Robert Shafran’s first inkling that his life would soon be turned on its head occurred on his first day at college in upstate New York in 1980. His fellow students greeted him like a long-lost friend. “Guys slapped me on the back, girls hugged and kissed me,” he recalls. Yet Robert had never set foot inside Sullivan County Community College until that day.

Another student, Eddy Galland, who had studied at the college the previous year, was the cause of the confusion, it transpired. Eddy was his spitting image, said classmates. Robert was intrigued and went to Eddy’s home to confront him.

“As I reached out to knock on the door, it opened – and there I am,” says Robert, recalling his first meeting with Eddy in the forthcoming documentary Three Identical Strangers.

The two young men had the same facial features, the same heavy build, the same dark complexions, the same mops of black curly hair – and the same birthday: 12 July 1961. They were identical twins, a fact swiftly confirmed from hospital records. Each knew he had been adopted but neither was aware he had a twin. Their story made headlines across the US.

One reader – David Kellman, a student at a different college – was particularly interested. Robert and Eddy also looked astonishingly like him. So he contacted Eddy’s adoptive mother, who was stunned to come across, in only a few weeks, two young men who were identical in appearance to her son. “My God, they are coming out of the woodwork,” she complained.

Eddy, Robert and David were adopted by different families when they were babies. They possessed the same complements of genes and, as young adults, they were indistinguishable. They were the same person trebled, as one commentator put it.

The story behind the triplets’ separation and subsequent reuniting forms the dark core of Three Identical Strangers, which emerges as a tale of grotesque medical manipulation that today would have led to prosecutions for malpractice. It is also a poignant tale of lives reunited. But most importantly of all, the documentary is a timely illustration of the unexpected ways that genes and life events interact to shape us.

And that is crucial – for once again genetics is back in the news. On one hand, scientists are launching new onslaughts in the battle over the influence of nature versus nurture in human affairs. At the same time, there has been the attempted hijacking, by the far right, of the findings of modern genetics to support their own extreme views.

In these circumstances, this month’s release of Three Identical Strangers, which won a special award for documentary storytelling at the Sundance film festival this year, could not have been more opportune, said medical historian Professor Nathaniel Comfort, of Johns Hopkins University, Baltimore. “The film is particularly well-timed, when genetic essentialism is on the rise and divisiveness and polarisation is sweeping the globe,” he said.

The vital point that affects the current nature-nurture debate and which is outlined in Three Identical Strangers is the seemingly intense impact of genetic influences on Eddy, Robert and David. When they were reunited, they not only looked like doppelgangers, they displayed an uncanny number of shared habits. All liked the same films, smoked the same cigarettes (Marlboros) and had been wrestlers at college. Yet their childhoods had been very different. Robert’s parents were prosperous, Eddy had grown up in a middle-class suburb, and David’s parents lived in working-class Queens.

Only their close genetic heritage could explain their powerful similarities, it was argued – a notion that the brothers milked to its limit. They appeared on TV, answering questions in unison; became celebrities on the New York club scene; had cameo roles in Madonna’s 1985 film Desperately Seeking Susan; and eventually opened their own restaurant, called Triplets.

The triplets with their adoptive brothers and sisters. Photograph: New York Daily News Archive/NY Daily News via Getty Images

But slowly Eddy, Robert and David’s story changed trajectory. The boys began to look less and less like each other. Their physiques changed as they aged. Then their restaurant business broke up. They veered down separate paths, and without spoiling the plot of this remarkable and powerful film, they ended up by having very different outcomes in life.

The varied fates of the triplets suggest other important, non-genetic forces must also have been involved in determining how their lives unravelled. Hence the interest that the film has stirred among nature-nurture protagonists. As a study of identical siblings raised apart, it shines remarkable light on the interaction of genes and environmental factors on human upbringing.

And while the triplets’ story did suggest at first that DNA looks like an overwhelmingly powerful determinant of human destiny, by the film’s finale, that assumption looks strained and unlikely. Nurture is certainly involved in the story of Robert, Eddy and David.

