Aaround one in 65 pregnancies in the UK results in a multiple birth, a figure which has increased since the advent of fertility treatments, and almost all of these multiples will be twins. The unusual character of twins has captured the creative imagination since the oldest of myths.
Take the case of identical twins Romulus and Remus, the apocryphal founders of Rome, to June and Jennifer Gibbons, the real sisters who are the subject of the new film by Polish filmmaker Agnieszka Smoczyńska, The Silent Twins. Gibbons girls, like all identical or monozygotic (MZ) twins, come 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 not only present a startlingly similar image of themselves, but also provide a mirror for society to examine age-old questions of nature and nurture. But their resemblance has often been treated not as a biological oddity but as something bizarre, even supernatural.
Think of the disturbing Grady girls, the ghost twins of Stanley Kubrick’s film the brilliant, who speak in unison and terrify Danny, the young boy locked in a snowy haunted hotel with his increasingly deranged father. They represent not only the fear of the extraordinary but also perhaps the duplicity of fate, the idea that there are alternative options parallel to the chosen paths.
The Gibbons twins achieved 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 South West Wales community and suffered from social isolation.
They withdrew from the outside world, invented their own language, or idioglossia, and refused to speak to anyone. They attended school but did not study, and eventually the medical authorities became so concerned about their emotionlessness that they were separated and sent to different boarding schools, but this only made them catatonic.
At 16, they began 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 on 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 in Wales. “It was a very sad story,” says Audrey Sandbank, author of Double and Triplet Psychology. “They never managed to separate until Jennifer died.”
Separation can sometimes be a major problem with twins, especially identical twins. “There are situations where twins are so bonded that they control each other,” says Sandbank, who has treated many twins during a long career as a psychotherapist specializing in siblings of the same age.
Hers is a self-selected group because people seek her out for the struggles 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. “A twin can become independent and the previously dominant one is upset that they can no longer assert themselves as they once were,” says Sandbank. “It’s a matter of having to restore the relationship.”
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 a single entity – dressing them the same, placing them in the same class at school and generally putting the emphasis on their interdependence.
However, from the 1960s there was a growing realization that, like all siblings, twins should be treated as separate individuals, allowing for healthy separation and independence from one another. That said, there is undoubtedly a special bond between the twins and it’s one they often find empowering, and sometimes frustrating.
Last week Van Tulleken’s ubiquitous twins Doctors Chris and Xand appeared on Radio 4 Today programme, promoting the second series of their BBC podcast, A thorough examinationwhich examines the opportunities and challenges associated with behavior change.
“It’s very complicated to have a clone wandering around the world with your face and your genetics,” Chris said. “Xand and I are very close, we get along well, but we fight quite often physically and bitterly. It’s not easy to be represented by someone else, with all its faults and good sides. He is like a living mirror.
Sure, he might look less like a mirror if they didn’t spend all of their time together doing TV shows and podcasts, but Xand said they have some rules in place to limit arguments. One included a cool-down period in which they had to leave the room and have nice thoughts about each other — “which is very hard to do,” he said.
They looked a bit like two nine-year-olds bickering, although, in fact, they have a serious purpose. As Chris said: “For us, this is the central question of our whole life: how much is each of us our genes, what was passed on to us at birth and how much are we shaped by the world around us. we?”
In other words, it is still this nature-nurture question, which remains the object of a permanent intellectual and political debate. At the heart of any inquiry into what most determines our lives – biology or the environment – is the study of twins, particularly the comparison of identical and non-identical twins (also known as fraternal or dizygotic twins).
In quantitative genetics, the twin method is a key way to estimate genetic and environmental influence. As Claire Haworth, Philip Dale and Robert Plomin, the authors of a widely cited study of these dual influences on the academic performance of nine-year-old boys and girls in science, put it: “To estimate genetic and environmental parameters individual differences, the twin method requires both identical twins (monozygotic [MZ]) and non-identical twins (dizygotic [DZ]). MZ twins are 100% genetically similar, while DZ twins are, on average, only 50% similar for segregating genes. At a gross level, this means that if a trait is influenced by genetics, then the within-pair resemblance for that trait should be higher in MZ twins than in DZ twins.
That is, since both types of twins are likely to be raised in the same environment very similarly, if identical twins exhibit more pronounced trait sharing, it is almost certainly due to genetic factors – especially when studies involve a large number of twins.
In the study above, the conclusion stated: “The results indicate that genetic influences account for more than 60% of the variance in scientific achievement, with environmental influences accounting for the remaining variance.”
Plomin, a psychologist and geneticist, later wrote a book titled Plan in which he argued that genetic heritability accounts for 50% of psychological differences between people and environment accounts for the other half. But he argued that most of that environmental 50% was random, not the kind that could be moderated socially, and much of it was an expression of genetics anyway.
The Van Tulleken seem to disagree. As Chris said: “The most urgent thing we need to change is the circumstances in which many people are born, because it is the environment [that] has a huge effect on… outcomes for children.
It would certainly be difficult to argue that the environment did not play a significant role in the fate of the Gibbons twins. If they had known the kind of comfortable first lives the Van Tullekens enjoyed, rather than the isolation of being two young black girls in an all-white world, then maybe they wouldn’t have developed a secret language. retired but rather thrived at school, university, even the BBC.
We may never have the definitive answer to this question, but the most promising way to approach it seems to be the study of twins in all their singularity and their infinitely fascinating commonalities.