Handedness

For other senses of this word, see handedness (disambiguation).

Handedness is an attribute of human beings defined by their unequal distribution of fine motor skill between the left and right hands. An individuals who is more dextrous with the right hand are 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 doing any task.

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

Mixed-handedness, also known as cross-dominance, is being able to do different tasks better with different hands. For example, a mixed handed person might write with their right hand but throw a ball with their left hand. However some people often refer to handedness as the hand that a person uses to write, so they don't refer to this type of handedness.

Ambidexterity is extermely rare. An ambidextrous person is able to do any task equally well with either hand.

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

Evolutionary theories

Evolution by natural selection tends to reinforce prevailing behaviours and deselect minority traits. In spite of this, 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 [1]). 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. 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.

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. 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 - many members of the British royal family are left-handed but, in 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 counter-clockwise 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 16% 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.

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 resulted in left hemisphere aberration. If the verbal processing area in the left hemisphere was 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 lefthanders 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. This individual seems to function normally but is the reverse of a normal right-hander; both brain hemispheres are intact, yet verbal processing takes place 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-handeders.

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. 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 that 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 phenomenum 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.

Objections

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.

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 right-handedness is an extremely old phenomenon.

The Inuit 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" depend on the person using those 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. Conversely, Arabic and Hebrew, which go from right to left, would be easier to write with the left hand. Again, "easier" and "harder" are subjective.

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 (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 judgments.