(CBS) Elyse Schein always knew who she was: the adored little sister adopted by a couple who wanted nothing more than to raise a beautiful little girl in their loving home near Long Island Sound.
It never seemed strange to her that she was adopted.

"Our next-door neighbors were adopted, and I also had two cousins who were adopted," Schein told CBS News correspondent Joie Chen. "So, it seemed very normal."

Paula Bernstein also grew up in a happy home in suburban New York. She loved her adopted parents and older brother and even wrote an article explaining "Why I Don't Want to Find My Birth Mother."

"I always say, you know, the people who haphazardly created me are not my real parents," she said. "My real parents are the ones who took me in, and I always felt that, you know, family is what you make, not necessarily what you're born into."

Schein's normal childhood was followed by college, then film school abroad. The year was 2003: Schein found herself in Paris, an aspiring filmmaker working as a receptionist to make ends meet. Then, acting on a whim, she opened a door that changed her life.
"I was Googling information and I said, well, 'What's out there? Here I am in Paris, France. What's going on with my old friends in college who I lost touch with?' And I said, 'Huh, what's going on with those biological parents?" she said.

It was a casual inquiry but it brought this letter from the adoption agency.

"Well, it said, 'You were born on October 9, 1968 at 12:51pm, the younger of twin girls,'" she said.

"Oh my God, I'm a twin! Can you believe this? Is this really happening?"

Researchers have long been intrigued by the ties that bind twins. In the darkest days of World War II, Nazi scientists performed horrific medical experiments on twins, attempting to advance their disturbed notions of genetic superiority.

Fast forward to today: Just last month, as they do every year, thousands of twins gathered in Twinsburg, OH. for the annual Twins Day Festival. A happy occasion, to be sure, but also an opportunity for forensic scientists to explore why it is that twins with identical DNA still have unique fingerprints.

But the only twins research that interested Schein was any information that would help her find the sister she never knew she had.

"I knew that we were both born in New York," she said. "And the adoption agency was located in New York. So it seemed like a good place to start."

When Bernstein began grappling with bouts of depression in college, she turned to the adoption agency that placed her hoping heredity might help explain her illness.

"I remember looking in the mirror one day when I was a college sophomore and it struck me," Bernstein said. "I bet my birth mother would understand this. Something is wrong with me. Something is wrong with the way I'm thinking. And I think the only one who might have a clue about what I'm going through is my birth mother. I don't wanna meet her, but I want to find out some answers."
That was 1987. Little did Bernstein know that years later that anxious request would result in a fateful phone call from Louise Wise, the adoption agency that placed her with her family.

"I thought, 'Why would they be calling me now after all these years?'" she said. "And at that point, the woman on the phone said, 'Is this Paula Bernstein? Were you adopted from Louise Wise?'" And she said, "Well, I've got some news for you: You've got a twin sister and she's looking for you."

It was news Bernstein wasn't quite certain she was prepared to deal with ... so she took down her twin's phone number but also asked for the number of a social worker.

"I think in the excitement of the moment and perhaps it was a Freudian slip, I dialed Elise's number," Bernstein said.

"And I picked up the phone," Schein said. "And I hear 'Hello?'"

"So, I heard her voice on the end of the line and I realized what I did," Bernstein said.

"It's almost like I'm hearing my own voice in a recorder back at me," Schein said.

"It's funny because I feel like in a way I was talking to an old, close friend I never knew I had," Bernstein said. "Which is very funny that we had an immediate intimacy, and yet, we didn't know each other at all."

With each new detail, the reunited twins got to know one another, found out about their differences and their remarkable similarities. They'd both gone to graduate school in film. They both loved to write.

"I think, you know, when we met it was undeniable that we were twins," Bernstein said. "But we weren't sure what our relationship was to one another. And I think it's taken, you know, 3 1/2 years for us to become sisters."

While the women tried to learn how to become sisters, they also were learning more about the curious circumstances that surrounded their birth and separation.

Dr. Lawrence Perlman was a research assistant in the late 1960s for a study conducted by child psychiatrists. He confirmed what the women had come to suspect: that their separation was part of a study designed to test the impact of nature versus nurture.
"It was a study of adopted identical twins who were reared by different families without the families having knowledge of their twinship," he said. "So it was a unique study where you might be able to parcel out nature versus nurture influences, because they were genetically identical but they were raised in totally different households."

Bernstein and Schein believe that the research also focused on whether mental disturbances are hereditary.

"A good number of the twins and triplets who were involved, did come from families with history of mental illness," Bernstein said.

The study was never completed; the files were sealed.

"They didn't publish," Bernstein said. "Because, by the time it was ready to be published, they realized that the public outcry against them would be too strong. We've spoken with one of the key researchers in the study who still has absolutely no reservations about what they did."

"They express no remorse," Schein said. "It's actually shocking."
In all, five pairs of identical twins and one set of triplets were separated, though it's not clear if that decision was made solely so they could be studied. Ronny Diamond of New York's Spence Chapin services, who reviewed the records of the now-defunct Louise Wise Adoption Agency, doesn't believe the separation was meant to be harmful.

"I have to believe that their intentions were good," she said. "And there are studies that look at twins reared apart. It's given us so much rich information on genetics versus environment. So the study would have been fabulous."

Nevertheless, he admits, this isn't something that would happen today. Still, Diamond expects the questions will keep coming - questions she may not be able to answer because of strict privacy regulations.

"I'm aware of, certainly, at least one situation of separated identical twins who do not know," Diamond said.

She isn't sure if she has the right to tell them.

Paula Bernstein and Elyse Schein began their journey from "Identical Strangers" three and a half years ago. They're now confidantes and co-authors of a book that chronicles their experience.

"I am proud that I found my twin and that we wrote a book together, and we shared this journey together," Schein said.
Now they finally feel like sisters.

"But it's perhaps even closer than sisters," Schein said, "Because we're also twins."

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