous tests, looking at a range of traits related to intelligence and personality. They were also filmed and photographed. Seckler recalls how she felt self-conscious when the researchers came to the house. "My mother, she agreed because she was a psyche major and knew the importance of child development studies," she says. "But the fact that it was a twin study, they weren't told the truth."

From the outset, the experiment had problems. We reached out to Lawrence Perlman, one of the few researchers who has spoken about his brief involvement in the study when he was a postgraduate student. In his role, he would visit the twins, test them and film them. He remembers being surprised at just how similar the estranged twins were. "Not just the physical appearance, but their whole personalities. It was quite clear to me that the genetic influences were very strong," he says. One young twin pair, for instance, both loved ketchup, to the delight of one adoptive mother and the frustration of the other, notes Perlman.

The twins were placed with carefully selected families based on several key factors, such as their parents' ages, socioeconomic status, education, religion, and their other children. "They all had an older sibling who had been adopted from Louise Wise, and that was kind of the hook that they had in terms of getting parents to agree," says Perlman. And according to Segal, it was also a way to create constant conditions across families.

The study soon ran into issues. Funding ran out and there were ethical concerns in the 1970s, regarding informed consent. Parents were retrospectively asked to sign consent forms, but some refused. I spoke with Arthur Caplan, a professor at New York University and a medical ethics expert, who told me that the study took place during a time when ethical violations in scientific research were all too common and he described this study as a clear case. "You could really cause severe harm, marital disruption, battles down the road between children and their parents," says Caplan. "The potential for harm is real, the potential for violation of basic rights, absolutely present."

The distance between the twins was also ill-thought-through – as was the likelihood of their meeting later in life. The children were all placed with families living in the New York metropolitan area at a time when communities were far closer knit than they are today.

Seckler and her sister had been adopted by families living in similar social circles. In fact, their parents had known about the other twins' existence for over a decade before they met, but were asked to keep the news a secret for the good of the girls. Viola Bernard specifically advised both sets of parents not to tell their daughters, suggesting it might be "too damaging", but offered little else by way of explanation. Other twins who were split up also met by chance, often through mutual acquaintances, as was the case for the identical triplets who met aged 19.

Scientifically, the research itself was fundamentally flawed. Perlman, looking back, says that the data they collected on the children were "a mess" and that the study was not well organised. And no scientific papers were ever published by Neubauer and his team. "They didn't really seem to have an understanding of the proper way to handle it from a scientific viewpoint," says Perlman. "They were threatened with lawsuits and nothing was published."

The study did not include fraternal twins, which would have been a natural control group. Comparing identical twins to fraternal twins can help tease out the role of genetics versus environment. Even so, fraternal twins were also placed in separate families by the adoption agency.

We spoke with Allison Kanter who was separated from her fraternal twin. Kanter was also a Louise Wise adoptee, but only discovered her twin recently after she watched a documentary featuring the story of the identical triplets and curiosity led her to take a genetics ancestry test. “I remember getting a shiver all over my body thinking: 'Wow, what if this was real?'.”

There was a match with someone called Michelle Mordkoff. They met as soon as they could. Though brief, their relationship was deep. “It was like a piece of me that was always missing that I never knew,” says Kanter. “The more we got to know each other, the more that we realised we were similar, you know, emotionally and how we looked at life and how we lived our life.”

Only a few years later, Mordkoff died of pancreatic cancer, meaning that the twins had less than three years together. “I think being fraternal twins… we felt we were collateral damage in this whole Louise Wise scheme. You know, we were not identical. They were not going to find out anything about us that would be the same as in identical cells. And they just kind of tossed us aside,” Kanter told me.

Was it, as Kanter questioned, all for nothing? What did happen to the data that were collected, and why are the other unwitting participants still looking for real answers about their involvement in this ill-fated study?

In the name of scientific research, they essentially exploited these families without ever using the data – Lawrence Perlman

Perlman only worked on the project for 10 months before his discomfort with the study led him to take a job elsewhere. But in the years that followed, he had wondered what happened. Only a few highly repetitive case reports containing scant details ever appeared.

