Only one out of every 10 people in the world is a lefty,but 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,chickens,toads,mice,rats and almost certainly thousands of other species. It is present in animals that dont have hands (fish) and in some that dont have backbones (honeybees).
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 importantalthough perhaps not necessaryconsequence of having a brain.
Like many structures in the body,the brain is bilaterally symmetrical. It is made up of two halvescalled hemispheresdivided 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 longtime researcher in the field who is an emerita professor at the University of New England in Australia.
The brains asymmetry is primarily in function,not structure (although careful measurement shows that 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 wiringand perhaps more subtle thingsmay 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. 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 testing this hypothesis.
Left-handed people are more likely to die from accidents. They may also be more likely to have neurological,immunological or psychiatric diseasesprobably because at least some left-handedness arises from prenatal damage or birth trauma. Whether left-handed people have shorter lives on average is in dispute.
Despite those costs,left-handedness has persisted through human evolution. One of the reasons is simple: lefties do better in fights.
A 2004 study by Charlotte Faurie and Michel Raymond,of the 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. This does not mean that 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 ability to surprise.