Handedness

Handedness is an attribute of human beings defined by their unequal distribution of fine motor skill between the left and right hands. An individual who is more dextrous with the right hand is called right-handed, and one who is more skilled with the left is said to be left-handed. A minority of people are equally skilled with both hands, and are termed ambidextrous. People who demonstrate awkwardness with both hands are said to be ambilevous or ambisinister. Ambisinistrous motor skills or a low level of dexterity may be the result of a debilitating physical condition. There are four main types of handedness:


 * Right-handedness is the most common. Right-handed people are more dextrous with their right hands when performing a task.


 * Left-handedness is less common than right-handedness. Left-handed people are more dextrous with their left hands when performing a task.


 * Mixed-handedness, also known as cross-dominance, is being able to do different tasks better with different hands. For example, mixed-handed persons might write better with their right hand but throw a ball more efficiently with their left hand.  This could also refer to pianists, because different sections of pieces are usually composed to fit the abilities of the different hands, considering that the right hand is usually for melodies and the left-hand, harmonies.  However, some people often refer to handedness as the hand that a person uses to write, so this form of handedness is not usually referred to.


 * Ambidexterity is extremely rare. A true ambidextrous person is able to do any task equally well with either hand. The condition is very rare, although it can be learned (those who master it in the end tend to sway towards their originally dominant hand, of course).

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

Theories of Handedness
Newer theories of handedness look at handedness in different ways than previously (see Guiard, Y. (1987). Asymmetric division of labor in human skilled bimanual action: The kinematic chain as a model.  Journal of Motor Behavior, 19(4), 486-517, and http://www.billbuxton.com/2hCHI94.html). The newer view is that handedness is not a simple preference for one hand, because the two hands actually work together in more subtle ways. For example, when writing on a piece of paper, it is not a simple matter of one hand being dominant and writing on the paper. For a right-handed person, the left hand is involved in important ways: it orients the paper and provides the context from which the right hand operates.

The principles by which the hands operate together (assuming a right-handed person) are:

1. The right hand operates in the context set by the left hand.

2. The motion is left to right. For example, the left hand grips the paper, and the right hand moves the pen.

3. The granularity of action is greater in the left than in the right. Thus the right hand is specialized for finer movements and the left for broader, contextual movements.

Evolutionary theories
Evolution by natural selection tends to reinforce prevailing behaviors and deselect minority traits (unless the minority traits are linked in some way with desirable traits). However, all human populations continue to 'maintain' a minority of left-handers. The implications are that:
 * any disadvantages associated with the minority trait (an increased likelihood of contracting certain diseases, for instance) are outweighed by a benefit to the left-handed individual.
 * there is some sort of frequency dependent cost/benefit of being left or right handed according to the relative frequency of each type in the population

This theory is explored in a study (Faurie & Raymond, 2004 ). The researchers complement ethnographic data with a discussion of the success of left-handers in certain sports, to demonstrate that left-handed individuals have a competitive advantage in combat. The rate of left-handedness appears to correlate with the amount of violence in a given society (taking homicide rates as a measure). It follows that the minority left-handed population has, historically, played a crucial role in the evolution of individual societies. The counter-conclusion - that increased violence in a society generates a larger left-handed population - is not, however, borne out by the researchers.

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.
 * It does not explain why there would be either right- or left-handedness to begin with.

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.

Brain hemisphere division of labor
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. 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.

Advantage in Hand-to-Hand Combat
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 and was employed to world-record effect on Nov. 4th 1947 when Mike Collins, a natural left-hander, emerged from his corner in a right-handed stance before suddenly shifting left and delivering the fight's first and last punch, knocking out opponent Pat Brownson in 4 seconds.)

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. To prove their theory, Faurie and Raymond surveyed nine primitive societies in five separate continents. Through a mix of direct observation and existing data, they estimated the number of left-handed people within each population. They also looked at murder rates, thinking that those communities with higher murder rates might favor populations with more left-handed people, if left-handedness is a trait associated with greater fitness with regard to combat. Among these samples, they found strong support for the idea that, at least in primitive societies with higher levels of violence, left-handed people are more numerous. Like the previous theory, the origin of handedness is not explained, nor is the prevalence of right-handedness among people.

Biological theories
There is strong evidence that prenatal testosterone contributes to brain organization. One theory is that high levels of prenatal testosterone results in a higher incidence of left-handedness.This could be why there are more left-handed males than females. See Geschwind-Galaburda Hypothesis.

Birth stress
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:
 * 1) 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.
 * 2) 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 that 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 that scanning produced an extra three left-handed babies per one-hundred births. This research has come under some criticism, since it implies that left-handed people are in some way "brain damaged."

