By Michael Corballis 18/08/2016


What’s special about twins?

I was recently asked to comment on a video clip of identical twin girls, who seemed to be thinking in parallel. They finished each other’s sentence, and sometimes seemed to gives the same simultaneous answers to questions, as though they communicated through telepathy. The clip was widely shown on television. Actually, it is not so unusual; if you Google “twins communicating telepathically” you can find lots of examples, with claims of psychic powers somehow special to twins.

I’m afraid I was not impressed. We humans in general seem adept at what is called theory of mind, knowing what others are thinking or feeling. This is what underlies empathy, social understanding, and even language itself. It is not uncommon for close pairs—husbands and wives, brothers and sisters, midfield rugby players—to know what’s in the other’s mind. They too finish each other’s sentences, sometimes to the despair of the person who spoke first. This is sustained by subtle cues and well-worn habits. None of it is evidence of telepathy.

Twin questions

Twins may well develop a heightened sense of what the other is thinking, especially if they have become inseparable. They are often confused for one another, treated as duplicates rather than separate individuals. They develop stereotyped answers to questions they are often asked: Do you think alike? Do you like doing the same things? Are your personalities different? And being twins, they probably do share traits more than non-twin siblings do. Some twins no doubt develop tricks to reinforce the idea that they communicate telepathically. And why not? It’s a good way to attract attention and publicity.

To some, being an identical twin may be a bit stifling, perhaps causing them to drift apart. I have identical-twin brothers, one living in Dunedin and the other in Sussex. They are on good terms with each other, but when one visits the other, the domiciled one runs the risk of being seen to be unfriendly. Even though they live far apart, they are still very much alike. They both took up painting in their 50s, apparently independently, but this may reflect no more than shared talent. It must be said, though, that their styles of painting are rather different. But telepathy? Nah.

While we’re at it, let’s look at another myth about twins. Some identical twins are so-called mirror twins, one right-handed and the other left-handed—like Tweedledum and Tweedledee. It’s sometimes suggested that the process of twinning caused the embryo to split into mirrored halves, like an apple cut down the middle. But mirror twin are seldom mirrored in ways other than handedness. A study of over 50,000 twin families showed no evidence of mirroring, with twins about as likely to be left-handed as the singly born. Some 23 percent of identical twins are mirror twins, a statistic which might misleadingly suggest that left-handedness is more likely among twins than among singletons. But this is not so. Very few twins are both lefties, so if you gauge the number of left-handers among twins it’s not much less than the proportion of left-handers in the general population.

Mirroring

We can take the proportion of left-handers in the general population to be about 12.5 percent, or about one in eight. Then if you take any two individuals at random, the binomial theorem says that probability of just one of them being left-handed is 14/64, or .219, while the probability that both are left-handed is 1/64, or .016. These values pretty much match those in identical twins. There’s nothing very special about left-handedness in twins, then.

Another form of mirroring is situs inversus, where internal organs are left-right reversed, so that the heart, for example is round the wrong way and displaced to the right rather than the left. Could this somehow explain mirroring in twins? No. Situs inversus afflicts only about 1 person in 10,000, and is no more common in identical twins than in the singly born.

How about brain asymmetry? Most twins mirrored for handedness are not mirrored with respect to brain asymmetry. Our own studies show that the left-handed member of a pair with opposite handedness is generally left-brained for language, just as the right-handed member is. Among left-handers in general, the incidence of left-brain dominance for language is about 75 percent, whereas among righties the incidence is around 95 percent. These figure apply as well to twins as to singletons.

So there’s nothing especially remarkable about a twin, except that she looks very much like her identical twin sister and may share some talents and personality traits. She may communicate with her sister a bit differently from the way she communicates with others, but she’s not telepathic.

As a person, she’s pretty much like anyone else.


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