About one in every 65 pregnancies in the UK results in a multiple birth, a figure that has grown since the advent of fertility treatments, and nearly all of those multiples will be twins. The unusualness of twins has captured the creative imagination going all the way back to the earliest of myths. Take the case of identical twins Romulus and Remus, the apocryphal founders of Rome, through to June and Jennifer Gibbons, the real-life sisters who are the subject of the Polish film maker Agnieszka Smoczyńska’s new film, The Silent Twins. The Gibbons girls, like all identical or monozygotic (MZ) twins, came from the same egg and sperm. When a zygote is split in two at an early stage of development, it creates two separate embryos that share the same genes. Worldwide, less than one in 300 people are identical twins. Effectively clones, they present not just a strikingly similar image of themselves, but also provide a mirror for society to examine the age-old questions of nature and nurture. But their likeness has often been treated not as a biological quirk but as something weird, even supernatural. Think of the disturbing Grady girls, the ghost-twins in Stanley Kubrick’s film The Shining, who speak in unison and terrify Danny, the young boy locked in a haunted snowbound hotel with his increasingly deranged father. They represent not just fear of the unordinary but also perhaps the doubleness of fate, the idea that there are alternative options running parallel to the paths chosen. The Gibbons twins gained notoriety not because they spoke in unison but because they remained silent in unison. Children of Windrush-era parents from the Caribbean, they were the only black girls in their community in south-west Wales and suffered social isolation. They withdrew from the outside world, invented their own language, or idioglossia, and refused to speak to anyone else. They attended school but didn’t study, and eventually the medical authorities became so concerned by their impassivity that they were separated and sent to different boarding schools, but this only led to both of them becoming catatonic. At 16, they started writing novels, one of which, written by June, was published by a vanity press. But then they turned to petty crime – vandalism, theft and arson – and were eventually sentenced to indefinite detention in Broadmoor, where they remained, in conditions June later described as “hell”, for 11 years. When they were released, Jennifer died almost immediately and June went on to live a quiet but independent life back in Wales. “That was a very sad story,” says Audrey Sandbank, author of Twin and Triplet Psychology. “They never did manage to separate until Jennifer died.” Separation can occasionally be a major issue with twins, especially identical twins. “There are situations in which twins are so tied together they control each other,” says Sandbank, who has dealt with many twins over a long career as a psychotherapist specialising in same-age siblings. Hers is a self-selecting group because people seek her out over the difficulties they have with their twins, or their own expectations of their twins. And for these people, she says, the shifting struggle for dominance is often the problem. “One twin may become independent and the one that was previously dominant is upset that they can’t be as assertive as they once were,” says Sandbank. “It’s a matter of having to re-establish the relationship.” How twins are brought up has dramatically changed over the past half-century. It used to be common for parents and society at large to treat twins as two halves of a single entity – dressing them the same way, putting them in the same class at school, and generally emphasising their interdependence. However, starting in the 1960s, there was a growing appreciation that, like any siblings, twins needed to be treated as distinct individuals, enabling a healthy separation and independence from each other. That said, there is undoubtedly a special bond between twins and it’s one that they often find empowering, as well as sometimes frustrating. Last week, the ubiquitous Van Tulleken twins, doctors Chris and Xand, appeared on Radio 4’s Today programme, promoting the second series of their BBC podcast, A Thorough Examination, which looks at the possibilities and challenges of making behavioural changes. “It’s very complicated having a clone wandering through the world with your face and your genetics,” said Chris. “Xand and I are very close, we do get along well, but we fight bitterly and physically quite often. It’s not easy having someone else represent you, with all your flaws and good points. He’s like a living mirror.” Of course, he would perhaps seem less like a mirror if they didn’t spend all their time together making TV shows and podcasts, but Xand said they had some rules in place to limit arguments. One included a cooling down period in which they had to leave the room and think nice thoughts about each other – “which is very difficult to do,” he said. They sounded a bit like two squabbling nine year olds, although, in fact, they do have a serious aim. As Chris put it: “This, for us, is the central question of our whole lives: how much of who any of us are is our genes, what’s handed to us at our birth, and how much are we shaped by the world around us?” In other words, it’s that nature-nurture question again, which remains the subject of ongoing intellectual and political dispute. Central to any inquiry into what most determines our lives – biology or the environment – is the study of twins, specifically the comparison of identical and non-identical (also known as fraternal or dizygotic) twins. In quantitative genetics, the twin method is a key means of estimating genetic and environmental influence. As Claire Haworth, Philip Dale and Robert Plomin, the authors of a widely cited study into those dual influences on the academic performance of nine-year-old boys and girls in science, phrased it: “To estimate both genetic and environmental parameters of individual differences, the twin method requires both identical twins (monozygotic [MZ]) and non-identical twins (dizygotic [DZ]). MZ twins are 100% genetically similar, whereas DZ twins are, on average, only 50% similar for segregating genes. At a crude level, this means that if a trait is influenced by genetics, then within-pair resemblance for that trait should be higher in MZ twins than in DZ twins.” That is to say that given that both types of twins are likely to be brought up in the same environment in very similar ways, if identical twins show a more pronounced sharing of traits, this is almost certainly due to genetic factors – particularly when studies involve a large number of twins. In the above study, the conclusion stated: “Results indicate that genetic influences account for over 60% of the variance in scientific achievement, with environmental influences accounting for the remaining variance.” Plomin, a psychologist and geneticist, went on to write a book called Blueprint in which he argued that genetic heritability accounted for 50% of the psychological differences between people, and environment accounted for the other half. But he argued that most of that environmental 50% was random, not of the kind that could be socially moderated, and that much of it in any case was really an expression of genetics. The Van Tullekens seem to disagree. As Chris said: “The most urgent thing we need to change is the circumstances of many people’s birth because it is the environment [that] has an enormous effect on … outcomes for children.” Certainly it would be difficult to argue that environment didn’t play a significant role in the plight of the Gibbons twins. Had they experienced the kinds of comfortable early lives that the Van Tullekens enjoyed, rather than the isolation of being two young black girls in an all-white world, then perhaps they wouldn’t have developed a secret language of withdrawal but instead flourished in school, university, even the BBC. We may never have the definitive answer to that, but the most promising way of getting closer to it seems to be through the study of twins in all their endlessly fascinating exceptionalism and commonality.