Who has the upper hand?

Five of the last seven American presidents have been left-handed. Gerald Rudolph Ford, Ronald Reagan, George H.W. Bush the elder, Bill Clinton and now Barack Obama.

So what does this mean in a world where only one out of every ten people, roughly speaking, is a lefty?

The answer is ... nobody knows. It may be a fluke.

But even if it isn't, exactly what left-handedness has to do with skill, intelligence, popularity, wealth and luck is almost certainly a matter of subtle advantage, not dramatic benefit.

What is clear is that “handedness'' runs all through the animal world.

Once thought to be uniquely human, some version of this attribute has been seen in chimpanzees, marmosets, cats, chicken, toads, mice, rats and almost certainly thousands of other species.

It is present in animals that don't have hands (fish) and in some that don't have backbones (honeybees).

Links with the brain

In biology, this phenomenon is known as “lateralisation''. It is the preference for doing or perceiving things more with one side of the body than the other.

It appears to be an important — although perhaps not necessary — consequence of having a brain.

Like many structures in the body, the brain is “bilaterally symmetrical''. It is made up of two hemispheres divided by a plane that makes one the mirror image of the other.

Lateralisation saves space and, therefore, working capacity, by not requiring that both hemispheres do the same thing.

It diminishes the chance of interference and confusion, which might arise if each side of the brain independently analysed the same input from the environment and came up with its own decisions about what to do about it.

It also allows the brain to sometimes do two things at once.
“Any brain seems to lateralise if it can,'' said Lesley Rogers, a researcher in the field who is an emerita professor at University of New England in Australia.

The brain's asymmetry is primarily in function, not structure (although careful measurement shows certain regions are bigger on one side than the other in nearly everyone). The most dramatic example involves language.

The ability to produce and comprehend language emanates from the left side of the brain in more than 95 per cent of right-handed people and in about 70 per cent of left-handed ones.

That difference is a big clue that the neural wiring — and perhaps more subtle things — may be different in lefties.

Curiously, there is no anatomical “home'' for handedness the way there is for language. For most voluntary movement, however, each side of the brain controls the opposite side of the body.

A part of the right side of the brain drives the left arm and leg, and vice versa.

Similarly, sensation, including vision, that is perceived on one side of the body is projected to the opposite side of the brain for processing.

What determines whether a person is right- or left-handed is not really known, although it seems to be a combination of genes, environment and culture.

A more basic question is: Why does handedness exist?
Left-handed people are more likely to die from accidents.

They may also be more likely to have neurological, immunological or psychiatric diseases — probably because at least some left-handedness arises from prenatal damage or birth trauma.

Whether left-handed people have shorter lives is in dispute.

Element of surprise

Despite those “costs'', left-handedness has persisted through human evolution. One reason: Lefties do better in fights.

A 2004 study by Charlotte Faurie and Michel Raymond, of University Montpellier II, in France, found that among eight traditional societies, those with the highest homicide rates had the highest proportion of left-handed people.

For example, among the Eipo people in the Indonesian province of Irian Jaya, where the homicide rate is 3 per 1,000 people per year, 20 per cent of people are left-handed.

Among the Dioula people of Burkina Faso, where the homicide rate is 0.01, 3 per cent are left-handed.

This does not mean lefties are more violent. It means that in violent societies, lefties may fare better.

“When it is important in a society to be a winner of a fight, then left-handers have an advantage,'' Faurie said.
Of course, there are limits.

The low prevalence of left-handedness everywhere suggests that the advantage holds only when lefties are rare, preserving their unfamiliarity and the ability to surprise.

Hormone decides side

About 27 per cent of sons of left-handed parents are left-handed, compared with 10 per cent of sons of right-handed parents.

However, about 20 per cent of identical twins have different handedness. So genes count for something but not everything.

Because left-handedness is more common in men than in women, many scientists have speculated that testosterone has something to do with it.

Lots of research has been done (and more is under way) testing this hypothesis.

The best evidence at the moment supports the idea that testosterone may “prune back'' some of the fibres in the corpus callosum, the huge bundle of nerves that connects the right and left halves of the brain across the midline.

That, in turn, may diminish traffic between the two hemispheres, enhancing lateralisation of function.