Left-handed

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People who are left-handed are more dextrous with their left hand than with their right hand: they will probably also use their left hand for tasks such as personal care, cooking, and so on. Writing is not as good an indicator of handedness as it might seem, because many people who are left-handed write with their right hand and use their left for everything else.

Statistics of left-handedness

Approximately 9–10% of the population is left-handed. People who can use both hands equally well are ambidextrous. True ambidexterity is rare. See laterality.

Generally, there are about 10% more male left handers than female left handers. Statistically, the twin of a left-handed person has a 20% chance of being left-handed.

Causes of left-handedness

No one knows for certain why the human population is right-handed-dominant, but a number of theories have been proposed.

Evolutionary theories

The warrior and his shield

This theory attempts to explain left-handedness by the position of a warrior's shield and his heart. Basically, since the heart is on the left side of the body, a right-handed warrior (who holds his shield with his left hand to free the right hand for a weapon) would be better able to protect his heart and therefore more likely to survive.

There are a number of objections to this theory:

The heart is not that far off center. Protecting it with a shield would result in a weak selective pressure.
There have not been enough generations since the Bronze Age to make a difference.
Analysis of ancient cave paintings indicate that humanity was right-handed long before the Bronze Age.
Some believe it predicts that more men would be right-handed than women. However, data indicates that more males are left-handed than females. This objection also demonstrates a misunderstanding of heredity. The theory would predict that fewer left-handed males would survive but says absolutely nothing about ratios of male:female left-handedness after that time. Nor does it explain why there would be either right-or left-handedness to begin with.

Advantage in fist-fighting

A variant of the above argument says that left-handed people have an advantage in fighting without weapons, because of the "surprise" factor. (This fact is well known to boxers.)

A 2004 study by Charlotte Fauriet and Michel Raymond of the University of Montpellier II in France, published in the Proceedings of the Royal Society, argues that there is such a link. The researchers hypothesize that left-handed inheritance is likely to be associated with violence, because violent left-handed people would be more likely to benefit from the advantage in fighting. They found a positive correlation between murder rates and percentage of left-handed people in several traditional societies: The more left-handed people, the higher the homicide rate.

The researchers argue that left-handed people are not more violent than right-handed, but that violent left-handed people are more likely to be successful than violent right-handed people[1].

Brain hemisphere division of labour

This is the most commonly accepted theory of handedness. The premise of this theory is that since both speaking and handiwork require fine motor skills, having one hemisphere of the brain do both would be more efficient than having it divided up. And since in most people, the left side of the brain controls speaking, right-handedness would prevail. It also predicts that left-handed people would have a reversed brain division of labour. Lastly, since other primates do not have a spoken language (at least of the type we have) there would be no stimulus for right-handed preference among them, and that is true.

Objections:

It does not explain why the left hemisphere would always be the one controlling language. Why not 50% of the population left and 50% right?
While 95% of right-handers do indeed use the left side of the brain for speaking, it is more variable for left-handers. Some do use the right for linguistic skills, some use the left hemisphere, and others use both.

On the balance, it appears that this theory could well explain some left-handedness, but it has too many gaps to explain all left-handedness.

Is left-handedness genetic?

Handedness runs in families, although even when both parents are left-handed, there is only a 26% chance of their child being left-handed. Thus, it is clear that genetics is not the only cause. Handedness must also be influenced by some of the other theories presented here.

Apparently, the Clan Kerr of Scotland built their castles with counter-clockwise staircases, so that a left-handed swordsman would be better able to defend it. However, a 1993 study found no statistically significant increase in left-handedness among people with the family name Kerr or Carr.

Many members of the British royal family are left-handed. Genetics is usually used to explain this.

Biological theories

There is strong evidence that prenatal testosterone contributes to brain organisation. One theory is that high levels of prenatal testosterone results in a higher incidence of left-handedness.

Environmental theories

Birth stress

Left-handed people cringe at this theory, because its basic premise is that left-handedness is due to brain damage during the birth process. Some statistics support this theory.

Difficult or stressful births happen far more commonly among babies who grow up to be left-handed or ambidextrous. Birth stress is also associated with a number of birth defects and complications, including cerebral palsy and autism.

There are objections to the birth stress theory:

Throughout history and throughout the world, the level of medicine and technology to assist with childbirth has improved. In spite of that, the proportion of left-handed people has not decreased.
It does not explain why humans are right-handed by default, with only birth stress making them left-handed. It could, however, explain left-handedness in combination with some of the other theories presented here.

Ultrasound

Scientists in Sweden say they have found evidence ultrasound scans may cause brain changes in unborn babies after they found men whose mothers had tests were more likely to be left-handed. The study suggested scanning produced an extra three left-handed babies per 100 births. This research has come under some criticism since it implies that left-handed people are in some way "brain damaged".

Parental pressure

This theory explains right-handed dominance by claiming that since the parents who raised us are mostly right-handed, we came to be mostly right-handed and so on.

Objections:

It does not explain how right-handed dominance started in the first place.
The handedness of children is more closely related to their biological parents than to adoptive parents.
It does not explain why left-handedness has persisted for so long.
This theory predicts the existence of isolated societies dominated by left-handed people, although none have been discovered.

