Handedness

Handedness (also referred to as chirality or laterality) is an attribute of humans defined by their unequal distribution of fine motor skill between the left and right hands. An individual who is more dexterous 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 most common. Right-handed people are more dexterous with their right hand when performing a task. A variety of studies suggest that 70-90% of world population are right-handed rather than left-handed or any other form of handedness.


 * Left-handedness is less common than right-handedness. Left-handed people are more dexterous with their left hand when performing a task. A variety of studies suggest that 8–15% of world population are left-handed.


 * 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 left hand but throw a ball more efficiently with their right hand. However, many writers define handedness by the hand used for writing, so mixed-handedness is often neglected.


 * Ambidexterity is exceptionally rare, although it can be learned.  A true ambidextrous person is able to do any task equally well with either hand.  Those who learn it still tend to sway towards their originally dominant hand.

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

Theories of handedness
The prevalence of right-handedness is universal across human cultures, although the percentage of right-handedness is less in primitive cultures (Previc, 1991). Newer theories of handedness look at handedness in different ways than previously. 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 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 and grips the paper and provides the context from which the right hand operates. Thus the right hand appears specialized for finer movements and the left for broader, contextual movements.

Sociological proofs
Evolution by natural selection is asserted 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.
 * handedness is inextricably linked to some other cost/benefit expressed in inherited traits.

This theory is explored in a 2004 study by Faurie and Raymond. 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 is argued 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, and it should be borne in mind that correlation does not necessarily indicate causation.

Brain hemisphere division of labour
Division of labour 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. Also, if all functions were carried out in both hemispheres, the size of the brain and its energy consumption would increase, which is not affordable. 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.

Objections

 * It does not explain why the left hemisphere would always control language.
 * "Approximately 95 percent of right-handed individuals primarily process speech in the brain's left hemisphere." "More than half of left-handers process speech in their left hemisphere, just like right-handers. However, about one fourth of left-handers process speech equally in both hemispheres." 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.
 * In primates and even sheep and canines, brain lateralization has been found (e.g., right hemisphere dominance for face processing).

Advantage in sports
The advantage to players in one-on-one sports such as tennis, boxing, fencing or judo is that in a population containing perhaps 10% left-handers and 90% right-handers, the left-hander plays 90% of his or her games against right-handed opponents and is well practiced at dealing with this asymmetry. The right-hander plays 90% of their games against other right-handers — thus when confronted with a left-hander is less practiced (see Rafael Nadal). When a left-hander plays another left-hander, they are both likely to be at the same level of practice as when right-handers play other right-handers. This explains why a disproportionately high number of left-handers are found in sports where direct one-on-one action predominates.

Other, sport-specific factors may increase or decrease the advantage left-handers usually hold in one-on-one situations:
 * In cricket, the overall advantage of a bowler's left-handedness exceeds that resulting from experience alone: Even disregarding the experience factor (i.e., even for a batter whose experience against left-handed bowlers equals his experience against right-handed bowlers), a left-handed bowler challenges the average (i.e., right-handed) batsman more than does a right-handed bowler because the angle of a bowler's delivery to an opposite-handed batsman is much more penetrating than that of a bowler to a same-handed batsman (see Wasim Akram).
 * In baseball, left-handedness gives a pitcher an advantage over batters smaller than the advantage one would expect if one's calculations used experience as the only variable, but it confers yet other advantages that, depending on other aspects of a game scenario, may or may not bring the overall advantage back in line with expectations:
 * All else being equal (i.e., in a hypothetical situation where batters' experience against left-handed pitchers is equal to their experience against right-handed pitchers), a left-handed pitcher would challenge the average (i.e., right-handed) batter less than does a right-handed pitcher because an opposite-handed pitcher's curve ball is easier to hit than is a same-handed pitcher's. (Because a) a curve ball breaks toward the side of a pitcher opposite his pitching hand and b) a curve ball breaking toward a batter is easier to hit than one breaking away from him, right-handed batters find left-handed pitchers' curve balls less difficult than right-handed pitchers' to hit.)
 * However, because a left-handed pitcher faces first base when throwing from the stretch position, whereas a right-hander throwing from "the stretch" has his back to first and instead faces third base, left-handed pitchers are generally better than right-handers at noticing and picking off baserunners attempting to steal second base. This advantage substantially outweighs the corresponding advantage that right-handers enjoy at picking off runners attempting to steal home plate:  Although a runner's successfully stealing home does far more damage than does his stealing second, the expected benefit of an attempt to steal home (averaged across all situations featuring runners on third base) remains less than that of stealing second (averaged across all situations with a runner on first) because the catcher needs not throw to second base, the baserunner must avoid the batter's swing, and the swinging batter must avoid the runner.  Moreover, the resulting rarity of attempts to steal home further diminish the advantage a right-hander enjoys.
 * Whether the sum of a left-hander's advantages exceed the sum of a right-hander's advantages depends on the game scenario, which includes not only the handedness of the pitcher, that of the batter, and the baserunner situation but also innumerable other factors including the game score, the specific hitting abilities of the batter in question, and the handedness, specific abilities, and lineup order of the opposing team's other players.

