Handedness (also referred to as chirality or laterality) is a human attribute defined by unequal distribution of fine motor skills 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:
No one knows for certain why the human population is right-hand-dominant, but a number of theories have been proposed.
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The prevalence of right-handedness is universal across human cultures, although the percentage of right-handedness is smaller in primitive cultures[4] (Previc, 1991). Newer theories of handedness look at handedness in different ways than previously.[5][6] 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.
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:
This theory is explored in a 2004 study by Faurie and Raymond.[7] 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.
Division of labor 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 labor.
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. Right-handers play 90% of their games against other right-handers. Thus, when confronted with left-handers, they are less practiced (see Rafael Nadal). When two left-handers compete against each other, 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 in which direct one-on-one action predominates.
Other, sports-specific factors may increase or decrease the advantage left-handers usually hold in one-on-one situations:
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.
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.[9]
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.
Spiral staircases and towers in castles typically spiral clockwise going up. This was meant to ensure 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, left-handed attackers gained an advantage.
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.
Some studies have shown that "...left-handers also tend to have unusually good visual-spatial skills and the ability to imagine spatial layouts."[8] Santrock goes on to point out that mathematicians, musicians, architects, and artists are more commonly left-handers than would be expected.[8] "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%).[8] Left-handedness may also reduce the risk of developing arthritis.[8] However, Hardyck and Petrinovich[10] reviewed a large literature and found no overall differences in mental ability in right-handers and nonright-handers.
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.[8] 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.[8] 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.
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.[11]
Increased pre-natal estrogen exposure in men, and its left-handed effect, may enduce lower visual-spatial skills.[12]
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.[8]
The "pathological left-hander" theory's basic premise is that left-handedness is due to brain damage during the birth process.[13] Some statistics support this theory, yet there is no hard evidence and genetic causes are thought to be more important.[14] Difficult or stressful births happen far more commonly among babies who grow up to be left-handed or ambidextrous.
Previc[4] 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[4] 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,[15] and asymmetry of the vestibular cortex is strongly correlated with the direction of handedness.[16]
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.[17][18]
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.[19][20] 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.[21] 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.[22]
The Right Shift theory[23] 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.[24][25]
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.[8] 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."[8] This may disprove the idea of "teaching" handedness by modeling parental behavior, but more research needs to be done in this area to provide stronger evidence.
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."[8] 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 Hinduism, usage of left hand for most of the ceremonial activities is strictly limited. Orthodox Hindus avoid giving or receiving objects of worth with left hand. They strictly discourage people from using left hand while having food. Usage of either right hand or both hands is often observed. (A or possibly the reason for usage of the left hand is because Orthodox Hindus use their left hand when "using a bathroom", to clean themselves, opposed to using paper towels.)
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| Side | Left | Both | Right |
|---|---|---|---|
| General | Ambidexterity | ||
| In cognitive abilities | Geschwind–Galaburda hypothesis | ||
| In brain | Brain asymmetry · Dual brain theory · Bicameralism | ||
| In eyes | Ocular dominance | ||
| In hands | Left-handedness | Cross-dominance | Right-handedness |
| Handedness in boxing | Southpaw stance | Orthodox stance | |
| Handedness in people | Musicians · US presidents | ||
| Handedness related to | Sex · Maths | ||
| Handedness measurement | Edinburgh Handedness Inventory | ||
| Handedness genetics | LRRTM1 | ||
| In heart | Levocardia | Dextrocardia | |
| In major viscera | Situs solitus | Situs ambiguus | Situs inversus |
| In feet | Footedness | ||