This implication is supported by the work of Professor Tim Spector, head of twin research at King’s College London. His studies have uncovered many examples of identical twins who have been raised together and whose early life experiences were also very similar, but who still evolved into very different people. Identical sisters – raised by parents who treated them as two versions of the same person and gave them the same clothes and hairstyles – nevertheless ended up with very different personalities and careers. In some cases, one suffered from serious depression while the other was unaffected. Yet they shared exactly the same genes and exactly the same upbringing. Some critical environmental influences in later life must surely be involved.

Spector believes these influences are triggered by epigenetic changes in which major life events – diet, illnesses, drugs, smoking and other factors – can produce temporary alterations in the behaviour of one twin’s genes but not the other’s, and have major health and behavioural consequences. “There appears to be no similarity in age at death between identical twins, for example,” said Spector. “In addition, our work shows there is only a 30% chance that if one identical twin gets heart disease the other one will as well, while the chance of getting rheumatoid arthritis is only 15%. Yet these twins share so many other features – the same height, hair and build.”

Until recently the study of twins – or triplets – was the only direct method available to scientists who were seeking to separate the influences of our environments and our genes. And seemingly the most promising type of this research is that involving the study of identical twins who are separated at birth and raised by different sets of adoptive parents. These siblings have the same sets of genes but have different backgrounds and so should be invaluable for prising apart the influences of nature and nurture. And many biologists have indeed used such studies to make striking claims for the overwhelming influence of genes on our behaviour.

But there are only a relatively limited number of sets of identical twins reared apart that are known to scientists, and most live in the US or Europe. This means results will be limited to relatively small data sets and are skewed by cultural influences, a point emphasised by Comfort.

Genetics professor Tim Spector with six sets of identical twins. Studies of twins have often relied on western subjects. Photograph: David Parker/Shutterstock

“If you look at all the twin studies that have been carried out over the years and ask how many involved sub-Saharan African twins who were reared apart, you know what the answer will be. Absolutely none,” he said. “And this is true of the vast majority of DNA sequencing surveys that have recently been carried out. They are nearly all done using people of European ancestry as their subjects. That has to have major consequences for influencing results.”

In addition, geneticist Steve Jones of University College London questioned the inferences that could be drawn from studies of identical twins in the first place. “Identical twins, from the very start of their existences, live very different lives, sharing cramped wombs in which single children normally come to term. Again that will skew results, but genetic determinists ignore that.”

On the other hand, new techniques for studying the influences of genes have recently been developed, and these do not require the participation of twins. They are known as genome-wide association study (GWAS) and they employ thousands – sometimes hundreds of thousands – of subjects to delineate traits and determine whether they have high genetic components. Used in the proper way, they can provide powerful insights in genetic influences, Comfort acknowledged. “I don’t think you can ignore some of the science they are producing, although one should be careful of interpretation.”

A good example of the work of these analyses was provided by a study of 4,000 students in England and Wales. It concluded that the type of school – selective, grammar or comprehensive – that a child attended had little influence in their scholarly performance in later years, but that their genetic makeup did play a part. This was not a matter of one or two genes having major influences, however, but hundreds of them, each having a small impact on academic achievement but combining to have significant influences.

The study’s take-home message was outlined by its senior author, Professor Robert Plomin of King’s College London, who said that the research showed selective schools were simply not adding value to children’s education. “They take the kids that do the best at school and show they do the best at school. It’s an entirely self-fulfilling prophecy,” he said in an interview in the Guardian.

By contrast, the influence of genes on achievement was important, he added. As a result, Plomin proposed that genetic testing of children should one day be used to predict academic potential. “It will probably happen,” he claimed.

