What makes twins so fascinating? Last week, De Aston School in Market Rasen announced it had 20 set of twins, and was applying to Guinness World Records for recognition as the most twin-heavy school in the world. The school's twins gathered for a photoshoot, and in the newspapers appeared uniformed blond twins and auburn twins, white twins and black twins, long-haired twins and short-haired twins. All absolutely ordinary-looking kids, save for the fact that there was another one just like them, right by their side.
Not all the twins at Market Rasen are identical – although there are six identical sets in year seven alone. But it's probably true to say that while all twins have a certain wow factor, it's identicals, formed when one fertilised egg splits, creating two babies who each have the same genetic make-up, that intrigue us most. The others are fraternal, or two-egg, twins, created when two ova are fertilised separately.
Maybe it's because our individual uniqueness is so much a part of being human that someone who seems to break that mould and isn't entirely unique is such a compelling idea. Maybe it's to do with the fact that human beings are physically symmetrical that draws us to a sense of "the other half". It's even possible that more of us were "meant" to be twins than actually are: apparently up to 15 per cent of conceptions produce two embryos, but only one in 10 of those lead to two live babies. In the rest, one twin is "lost" – sometimes reabsorbed into the mother's body. Some believe that people with a sense of another "self" may be a twin whose sibling was lost but deep inside knows they were originally destined to be one of a pair.
Twin births in the UK are on the up: about one in every 32 babies born are twins, compared with one in 52 babies 30 years ago. That's down to two factors: fertility treatments (two fertilised eggs are often transferred to the uterus to maximise the chance of at least one baby) and the fact that mothers are getting older. The older a woman is, the less efficient ovulation is – and the more chance she has of releasing two eggs instead of one. A woman of 20 has a 7 per cent chance of having twins; a woman of 45 has a 55 per cent chance (if she manages to conceive at all).
But that's in Britain: in Africa, the likelihood of twins is much higher at any age. Visit the sleepy farmland region of south-west Nigeria and you'll be greeted by a sign welcoming you to "Igbo-Ora, the land of twins". Here more than 6 per cent of births are twins. No one is quite sure why, though some blame it on the yams: they're the staple food, and they contain a natural hormone, phytoestrogen, that may stimulate the Yoruba women's ovaries to release more eggs.
But giving Igbo-Ora a run for is money is Candido Godoi, a village in Brazil where the rate of twin births is 10 per cent. The profusion of twins there goes back to the early 20th century, but that hasn't stemmed a rumour that the notorious Nazi doctor Josef Mengele could be behind the statistics: he is known to have fled to South America, giving rise to a story that he conducted experiments on local women.
Part of why twins fascinate is because they can do things the rest of us can't do. Some people believe they have psychic powers and can sense what's happening to the other when they're apart. Many twins have their own language when they're growing up – although according to educationalists this is based on their understanding one another's grammatical mistakes, and isn't to be encouraged. And then there's the fact that identicals can get away with things the rest of us wouldn't be able to – like murder. In February last year, eyewitnesses saw either Orlando Nembhard or his twin brother, Brandon, shoot a man outside a nightclub in Arizona. But after holding Orlando for eight months prosecutors dropped the charges, because they couldn't prove that Orlando, rather than Brandon, was the culprit. In 2009, Malaysian twins Sathis and Sabarish Raj, 27, were spared from execution for drug trafficking because it was unclear which of them was the criminal. There was DNA evidence, but of course DNA – in the case of identical twins – is identical. Fingerprints, interestingly, are not. That's because, although the shapes on the fingertips are the same, subtle differences in the babies' womb nutrition mean the gaps between the swirls in the pattern aren't the same.
Being identity tricksters has captivated Hollywood, too. The Parent Trap, which tells the story of identical twins separated at birth who swap places to try to entice their parents to get together again, was a huge hit in its original 1961 version, and another box office hit in 1998 – although in both cases the identical twins were played by one actor (Hayley Mills in the original, Lindsay Lohan in the remake). In The Shining, the Grady twins – a pair of identical young girls rumoured to be dead – make only a brief appearance, but everyone who has seen the movie knows its terror owes much to those lookalike little girls in their powder-blue dresses who haunt the corridors of the Overlook Hotel. And then there are the Weasleys – Fred and George, some of the best-loved characters in the Harry Potter lexicon, a pair of confident pranksters who always seem to have the upper hand.
In real life too, being a twin can build a mythology. Plenty of gangsters terrorised London's East End during the 1950s and '60s, but Ronnie and Reggie Kray are the ones history remembers. And part of the enduring passion of the Bee Gees was that two of the three brothers, Robin and Maurice, were twins.
Robin Gibb died earlier this year of colorectal cancer, but his ill-health began with a blocked intestine – the same condition that killed Maurice in 2003. But if that suggests twins are likely to suffer the same diseases as one another, think again. Professor Tim Spector, who founded Twins UK 20 years ago to research what twins' experience of disease could reveal, is now doing groundbreaking research to show that surprisingly few twins die of the same disease as their sibling. "What we're discovering is that, though twins share the same genes, lifestyle choice and events can switch those genes on and off, a bit like a dimmer switch on a light. That's a very empowering message for the rest of us, because it means we're not prisoners of our genes. So healthy eating and exercise, for example, can make all the difference to our lives even if we've been born into a family with a genetic tendency to, say, heart disease or obesity."
But what's it like to raise twins – to carry not one but two babies in your belly, and then to be part of a three-way bonding process? (As well as bonding with their mother, twin babies are bonding with one another – and indeed, their bond predates the one with their mother and may seem to compete with it.)
Una MacDubhghaill, 45, had twins – and then had twins again. The first set, 14-year-old Dara and Cuan are identical, while the younger pair – nine-year-old Mike and Aobh – are fraternal. (The odds of having two sets of twins are about 50,000 to one.) "I couldn't believe it when they told me we were having another set of twins," she says. "For a long time we lived in a state of total chaos: I had five children under the age of five."
These days, though, the family benefits from so many children across a tight age range – and there's no doubt, says Una, that the bonds between the twins are incredibly close. "When the older set are watching television I sometimes come into the room and their legs are wrapped round one another," she says. At times, it must be hard to remember where you end and your twin starts: after all, for an identical there was a moment in your history when you really were part of another person.