Eventually, in 2004, Segal and Perlman met after corresponding with each other as each of them searched for answers. Together they went to meet a 91-year-old Neubauer at his Maddison Avenue apartment in New York City. Even then Neubauer did not express any regrets. "He defended the practice saying that it was Viola Bernard's idea," says Perlman. "He was not going to acknowledge any responsibility for having done anything wrong. So that was just his stance and he dug his heels in. In the name of scientific research, they essentially exploited these families without ever using the data."

Louise Wise Services, a once well-respected agency, shut down in 2004, passing on its adoption and research records to another agency called Spence-Chapin. Control of the records relating to the study, however, belongs to The Jewish Board of Family and Children's Services. In a comment to us the Jewish Board strongly denied any responsibility for the Neubauer study. A spokesperson told the BBC that "because of confidentiality laws, and out of consideration for the extremely private and personal nature of the information contained in these study records, we have limited access to the records to the study subjects themselves". They added that all of the living study subjects are now aware of their involvement.

With permission from Seckler and Pritzl, I requested access to footage filmed of them as young children – but was told that the twins would have to request it themselves. They were then informed that should they get access, they could not share the files with anyone else, as they may contain "sensitive information about people other than the study subjects themselves". Due to the emotional toll of revisiting their past, the twins did not wish to pursue it any further.

As it stands, the data collected in the study remains sealed at Yale University and cannot be opened until 2065. Neubauer arranged for the records to be locked at Yale in 1990, claiming he did this to protect the twins.

"I do not believe that for a minute. I believe it was done to protect themselves," Segal says. Caplan wonders if the reason was simply to hide incompetence. "Why keep the records of the research under seal? I think that the only explanation I can come up with is embarrassment."

But even if that data exists in any meaningful way, due to the ethical concerns and the flawed nature of the study, it is questionable whether it should ever be used. Segal for instance, stresses that this is a study that should never have been done in the first place. "There really are no insights," she says. We don't know a great deal about what's in there. And if we were to get access to it and publish it, perhaps, what kind of message does that send to future researchers?"

For the families, questions persist without answers, and the experiment has cast a long shadow. No living individual has ever been held accountable. One unintended legacy of the experiment is that it provides an example of how science should not be done, and how important ethics considerations are at every stage. (Learn about other research that has strayed from the ethical tightrope in this film and this article.)

And for Seckler, on a personal front, she hopes that by telling her story it will make the painful twists and turns of her discovery easier to bear. Until recently, when people learned she was a twin, it would lead to inevitable questions such as: "Oh that must have been so much fun growing up together, did you dress the same, look the same…?"

Seckler says it was often easier not to revisit the emotional turmoil of the story. "I would lie and say, oh yes, we dressed differently… I had to continue the legacy of this secret sister, it's been difficult," she says. "So I'm sort of glad, that hopefully people will see this and get the story out there."

While the study may have been trying to unravel the role of genes and environment on their identities, it instead exerted a toll on their lives – and that of their families – that is hard to imagine. Finding their secret identical siblings changed their lives forever.

The triplets involved in the study struggled with mental health problems for years after their discovery (though they also had psychiatric problems as teenagers) and one of them committed suicide. It is believed their biological mother had a history of mental ill-health. Another woman from a twin pair, who was separated but not studied, is also thought to have committed suicide – her biological family also had a history of depression. (It's worth noting that while stressful experiences do not necessarily cause mental ill-health, severe stress can exacerbate previous mental health issues – especially for with those who have a genetic predisposition to it.)

Others have experienced anger, sadness and regret at their involvement in the experiment. For some, it affected their relationships with their adoptive parents. And most of all it affected their relationship with their twin.

"We could never go back because we were twins but we weren't sisters," adds Seckler. "We didn't grow up together and even to this day that has been a very difficult part of our relationship."

Most of all it has left those involved asking a deep question about the very topic of the intended research: how much their nature was impacted by those who split them up?