Social stigma and repression of left-handedness

Linguistic suggestion

Some left-handed people consider themselves oppressed, even to the point of prejudice. Etymology often lends weight to the argument:

In many European languages, "right" (German: recht; French: droit) stands for authority and justice; and being right-handed is being skillful: the Latin word for right-handed is dexter, as in dexterity; the Spanish derecha also means right.

On the other hand, the English word "sinister" comes from Latin, where it means simply "left". The modern Italian sinistra has both meanings of sinister and left. The Spanish siniestra has both, too, although the second meaning is less common. A left-hander was supposed not only to be unlucky, but also awkward and clumsy, as shown in the French gauche and German links. As these are all very old words, they support theories indicating that the predominance of right-handedness is an extremely old phenomenon.

In ancient China, the left has been the "bad" side. The adjective "left" (左 Mandarin: zuo) means "improper", "out of accord"; for instance, the phrase "left path" (左道 mandarin: zuodao) stands for illegal or immoral means.

In some parts of China, some adults can still remember for the "crime" (with suitable traumatic punishments) of not learning to be right-handed in both primary and secondary schools, as well as in some "Keeping-good-face" families.

Even the word "ambidexterity" reflects the bias. Its intended meaning is "skillful at both sides". However, since it keeps the Latin root dext, which means right, it ends up conveying the idea of being "right-handed at both sides".

Daily suppression

In many parts of the world, right-handed people consider it impolite to eat with the left hand. Until the latter part of the twentieth century, Roman Catholic nuns in American elementary schools (and possibly elsewhere, for example Dutch primary schools) would punish children for using their left hand to write, typically by slapping their left hand with a ruler if they attempted to pick up a pen with it.

Left-handedness was often interpreted as a sign of Satanic influence, and thus prohibited. The Eskimos also believed that every left-handed person was a sorcerer.

The use of left hand was also frowned upon in Asia. Allegedly, though there were few examples of its happening, a Japanese man could once divorce his wife if he discovered that she was left-handed.

Until very recently in Chinese societies, left-handed people were strongly encouraged to switch to being right-handed. However, this may be in part because, while Latin characters are equally easy to write with either hand, it is more difficult to write legible Chinese characters with the left hand. The prescribed direction of writing each line of a Chinese character is designed for the movements of the right hand, and some shapes tend to feel awkward to follow with the left hand's fingers. It results in a less soft writing than it would be with the right hand.

Until recently left-handed children in parts of Africa would have their left hand covered in boiling water. The hand was then buried in mud until all the nerves were killed, thus forcing the child to be right handed.

Despite the suppression, there have been, however, many famous left-handed people, and the associated right brain hemisphere that is said to be more active in left-handed people has been found in some circumstances to be associated with genius and is correlated with artistic and visual skill. As visual thinking is much promoted nowadays, left-handers cannot help but begin to gain more and more respect. As well, in certain fields, left-handedness is advantageous; for example, in baseball, it is commonly known that a left-handed pitcher is harder to to hit against successfully.

Left-handedness and health

A 2005 study by researches at the University Medical Center in the Netherlands found that left-handed women were more than twice as likely to develop breast cancer before going through menopause. No reason is known; researchers speculated that it may come about because left-handed women were exposed to higher levels of certain hormones in the womb, which may also explain their handedness.

Left-handedness and intelligence

A common belief exists to suggest that left-handed people are more intelligent or creative than right-handed people; yet the theory is largely unfounded.

"Disappearing" Left-Handers

Statistics show that older people are less likely to be left-handed than their younger counterparts — the percentages of lefties sharply drop off with increased age. In America, 12% of 20 year olds are left-handed, while only 5% of 50 year olds and less than 1% of people over 80 are. These numbers are surprisingly divergent — how can they be explained?

A study published in 1991 claimed that these statistics indicate that left handed peoples' lifespans are shorter than those of their right-handed counterparts by as much as 9 years. They explained this gap by asserting that left-handed people are more likely to die in accidents as a result of their "affliction," which renders them clumsier and ill-equipped to survive in a right-handed world.

This theory is in contention, as some researchers now attribute the different percentages among different age groups to the fact that older people would be more likely to have experienced pressure to switch hands, a factor not affecting the younger generations. This is supported by the fact that more women than men switched hands, and women live longer than men. However, this reasoning cannot explain all variation, and "the case of the disappearing southpaws" remains a mystery. Presently all explanations are incomplete and speculative, including such ideas that eventually lefties succumb to conformist pressures, or a "biological imperative" grips them late in life and they lose their left-handedness.

It could also be attributed to the fact that the percentage of left handed children born has been increasing over time.

Left-sidedness

In humans

Studies show that left-handedness does not necessarily correspond with "left-sidedness" (using your left foot to kick with, for example). The same thing holds with "eyedness".

In animals

Most primates also exhibit a preference for using one hand over the other although their populations are not right-hand preferential.

Source

External links

Left-handed – ambidexterity – Right-handed

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