Left-handed batters have a slightly shorter run from the batter's box to first base than right-handers. This gives left-handers a small advantage in beating throws to first base on infield ground balls.

Left-handed throwers are generally at a disadvantage to right-handers when playing the infield positions; the exception being first base. A left-handed thrower playing second base, third base or shortstop would need to pivot before making a throw to first base, thereby losing time and possibly accuracy on the throw. Conversely, a left-handed first baseman would not need to pivot when throwing to second or third base, as a right-handed first baseman would.

Few left-handed throwers have successfully played the catcher position (although in his youth, Babe Ruth, who threw left-handed, once played catcher for the St. Mary's Industrial Home's baseball team). Many attribute this to a perceived tendency for a left-hander's throw to "tail" in a manner not commonly associated with right-handed throwers, which presumably puts a left-handed catcher at a disadvantage when attempting to throw out would-be base stealers.

In sports in which one competitor's performance does not affect another's (except indirectly through subjectively perceived psychological pressure), a particular hand preference confers little or no advantage. Golf and miniature golf feature occasional situations when obstacles on one side of the ball but not the other interfere with the stance and/or swing of a right- or left-handed player but not the other's. Even so, the "favoritism" on any given course is likely minimal, especially at high levels of play:  A layperson such as the owner of a small miniature golf business may when placing obstacles assess the results from only his/her own-"handed" perspective, such that more courses would be made difficult for right-handers than for left-handers. However, a thoughtful designer — especially a professional in the field — will likely ensure game balance by making sure to add handedness-specific obstacles in equal numbers and in places of similar tactical importance.

Advantage in combat
A variant of the above argument says that left-handed people have an advantage in combat, because combatants would encounter left-handed opponents less frequently. This tactic is well-known to striking combat sport fighters, and was employed to world-record effect in a boxing match on November 4, 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 Faurie 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 undeveloped 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.

In neither of the previous two theories is the origin of handedness explained, nor is the prevalence of right-handedness. The data collected from studies of this type is also highly subject to observer bias.

Left-handed swordsmen were eagerly sought for castle invasions. Spiral staircases and towers spiral clockwise going up. Usually this meant that the (predominantly right-handed) defenders up the stairs had a good angle to swing swords and other weapons down at attackers. Conversely, right-handed attackers would find the weapons constantly colliding with the central pillar of the stairs. Hence the desire for left-handed attackers.

The Bible (Judges 3:12–4:1) includes the story of Ehud, an Israelite judge who exploits his left-handedness in successfully assassinating an oppressive king.

Other general advantages
Some studies have shown that "...left-handers also tend to have unusually good visual-spatial skills and the ability to imagine spatial layouts." Santrock goes on to point out that mathematicians, musicians, architects, and artists are more commonly left-handers than would be expected. "Also, in one study of more than 100,000 students taking the Scholastic Aptitude Test (SAT), 20% of the top-scoring group was left-handed, twice the rate of left-handedness found in the general population (10%). Left-handedness may also reduce the risk of developing arthritis. However, Hardyck and Petrinovich reviewed a large literature and found no overall differences in mental ability in right-handers and nonright-handers.

Possible disadvantages in learning
Although there is little association with children's school performance in regards to handedness, some studies have shown problems in language development in left-handers. Research has shown left-handers are more likely to have problems with reading and they also "...don't do as well on phonology (the sound system of language) tasks..." when compared with right-handers. Also, in left-to-right languages, as the left-handed writer moves their left hand across the place where they have just written, smudging may occur, though this was mainly a concern in older technologies such as fountain pen usage. This situation is reversed in right-to-left languages such as Hebrew and Arabic.

Biological theories
There is strong evidence that prenatal testosterone and estrogen contributes to brain organization. In a study endorsed by the Centers for Disease Control (CDC), it is suggested that men who were prenatally exposed to diethylstilbestrol (a synthetic estrogen based fertility drug), are more likely to be left handed.

Increased pre-natal estrogen exposure in men, and its left-handed effect, may enduce lower visual-spatial skills.

Some other studies have been done that show the possibility of handedness occurring as early as in the womb which would indicate a biological process. For example, in one study, ultrasound observations of fetal thumb sucking showed that nine of ten fetuses were more likely to be sucking their right hand's thumb. Newborns also show a preference for one side of their body over the other. In one study, 65 percent of the infants turned their head to the right when they were lying on their back in a crib. Fifteen percent preferred to face toward the left and the remaining 20 percent showed no preference. These preferences for the right or the left were linked with handedness later in development.

Birth stress
The "pathological left-hander" theory's basic premise is that left-handedness is due to brain damage during the birth process. Some statistics support this theory, yet there is no hard evidence and genetic causes are thought to be more important. Difficult or stressful births happen far more commonly among babies who grow up to be left-handed or ambidextrous.

Prenatal vestibular asymmetry
Previc reviewed a large literature that points to the role of prenatal positioning during the final trimester and subsequent birth position as affecting handedness outcome, with about two-thirds of fetuses presenting with their left occiput at birth. This partly explains why prematurity results in a decrease in right-handedness. Previc argues that asymmetric prenatal positioning creates asymmetric stimulation of the vestibular system, which is important for the development of handedness. In fact, every major disorder with reduced right-handedness is associated with either vestibular abnormalities or delay, and asymmetry of the vestibular cortex is strongly correlated with the direction of handedness.

