The new novel "Sisterland" by best-selling author Curtis Sittenfeld tells the story of twin girls who have a psychic connection. Though it's a fictional story, it draws upon a common belief that twins share some sort of unexplained mental or spiritual connection. It's an intriguing idea, but what's the truth behind it?
About one out of every 30 babies born in the United States is a twin. Identical twins are of particular interest to scientific researchers because they help control for genetic variation between patients in studies. It's not surprising that identical twins share much in common, since they have the same DNA. Scientists know that what makes us who we are - including, to some degree, our intelligence, personal preferences, and so on - is part nature and part nurture, a combination of heredity and environment.
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Stories about mysterious connections between twins - and especially identical twins - are ancient and legion. Some say twins share some psychic connection, or even feel each other's pain. There are stories of twins who happened to give birth to baby daughters on the same day, completely by chance, and twins who were separated at birth but reunited years or decades later to find that they both married spouses with the same name and drive identical cars. Psychic powers have never been scientifically proven to exist, and psychic twins are no exception. However as we move way from science toward anecdotal evidence, we find intriguing parallels between twins.
Playing the Odds While such stories, when confirmed, are amazing and intriguing, it's important to remember that such reports are anecdotes cherry-picked and highlighted specifically because they are examples of remarkable coincidence. Just by random chance, some pairs of siblings - twins or not - will end up leading lives that are parallel in some significant ways. For example many mental and physical diseases run in families, so of course they may struggle with similar health issues.
Consider the likelihood that twins who were separated at birth - or perhaps estranged for years - may end up independently marrying a lawyer in his mid-40s named Steve. At first glance that may seem very unlikely to happen just by chance; surely there's some psychic or spiritual connection that guided the women to similar mates?
However a closer look at statistics shows that while such a coincidence is incredibly unlikely for any particular set of twins, it is actually quite likely for some set of twins somewhere. The same principle applies to lottery winners: The chance of any given person winning the lottery is incredibly small, perhaps hundreds of millions to one. But the chance of some person winning the lottery is 100 percent.
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There are finite probable pools of significant characteristics in all our lives. There are only so many boy's names, only so many careers, and so on. The likelihood of any woman, twin or not, marrying a lawyer named Steve - or Stephen, or Stefan, etc. - is far higher than the likelihood of that same woman marrying a professional magician named Adolph. People also tend to marry within their racial and socioeconomic class, further narrowing the pool of likely mates.
Other hidden factors also contribute to these otherwise extraordinary coincidences. For example, American male twins in their 40s are more likely to be married to a woman named Jennifer than to a woman of any other name. This has nothing to do with the men being twins, because the same is true for non-twin men in their 40s. Nor does it have anything to do with any supernatural bias or attraction to women named Jennifer. Instead, it has everything to do with the pool of available women; Jennifer was the single most popular name for newborn American girls for every year between 1970 and 1984. More Jennifers in the population means that more men born around that same time will marry Jennifers than Joans or Jills.
Are Twin Coincidence Stories Unusual?
The stories of twin coincidences that make the news - and are collected in books about twins and unexplained phenomena - are, of course, the most dramatic ones. Family lore about identical twins in different states unknowingly each buying a used 1992 Honda with a bad clutch don't make the cut. There are only a few dozen of these remarkable, seemingly inexplicable stories collected over the years, but for the sake of argument let's round it up to 100.
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By one estimate there are about 100 million twins worldwide, and the number of twins being born is on the rise, due in part to the increased use of fertility drugs. So that means that over 99 percent of twins have not experienced these sorts of dramatic events and amazing coincidences. If the "twin connection" is as strong and common as often claimed - and cannot be explained merely by chance or genetic-based lifestyle choices and preferences - then we should reasonably expect millions of these amazing accounts, not a few dozen or a few hundred.
There is also the problem of what scientific researchers call selection bias. Is the rate of these amazing coincidences really any higher among twins than other siblings? There's no way to know because there have been no studies that have taken the considerable time and effort to comb through hundreds of millions of siblings to find out if two non-twin sisters or brothers ended up marrying partners with identical (or near-identical) names, careers, and lifestyles. In other words, nobody is interested because it's only a story. And it only fuels the "special-twin connection" mythology when the two people involved happen to be twins.
There's no doubt that the personal bonds between twins - and especially identical twins - can be strong, but there's no evidence that it's mysterious or unexplained.