Paula Bernstein (left) and Elyse Schein, identical twins who were given up for adoption to different families at birth and only discovered this in their mid-30s. (Random House)

Twins have long fascinated the human imagination. This would apply to the mythical beings that supposedly founded Rome, to TV's paparazzi-prone Olsen twins, to the two identical sisters who were given up for adoption to different families at birth and made headline news when they were reunited recently as grown women.

But twins have also long been the subject of widespread scientific research. Indeed, in many ways they have become science's favourite human guinea pigs.

Because of their common genetic makeup, twins provide scientists the ability to conduct comparative research that may not otherwise be possible, on everything from heritable disease to how the brain develops to when infants learn to speak.

As a result, many twins (and triplets) undergo a barrage of DNA and IQ testing, extensive questionnaires about their tendencies, and basic observation beginning as infants and continuing, if they are willing, throughout much of their lives.

Celebrity twins Mary-Kate, left, and Ashley Olsen on the Hollywood Walk of Fame. (Nick Ut/Associated Press)

Twin research, in fact, is so extensive that there is even a group, the International Society for Twin Studies (ISTS), whose job it is to keep track of all these studies for the benefit of twins, their families and the medical community at large.

In 2006, its medical journal, the Twin Research and Human Genetics listed 52 different twin registries that they consider to be advancing genetic research worldwide. Some of these registries follow as many as 15,000 sets of twins.

Those are the legitimate ones, of course. Sometimes, it seems, the fascination with twin research is so intense that scientists may cross the line.

Nature or nurture

A case in point: The story of identical twins Paula Bernstein and Elyse Schein, which made news headlines around the world late last year.

Given up for adoption at birth, the two were separated and placed with different families, ending up living part of their lives on different continents. The impetus was, at least in part, a psychiatric experiment into what role nature, versus nurture, played in developing their personalities.

When, in 2002, Schein contacted the New York State Adoption Information Registry for more information about her biological mother, she made a shocking discovery.

At 33 years of age, she learned that she had a twin sister and, worse still, that they had been subjects of a psychiatric research experiment — its results reportedly locked away until 2066 — that was designed to examine the behaviour of several sets of twins nurtured in different environments.

Bernstein and Schein were essentially unknowing lab rats for well-known New York psychiatrist, Peter Neubauer, who has refused to talk publicly about the research. New York state banned the separation of twins during adoption in 1980. But until then, many psychiatrists, including apparently Neubauer, felt that splitting twins up would only help their social development.

The sisters have co-written a book about their experiences called Identical Strangers: A Memoir of Twins Separated and Reunited. Both are writers, one is also a filmmaker, and both, it turns out, were editors of their respective high school newspapers.

In their book, they say they are happy to have found each other but troubled knowing such a study could have been authorized in the first place.

"We felt our lives had been orchestrated by these scientific researchers who put their career objectives before our needs as children," Schein told CBC's George Stroumboulopoulos on The Hour.

The rules today

These days, twins continue to be extensively studied, though under more sensible guidelines. Consent by twin study participants or their parents is mandatory, but general research practice still keeps the participants from accessing study records.

"We do notify patients if during our research we find they are at risk of something," says Kerry Jang, director of the Twin Project at the University of British Columbia's department of psychology.

He considers the support of the subjects involved to be a crucial aspect in the study of twins. "We rely on parents and participants to provide accurate reports," he says.

Identical twins share at least 99 per cent of their genetic variation, and because they are born at the same time and into the same environment, there is virtually no generation effect or cultural difference between the two siblings.

For this reason, identical twins are extremely helpful in setting the external factors that affect physical and mental development, explains Jang.

On the other hand, fraternal twins have approximately 50 per cent of their genetic variation in common and, again, because of their similar environments, can also be helpful in evaluating hereditary variations.

In other words, twins might hold the secret to the proverbial nature versus nurture argument. And with multiple births on the rise, thanks to the increased use of modern reproductive technologies, that gives science all the more options to explore.

Twins and Science

The largest twin registry in Canada is UBC's Twin Project, with data on more than 1,500 sets of twins.

Among the projects it has looked into are those seeking to evaluate the origins of personality traits, artistic talent and disorders such as paranoia, depression and thrill-seeking tendencies.

"I have found that not all attitudes are learned," says Jang. "And this is upsetting to some psychologists."

One of Jang's earlier studies examined personal attitudes on at least 30 different subjects. He found that attitudes concerning the preservation of life (such as thoughts on abortion), equality (such as views on liberal immigration policies), and athleticism (such as opinions on exercise) were more likely to be influenced by genetic predisposition than views accumulated throughout life.

On the other hand, attitudes concerning intellectual pursuits, such as reading for example, showed no signs of being inherited.

"The prevailing theory in psychiatry and psychology is that you can change any behaviour. But genetics also affects attitudes, it's not just environment," explains Jang.

Jang also found that twins who share a genetic inclination to certain personality disorders do not always develop them. That opens the door for comparative twin studies to help determine what external factors cause a personality to go off the rails.

Twins and language

Headed by Canadian-born psychologist Steven Pinker, the Twin Study at Harvard University has been examining language development among twins. One of its key findings is that twins are more prone to experience slight delays in language development within the context of average childhood development.

As part of her studies, research supervisor Jennifer Ganger, found that twins tend to say their first word three to six months later on average than other children, although twins do tend to catch up by the time they reach primary school.

"Early language delays in twins seem to be largely due to environmental factors, and this surprised me," says Ganger.

One common explanation for this is that twins typically have less one-on-one conversation with adults during their early years because their rearing tends to be more shared. She found that both fraternal pairs and identical pairs reach verbal milestones together, which suggests that early language development is influenced by the amount of face-time they have with adults and not genetics. This finding is important because it is contrary to the popular notion that a predetermined maturation timetable exists within each of us.

The future of twin studies

Nancy Segal, who directs the Twins Study Centre at California State University, says that one of the most important developments to come out of this kind of research is a deeper understanding of the new science of epigenetics — basically how external factors or chemicals interact with human genes.

Epigenetics, Segal says, is the environment's ability to adjust the "on" and "off" position of genes and helps explain how one identical twin can develop a serious genetic disorder such as cancer or autism while the other does not.

In 2005, Mario Fraga and his team at the Spanish National Cancer Centre published a study in which 80 sets of identical twins were examined to assess how and why vulnerability to certain diseases can differ among identical twins.

They found that during the early years of life the identical siblings were genetically indistinguishable. But as time passed, 35 per cent of the twins they studied grew to show significant differences in their DNA portraits.

Their genetic map had not changed, but the ways in which these maps were being expressed had.

This groundbreaking study suggested that external factors such as diet, stress and perhaps toxins can either silence or promote heritable disease by their affect on the disease-prone gene.

The completion of the Human Genome Project in 2003, which successfully mapped the human chromosome sequence, was expected to answer many outstanding questions surrounding heritable diseases. But while it is one of the greatest scientific feats of our time, it has only confirmed the complexity of our genetic makeup and is likely to make twin studies more important than ever before.

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