what the special bonds of twins reveal about nature, breeding and genetics

Around one in 65 UK pregnancies result in a multiple birth, a figure which has risen since the advent of fertility treatments, and nearly all of these multiples will be twins. The unusualness of Gemini has captured the creative imagination since the very first myths.

From the case of identical twins Romulus and Remus, the apocryphal founders of Rome, to June and Jennifer Gibbons, the real sisters who star in the new film by Polish filmmaker Agnieszka Smoczyńska, 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, fewer than one in 300 people are identical twins. Effectively clones, they not only present a strikingly similar image of themselves, but also provide a mirror for society to examine the secular issues of nature and nurture. But their resemblance has often been treated not as a biological quirk but as something strange, even supernatural.

Think of the creepy Grady girls, the ghost twins in Stanley Kubrick’s film The brilliant, who speak in unison and terrify Danny, the little boy locked up in a snow-infested hotel with his increasingly deranged father. They represent not only a fear of the unusual, but perhaps also a duplicity of fate, the idea that there are alternative options that run parallel to chosen paths.

The Gibbons twins gained notoriety not because they spoke in unison but because they were 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 from social isolation.

Author Marjorie Wallace with Jennifer, left, and June Gibbons during a visit to Broadmoor in 1993. Photograph: PA Images/Alamy

They withdrew from the outside world, invented their own language, or idioglossy, and refused to speak to anyone else. They attended school but did not study, and eventually the medical authorities became so concerned about their impassiveness that they were separated and sent to different boarding schools, but this only resulted in both of them becoming catatonic.

At 16, they began writing novels, one of which, written in 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 what June later called “hellish” conditions, for 11 years.

When they were released, Jennifer died almost immediately and June continued to live a quiet but independent life in Wales. “It was a very sad story,” says Audrey Sandbank, author of Twin and hat-trick Psychology. “They were never able to part until Jennifer died.”

Separation can occasionally be a big deal with twins, especially identical twins. “There are situations where twins are so bonded to each other that they control each other,” says Sandbank, who has dealt with many twins over the course of a long career as a psychotherapist specializing in siblings of the same age.

Hers is a self-selecting group because people seek her out because of the difficulties they have with their twins or because of their own expectations of their twins. And for these people, she says, the ever-changing struggle for dominance is often the problem. “A twin can become independent, and the previously dominant one is upset that they can’t be as assertive as they once were,” says Sandbank. “It’s about having to reconnect.”

The way twins are raised has changed dramatically over the past half century. It was common for parents and society at large to treat twins as two halves of one entity, dressing them alike, placing them in the same grade in school, and generally emphasizing their interdependence.

It was common practice for parents and society in general to treat twins as two halves of one entity

However, since the 1960s, there has been a growing appreciation that, like all siblings, twins should be treated as distinct individuals, allowing for healthy separation and independence from each other. That said, there is undoubtedly a special bond between twins and it is one they often find empowering, as well as sometimes frustrating.

Last week, the ubiquitous Van Tulleken twins Doctors Chris and Xand appeared on Radio 4 Today programme, promoting the second series of their BBC podcast, A thorough examinationwhich examines the possibilities and challenges of making behavioral changes.

“It’s very complicated to have a clone roaming the world with your face and your genetics,” Chris said. “X and I are very close, we get along well, but we fight bitterly and physically quite often. It’s not easy to be represented by someone else, with all your flaws and strengths. It’s like a living mirror.

Sure, maybe it would look less like a mirror if they didn’t spend all their time together doing TV shows and podcasts, but Xand said they had some rules in place to limit discussions. One included a cool-down period where they had to leave the room and think nice things about each other—”very hard thing to do,” she said.

They sounded a bit like two nine-year-olds arguing, though in reality, they have serious aim. As Chris said: “This, for us, is the central question of our whole life: how much of who we each are our genes are, what we were passed on at our birth, and how much we are shaped by the world around us we?”

In other words, it is that nature-nurture issue again, which remains the subject of an ongoing intellectual and political dispute. At the heart of any investigation into what determines our lives most—biology or the environment—is the study of twins, especially the comparison of identical and nonidentical (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 of those dual influences on the academic achievement of nine-year-old boys and girls in science, put it: “To estimate both genetic and environmental parameters of differences, the twin method requires both identical twins (monozygous [MZ]) and non-identical twins (dizygotic [DZ]). MZ twins are 100% genetically similar, while DZ twins are, on average, only 50% similar for gene segregation. At a crude level, this means that if a trait is influenced by genetics, then the within-pair similarity for that trait should be higher in MZ twins than in DZ twins.

That is to say, since both types of twins are likely to be raised in the same environment in very similar ways, if identical twins show more pronounced trait sharing, it is almost certainly due to genetic factors, particularly when the studies involve large numbers of twins.

In the above study, the conclusion stated, “The results indicate that genetic influences account for more than 60% of the variance in scientific outcomes, with environmental influences accounting for the remaining variance.”

Plomin, a psychologist and geneticist, went on to write a book entitled Planimetry 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 the kind that could be socially restrained, and that in any case much of it was indeed an expression of genetics.

The Van Tullekens seem to disagree. As Chris said: “The most pressing thing we need to change is the circumstances of a lot of people’s births because it’s the environment [that] it has a huge effect on…outcomes for children.”

Certainly it would be hard to argue that the environment didn’t play a significant role in the Gibbons twins’ plight. Had they experienced the kind of comfortable early life the Van Tullekens enjoyed, rather than the isolation of being two young black girls in an all-white world, then perhaps they would not have developed a secret language of withdrawal but instead would have blossomed into school , university, even the BBC.

We may never have the definitive answer to this one, but the most promising way to approach it seems to be through the study of twins in all their infinitely fascinating exceptionality and commonality.

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