Is left-handedness genetic?
Handedness runs in families - many members of the British royal family are left-handed but, even though left-handedness seems to be prevalent in some families, except for a small fraction of lefthanders, left-handedness is not "genetic." One of the many myths of left-handedness involves genetics and the Kerr family; the predominantly left-handed Kerr noblemen of the Scottish Borders built fortified homes with counterclockwise spiral staircases, so that left-handed swordsmen would be better able to defend them (but perhaps at the same time making it easier for right-handed swordsmen to attack them). However, a 1993 study found no statistically significant increase in left-handedness among people with the family name Kerr or Carr.

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 are not the only cause. Handedness must also be influenced by some of the other theories presented here.

The least controversial etiology of left-handedness is that of the pathological left-hander. Left-handers comprise almost 20% of the mentally retarded population and 28% of the severely and profoundly mentally retarded population. It is believed these individuals are both retarded and left-handed for the same reason: a massive insult to their left hemisphere as a result of a prenatal or postnatal event. It is also possible that a nutritional insult results in left hemisphere aberration. If the verbal processing area in the left hemisphere is damaged even partially, early in life, the right hemisphere would assume verbal processing functions, along with other hemispheric functions. This would account for the left-handers who process verbal material in their right hemisphere and, depending upon the severity of the brain damage, would also account for the higher proportion of left-handers found in the retarded population. There is no genetic component to this type of left-handedness.

The second type of left-hander is the natural or genetic lefthander. Such persons function normally but are more likely to process language (at least in part) in the right hemisphere. Because many of these left-handers are part of a monozygotic twin pair, the situs inversus phenomenon (mirror-image twinning) is probably involved. It has been suggested that when a left-hander is born as a singleton the conception was originally a monozygotic twin set, but one twin, the right-handed one, died and was reabsorbed in utero. This is plausible, but highly speculative. It has also been suggested that there is a genetic component to monozygotic twinning and, if this is true, it could provide a genetic basis for a small proportion of left-handers.

The third type of left-hander is the learned left-hander. This left-hander writes with the left hand but has relatively poor handwriting, and shows dual hemispheric activation during verbal processing. Because preverbal children are not lateralized for hand use, these left-handers may have initially chanced to successfully manipulate some toy with their left hand and continued to use their left hand for toy manipulation. When eventually given a pencil or crayon, because of past reinforcement, they employ their left hand, and continue to use their left hand when they write even when they may be naturally right-handed. This, of course, is quite inefficient neurologically, as described above, and because of the additional processing time required, may be the reason quite a few left-handers stutter when they are young and have notoriously poor handwriting. It is believed that eventually these left-handers develop verbal processing function in their right hemisphere too, and that these individuals become the left-handers who naturally show dual hemispheric activation during verbal processing.

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.) The reason for this seemingly contradictory situation is that improved medical technology has allowed more fetuses to survive through stressful births whereas in the past they would have been stillborn.

It does not explain why humans are left-brained for verbal processing by default, although this is almost certainly a developmental phenomenon similar to the fact that the vast majority of humans have a pair of feet, legs, hands, eyes, etc., and one nose, mouth, heart, liver, etc.

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.

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-handed dominated societies. There isn't word about one so far.
 * Objections:

Social stigma and repression of left-handedness
In many European languages, including English, the word for "right" (in a directional sense) also means "correct" or "proper". Throughout history, being left-handed was considered as negative - the Latin word sinistra (from which the English word "sinister" was derived, along with various Romance language derivatives) meant "left" as well as "unlucky". There are many negative connotations associated with the phrase "left-handed": clumsy, awkward, unlucky, insincere, sinister, malicious, and so on. The French word gauche means both "left" and "awkward" or "clumsy", whereas the French word droit (cognate with the English word "adroit") means both "right" and "dextrous". As these are all very old words, they would tend to support theories indicating that the predominance of right-handedness is an extremely old phenomenon.

The Inuit believed that every left-handed person 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). In general, it is considered more difficult to write legible Chinese characters with the left hand than it is to write Latin letters, though difficulty is subjective and depends on the person in question. Because writing when moving one's hand away from its side of the body can cause smudging if the outward side of the hand is allowed to drag across the writing, it is considered easier to write the Latin alphabet with the right hand than with the left. Conversely, right-to-left alphabets such as the Arabic and Hebrew are considered easier to write with the left hand in general.

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 hypothesized that some sun worshipers have grown to associate their left sides with evil, since people facing north would see the sun set 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 direction judgments.