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The science of sinistrality

Left-handers are a minority, but are not rare. A recent survey showed that almost one child in eight in Britain is left-handed and, in some schools, nearly one in four.

One reason for the variation is that few of us favour the left or right hand exclusively. Most babies shift their preference once or twice during their first year, and some children take most of their pre-school years, or even longer, to make up their minds. It's not a question of making a conscious choice, as left-handedness is merely the most obvious aspect of laterality - a characteristic of the human brain which means that the left and right hemispheres have different specialities, and that one hemisphere dominates the other. Scientists believe it was the development of laterality, around 137,000 years ago, that enabled us to acquire language and so make the leap from Homo erectus to Homo sapiens.

The 19th century French pathologist and anthropologist Paul Broca first identified the third convolution of the left frontal lobe as being mostly responsible for speech. It soon became apparent that other functions also tended to be controlled by one or other of the hemispheres.

Most people have a left or right prefernce, not only with respect to hands, but also for other parts of the body. A telescope or viewfinder will usually be held to one eye, and one foot will lead when we jump over an obstacle.

While these preferences are clearly connected to the lateral division of the brain highlighted by Broca, the relationship is far from simple. The dominant hand is not always on the same side as the dominant foot, and about one-third of left-handed, and three-quarters of right-handed, people are right-eyed.

From studies of people who have suffered injury to one side of the brain, the left hemisphere is dominant for language in around 99 per cent of right-handers, but two-thirds of left-handed and ambidextrous people also have left-hemisphere language. In fact, scientists can point to every combination of sidedness and dominance.

And while left-brain language and right-handedness often go together; and many people who have dyslexia are left-handed, no single set-up has been shown to be inherently superior or inferior in every case.

Many would argue that different combinations confer different types of ability - an aptitude for numbers, for example, or enhanced powers of creativity.

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