It was an idle day at the Parisian office where Elyse Schein was working as a temporary receptionist in late 2002. On a whim, she typed the words "adoption search" into Google. If those words seemed innocuous at the time, they couldn't begin to convey the dramatic turn of events they would incite, a chain of actions and reactions that would throw Elyse's identity into jeopardy, and expose a dark secret that would change her life.

While she'd known since childhood that she was adopted, Elyse had never sought information about, or contact with, her biological parents. She had grown up in a supportive family in New York and Oklahoma, and considered her adoptive mother, Lynn, and father, Marty, to be her "real" parents.

Now, at 33, the same age at which her adoptive mother had died from spinal cancer, Elyse felt her own human frailty come to bear. A mixture of instinct and curiosity beckoned her towards the archives of her unknown past. And something else, too - the unshakeable feeling she was "missing" something, a sense that had haunted her throughout her life. "I feel like I'm missing a twin," she used to confide to her friends, assuming her feelings of loss were intrinsic to everyone.

She registered with the New York State Adoption Information Registry and, a year later, a letter arrived in the post from Louise Wise Services, the private adoption agency that had handled Elyse's case. It contained astonishing news: "You were born at 12.51pm as the 'younger' of twin girls born to a 28-year-old Jewish single woman," it read.

"I walked around Paris in a daze," recalls Elyse, who at the time led a bohemian life, supplementing her work as a filmmaker with office temp jobs. "I was so excited to confirm this element of my identity. But I felt almost immobile. I was mesmerised with all these different scenarios in my mind. Not only did I have all of these questions about this person, I also wondered: why were we separated?"

But there was more surprises to come: Elyse and her identical twin, Paula, would soon discover that they had been split up as babies as part of a secret study on nature versus nurture. Their adoption agency, Louise Wise Services, had clandestinely separated them, allowing psychologists to study the girls during regular childhood visits to each of their homes under the guise of research for a "child development study". The true nature of the research was never revealed to them or their parents. The researchers did so apparently without concern for the fact they were "playing God" in their subjects' lives.

Learn more about the twins, their extraordinary similarities despite being separated for decades, and the study that changed their lives in the February issue of marie claire.

OTHER HUMAN EXPERIMENTS

The "Monster Study"
This experiment on 22 orphans hypothesised that children develop speech impediments because of psychological pressure. In 1939, speech pathologist Wendell Johnson, from the University of Iowa in the US, administered positive speech therapy to half of the children and negative therapy to the remainder. Many went on to suffer lifelong psychological problems as a result. In 2001, six participants sued the university on the grounds that the experiment had been damaging, and were later awarded $US925,000 ($1.1 million) in compensation.

Dr Josef Mengele
The most notorious human experiments ever recorded were those conducted between 1943 and 1945 at Polish concentration camp Auschwitz by Dr Josef Mengele. Known as the "Angel of Death", Dr Mengele singled out identical twins and subjected them to appalling tests. Of the 3000 twins studied, fewer than 200 survived. The tests resulted in the 1947 Nuremberg Code, the first international document to set out ethical regulations in human experimentation.

HIV Drug Trials on foster children
It has been alleged that between 1988 and 2001, New York City's welfare department forced 465 HIV-positive children in its care to be subjected to trials of experimental AIDS drugs made by GlaxoSmithKline and other drug companies. Almost all of the children are believed to be of African-American or Latino descent. The treatment is believed to have included forcing the children to take the drugs via surgical feeding devices.

David Reimer
Born in 1965, David Reimer was eight months old when a routine circumcision burnt off his penis. After being referred to Dr John Money, a gender identity expert, his parents agreed to raise him as a girl. Now called Brenda, she and twin Brian became the perfect experiment of nature versus nurture for Dr Money. Initially thought a success, an unhappy Brenda was told the truth as a teenager and began the process of converting to a man. In 2002, Brian died of an overdose and two years later, David killed himself with a shotgun.