Identical twins, which occur when a fertilized egg splits into two, often look like spitting images of each other.
Many identical twins — perhaps as many as one in five — claim to share a special psychic connection. About one out of every 30 babies born in the United States is a twin, and identical twins are especially interesting because they have the same genes and are alike in many ways. Brothers and sisters can be close, but some twins claim to know what the other is thinking or feeling. It's an intriguing idea, but what's the truth behind it? Coincidence, psychic powers or something else?
This sort of psychological connection isn't necessarily mysterious, of course: any two people who know each other very well and who have shared many common experiences — including non-twin siblings, old married couples, and even best friends — may complete each other's sentences and have a pretty good idea about what the other person is thinking.
The idea of twin telepathy has been around for well over a century. It appears, for example, in the 1844 Alexandre Dumas novella "The Corsican Brothers." It tells the story of two once-conjoined brothers who were separated at birth yet even as adults continue to share not only thoughts but also physical sensations. As one twin describes, "However far apart we are now we still have one and the same body, so that whatever impression, physical or mental, one of us perceives has its after-effects on the other." The 2013 best-selling novel "Sisterland" by Curtis Sittenfeld also tells the story of twin girls who share a psychic connection.
Most of the evidence for twin telepathy is not scientific but instead anecdotal. For example, in 2009 a British teenager named Gemma Houghton was in her home when she suddenly had a feeling that her fraternal twin sister, Leanne, needed help. "I just got this feeling to check on her, so I went up to the bathroom and she was under the water," she said. Gemma found Leanne in a bathtub, unconscious. She had suffered a seizure and slipped under the water, nearly drowning. Gemma called for help and administered first aid, saving her sister's life.
The story of Gemma and Leanne Houghton has been widely cited as an example of twin telepathy (the phrases "telepathic bond" and "sixth sense" appear in many news stories about them), and was cited by Guy Lyon Playfair, author of a book about twins, as a case where twin telepathy may have saved a life. [Countdown: 8 Fascinating Facts About Twins]
A closer look
However the case, as fortunate as it is, may not be as inexplicable as it first appears. Leanne had suffered many similar fits in the past, and the rest of her family (including her sister) had been warned to keep a watchful eye on her. Knowing that Leanne had a tendency to have seizures that could render her unconscious — and knowing that being alone in water (such as a bathtub, pool, or lake) could be dangerous or fatal for her — it's not surprising that Gemma might have become concerned when after a few minutes she heard no activity (such as splashing water) from the upstairs bathroom where Leanne was bathing alone. No "twin telepathy" is needed to explain this event; it's likely that any other (non-twin) member of the family who was home at the time would have reacted the same way.
Such stories of twin telepathy (or coincidences) that make the news and are discussed in the context of some unexplained phenomena are, of course, the most dramatic ones. Mundane coincidences that everyone experiences aren't newsworthy, and if Leanne's mother (and not her twin sister) had saved her it's unlikely we would have heard about it. By one estimate there are about 100 million twins worldwide, and the fact is that most twins do not report experiencing any sort of mysterious telepathic connection. If some special, psychic connection between twins is as strong and common as often claimed, then by chance alone we should expect millions of these amazing stories, not just a few dozen.
Despite decades of research trying to prove telepathy, there is no credible scientific evidence that psychic powers exist, either in the general population or among twins specifically. The idea that two people who shared their mother's womb — or even who share the same DNA — have a mysterious mental connection is an intriguing one not borne out in science.
Benjamin Radford, M.Ed., is deputy editor of Skeptical Inquirer science magazine and author of seven books including "Scientific Paranormal Investigation: How to Solve Unexplained Mysteries" and "Mysterious New Mexico: Miracles, Magic, and Monsters in the Land of Enchantment," which will be published later this year.