Ultrasound
A popular theory is that ultrasound may affect the brains of unborn children, causing higher rates of left-handedness in children whose mothers received ultrasounds during pregnancy. Research on this topic suggests there may exist a weak association between ultrasound screening (sonography used to check on the healthy development of the fetus and mother during pregnancy) and non-right handedness.

Is left-handedness genetic?
In 2007, researchers discovered LRRTM1, the first gene linked to increased odds of being left-handed. The researchers also found evidence that possessing one particular form of this gene slightly raises the risk of psychotic mental illnesses such as schizophrenia. However handedness is not inherited from parents in a simple way. Even when both parents are left-handed, there is only a 26% chance of their child being left-handed.

This rate of incidence is high enough that when members of the same family exhibit left-handedness by chance, it can look as though the trait is inherited. For instance, many members of the British royal family are left-handed, and their fame has led to observance of possibly inherited left-handedness. When a powerful family exhibits left-handedness they do not feel the same pressure to comply to the norm, and may instead glorify their difference, leading to a reverse discrimination. One of the many myths of left-handedness involves the genetics of the Clan Kerr. 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.

The Right Shift theory of handedness postulates a gene (RS) responsible for left cerebral hemisphere dominance, which would shift the probability distribution for handed R-L (right minus left) skill to the right; in the absence of the gene hemisphere dominance and handedness would be independently chosen at random. In other words, people with RS+ genotype would have left hemisphere dominance and this would increase the probability of right-handedness, while RS- would have both hemisphere dominance and handedness independently determined at random. The theory was developed from studies on aphasic families, and is consistent with the fact that monozygotic and dizygotic twins have the same pattern (RR,LR,LL) probabilities of left-handedness despite having different genetic sharing.

The second type of left-hander is the natural or genetic left-hander. Such persons function normally but are more likely to process language (at least in part) in the right hemisphere.

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. So far there is no clear explanation why humans are left-brained for verbal processing by default. Different modes of information-processing were studied in a joint project at Yale University and Ohio State University. Handedness and facility with intuitive and rationalistic modes of information processing was compared. No correlation was found.

Parental and societal pressure
This theory explains right-handed dominance by claiming that since people are mostly right-handed, parental pressure essentially teaches this behavior as normal. In this way, the right-handed dominance continues. This idea assumes that environmental pressures can dominate over a genetic tendency because the percentage of left-handed people has remained virtually unchanged. There are recent studies that indicate no heredity involvement in handedness. On the other hand, however, "...in another study, the handedness of adopted children was not related to the handedness of their adoptive parents, but it was related to the handedness of their biological parents." This may disprove the idea of "teaching" handedness by modeling parental behavior, but it is obvious that more research needs to be done in this area.

Social stigma and repression of left-handedness
Left handed people live in a world dominated by right-handed people, and many tools and procedures are designed to facilitate use by right-handed people, often without even realising difficulties placed on the left-handed."For centuries, left-handers have suffered unfair discrimination in a world designed for right-handers." However, as well as inconvenience, left-handed people have been considered unlucky or even malicious for their difference by the right-handed majority.

In many European languages, including English, the word for the direction "right" also means "correct" or "proper". Throughout history, being left-handed was considered negative. The Latin word sinistra meant "left" as well as "unlucky" and this double meaning survives in European derivatives of Latin, and in the English word "sinister.'

There are many negative connotations associated with the phrase "left-handed": clumsy, awkward, unlucky, insincere, sinister, malicious, and so on. A "left-handed compliment" is considered one that is unflattering or dismissive in meaning. In French, gauche means both "left" and "awkward" or "clumsy", while droit(e) (cognate to English direct) means both "right" and "straight", as well as "law" and the legal sense of "right". The name "Dexter" derives from the Latin for "right", as does the word "dexterity" meaning manual skill. 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.

Black magic is sometimes referred to as the "left-hand path".

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 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.

Left-to-right alphabets can be written smudge-free and in proper "forward slant" with the left hand if the paper is turned 45 degrees to the right. This prevents the painful bent-wrist "crab hand" often seen in left-handed writers, and it permits a clear view of what has already been written on the current line. It is also possible to do calligraphy in this posture with the left hand, but using right-handed pen nibs. Otherwise, left-handed pen nibs are required in order to get the thick to thin stroke shapes correct for most "fonts", and the left-handed calligrapher is very likely to smudge the text. Left-handed pen nibs are not generally easy to find, and strokes may have to be done backwards from traditional right-handed calligraphic work rules to avoid nib jamming and splatter. These issues have been made almost irrelevant by the near-universal adoption of fast-drying ballpoint and gel pens for everyday use (pen nibs are now a specialty item rarely stocked by office suppliers), and the widespread use of computers and other electronic devices for communicative purposes.

In many South Asian countries it is encouraged to use the right hand for acts like eating and drinking. The use of the left hand is usually seen while cleaning oneself after urination or defecation.