Left-handed

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 write with their right hand use their left for everything else.

Statistics of left-handedness

Approximately 10-13% 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, one twin of a pair has a 20% chance to be left-handed. According to one study, homosexuals may be 39% more likely to be left-handed than heterosexuals (Blanchard, Lalumiere and Zucker, 2000).

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 labor. 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.

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. (In a sense, it has increased because more people see left-handedness as the benign trait it is.)
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.
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 left-handeds dominated societies. There isn't word about one so far.
Social stigma and repression of left-handedness

Many European languages (including English) use the same word for "right" (in a directional sense) to mean "correct, proper". Throughout history, being left-handed was considered as negative — the Latin and Italian word sinistra (from which the English 'sinister' was derived) means "left". There are many negative connotations associated with the word "left-handed": clumsy, awkward, unlucky, insincere, sinister, malicious, and so on. French gauche, meaning "left", means "awkward or clumsy" in English, whereas French droit is cognate with English "adroit", meaning dexterous, skillful with the hands. As these are all very old words, they would tend to support theories indicating that the predominance of righthandedness is an extremely old phenomenon.

The Eskimos believed that every lefty was a sorcerer. A Japanese man could divorce his wife if he discovered that she was left-handed. 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.

Until very recently in Taiwan, left-handed people were strongly encouraged to switch to being right-handed (or at least, switch to writing with the right hand). It is more difficult to write legible Chinese characters with the left hand than it is to write Latin letters. Remember that "easy" and "difficult" are subjective terms, so your writing may be neater. Because it is supposedly easier to write when moving your hand towards its side of the body, it is easier to write the Roman alphabet with your right hand than with your left.

Some theorize that those languages that have a written language from right to left, such as Arabic and Hebrew, are derived from
cultures where the first writing was chisled by hand in stone, where right-handed masons would write from right to left. Under this theory, cultures where the first written words were on some form of paper use left to right. This is not universally accepted, however.

Until the latter part of the twentieth century, Roman Catholic nuns in American elementary schools (and possibly elsewhere) 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 interpreted as a sign of Satanic influence, and thus prohibited.

It has been hypothesised that some sun worshipersers have grown to associate their left sides with evil, since people facing north would see the sun set (disappear) on their left. The evidence for this is very weak, however, as the opposite conclusion can be drawn when one considers a person facing south (the opposite direction). It has been suggested that there may be a preference for northern hemisphere dwellers to face the fixed north star (i.e., north) when making directions judgements.

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".