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What causes left-handedness?

Oliver Stewart, Cobham

Being left-handed is the result of genes and environment. About 50 per cent more males than females are left-handed and 17 per cent of twins are, compared with about 10 per cent in general. The 'vanishing twin' hypothesis suggests that left-handers were originally a twin, but the right-handed foetus failed to develop.

Brain dominance also appears to play a part. Most people are right-handed and have language controlled by the left hemisphere, and most left-handers are the opposite. However, some left-handers have language processing in the left hemisphere, or in both.

The genetic basis for left-handedness is complicated. Even if both parents are left-handed there is only a 26 per cent chance of their child being left-handed. Possessing the 'LRRTM1' gene increases the chances, but only if it is inherited from the father.

But whether you become left-handed or not is also dependent on development. It may be influenced by levels of testosterone or oestrogen during pregnancy, and early experience with holding and throwing things can also affect later hand use. Finally, damage to the right hand can make people left-handed.

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