In denigrating the usefulness of private schools, Plomin gets brownie points from the left. But his call for gene testing of children has raised their ire. Plomin is unrepentant. “We now know that DNA differences are the major systematic source of psychological differences between us. Environmental effects are important but what we have learned in recent years is that they are mostly random – unsystematic and unstable – which means that we cannot do much about them,” he writes in his recent book Blueprint: How DNA Makes Us Who We Are.

Professor Robert Plomin argues that the excellence of selective schools is a ‘self-fulfilling’ process, endearing him to the left. Photograph: Martin Godwin/The Guardian

The book is uncompromising in its insistence on the importance of the influence of DNA in determining our lives. It has been heatedly attacked – and warmly praised – since its publication last month, and is seen by many as the most significant act in the recent reawakening of the old nature-nurture battle.

This re-emergence is also reflected in the uptake of genetic tests by neo-Nazi groups – particularly those in the US – who seek to use them to “prove” their white European ancestry (frequently with disappointing results, it should be noted). At the same time “alt-right” groups are celebrating their genetic “purity” by publicly swilling milk, which they believe is the nutritionally perfect white drink that only westerners can digest because they possess a genetic mutation, known as lactase persistence, that others lack.

Yet these beliefs – that genes are the primary driver of human nature – are simply not borne out by studies, as the US writer Carl Zimmer makes clear in his recent book, She Has Her Mother’s Laugh: The Powers, Perversions and Potential of Heredity, which has been shortlisted for the Baillie Gifford prize.

Consider the issue of IQ, Zimmer said. “While identical twins often end up with similar scores, sometimes they don’t. Equally, if you get average scores on intelligence tests, it is still entirely possible your children may turn out to be geniuses. And if you are a genius, you should be smart enough to recognise your children may not follow suit.”

The problems for those trying to separate environmental issues from those triggered by our genes are highlighted by Jones. “The trouble is that people look at the nature-nurture debate like a cake and assume you can have a slice of environment and then a slice of genetic factors. You can’t. They are baked together and cannot be separated without finding some way to unbake your cake.”

The crucial point is that by concentrating on the role of just one influence, our DNA, in controlling our behaviour and in determining our life stories, we risk ignoring the influence of free will and the role that humans have in determining their own affairs.

And this is why the release of Three Identical Strangers was important, said Comfort. It suggested that outcomes in life could be very varied despite common genetic heritages – and that was to be welcomed. “We need stories that say that ‘environments matter’, that ‘people matter’, and that ‘experience matters’. For if they don’t, what’s the use of living?”

‘We were treated like lab rats’

Three Identical Strangers took British director Tim Wardle five years to complete. The documentary follows the outcome of a sinister American experiment in which the triplets, Robert, Eddy and David, were deliberately separated from each other at birth and assigned to three different families. One was affluent, the second middle-class, and the third came from a working-class area of New York.

The adoptions were carried out as part of a secret study designed by child psychoanalyst Peter Neubauer, who wanted to uncover the influences of genes and the environment in children’s upbringing. Each family was told nothing of the other identical siblings who were involved in the experiment; nor were they informed about the nature of the follow-up studies that Neubauer set in motion in order to keep tabs on his subjects.

The adoption of the boys was arranged by the now defunct Louise Wise Adoption Agency in New York, which was set up primarily to serve the city’s Jewish community, an involvement that was highlighted by the US journal Science earlier this year.

“The irony of a Jewish researcher and a Jewish adoption agency conducting a twin study after the atrocities waged against Jewish people in Nazi Germany is clear, and was perhaps the reason that Neubauer never published [the results of] the study.”

Several other sets of identical siblings were separated and used in the experiment. Many remain bitter about their fates, arguing that had been treated like lab rats and were the victims of actions that amounted to “Nazi shit”.

Neubauer died in 2006 after he had sealed his data in vaults at Yale University, with orders that they remain there until 2066. During the making of Three Identical Strangers, Wardle and his subjects attempted to gain access to their files, but were able to see only heavily edited versions of Neubauer’s study.

Three Identical Strangers is released in the UK on 30 November