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Handedness 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 hands when performing a task.


  • Left-handedness is less common than right-handedness. Left-handed people are more dexterous with their left hands when performing a task. About 8–15% of people 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

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 specialised 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 labor

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.

Objections

  • It does not explain why the left hemisphere would always control language. Why not 50% of the population left and 50% right?
  • "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, 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. 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. In other sports such as golf, this advantage is not present since the one-on-one action is indirect; the handedness of one player has no effect on the other. In cricket, having a left handed bowler gives more of a challenge to right handed batsmen because the angle of the delivery is much more penetrating than a bowler who has the same handedness (see Wasim Akram).

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 IImarker 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.

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-hander 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 a successful plot to assassinate an oppressive alien ruler.

Other general advantages

Some studies have shown that, "...left-handers also tend to have unususally good visual-spatial skills and the ability to imagine spatial layouts." Santrock goes on to point out that more 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.

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 that 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.

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 and also the increased incidence of left-handedness in male twins. See Geschwind–Galaburda hypothesis.This has been contradicted by Elkadi, Shaymaa et al in their study where they have shown that there was no difference between the opposite and same-sex twins for either sex in the measures of the strength of hand preference and the incidence of sinistrality. (ref:Handedness in opposite and same-sex dizygotic twins: testing the testosterone hypothesis

Elkadi, Shaymaa; R. Nicholls, Michael E.; Clode, Danielle:NeuroReport Issue: Volume 10(2), 5 February 1999, p 333–336.

Some other interesting 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.


Environmental theories

Birth stress

This theory's 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.

Medical technology and obstetrics has failed to lower the proportion of left handed people, supporting the idea that routine obstetrical care is stressful to the neonate. Routine obstetric care in a hospital setting for low-risk women has not been shown to improve outcomes but has been shown to increase the use of obstetrical interventions during the birth process.Common routine interventions that may contribute to a baby's stress during birth include non-indicated induction of labor, pharmacological forms of pain relief, restriction of mother's mobility, restriction of mother's food and drink, administration of antibiotics during labor, continuous electronic fetal monitoring, artificial rupture of membranes, directed pushing or pushing in the supine position, immoderate use of forceps delivery or vacuum extraction, elective or excess cesarean section, restriction of access to vaginal birth after cesarean section, early clamping of the umbilical cord, separation of mother and baby, and non-emergency early newborn procedures.

Ultrasound

A popular theory is that ultrasound scans may affect the brain of unborn children, causing higher rates of left-handedness in mothers who have ultrasound scans compared to those who do not. This is probably based on a few studies where this relation is studied. In one of these the authors claim:

…we found a possible association between routine ultrasonography in utero and subsequent non-right handedness among children in primary school.


However later in the same article the authors state that "Thus the association … may be due to chance," and:

the result was not significant, suggesting that the study had insufficient statistical power to resolve the relationship between ultrasonography and subsequent left handedness in the child.


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 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 that in these individuals, both their left-handedness and their retardation are caused by brain damage 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 early in life, even partially, 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 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 Universitymarker and Ohio State Universitymarker. 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 as 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."

Left-handedness is still strongly discouraged in some cultures. In the Maswai culture in Africa, "almost 90 percent of teachers and parents said that if children show a left-handed tendency they should be forced to change to right-handedness." They believe that, "left-handers are less skilled and powerful."

Until very recently in Taiwanmarker, left-handed people were strongly encouraged to switch to being right-handed, or at least switch to writing with the right hand. Indeed, the words right/left (正/倒 ) in Taiwanese Hokkien also mean correct/backwards. 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 1/4 turn clockwise (90 degrees to the right), and the left hand is drawn toward the body on forward strokes, and left to right on upward strokes (as expressed in directionality of the text). 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.

In Christianity, right and left are taken up as agents of symbolism. Christian Scripture depicts Jesus sitting at the "right hand" of God; sin is depicted as deviation to the "left hand side" of Christ.

In Islam it is encouraged to use the right hand for acts considered good, like eating and drinking. The use of the left hand is encouraged when the act is considered base, such as cleaning oneself after urination or defecation. This is attested in various Hadiths by the Prophet Muhammad.

See also



Sources

  1. Faurie & Raymond, 2004 http://www.pubs.royalsoc.ac.uk/proc_bio_home_link_6.shtml
  2. Johnson, K.; Daviss, B. (2005). “Outcomes of planned home births with certified professional midwives: large prospective study in North America” British Medical Journal 2005;330:1416 (18 June), doi:10.1136/bmj.330.7505.1416. http://www.bmj.com/cgi/content/abstract/330/7505/1416
  3. Sakala, C.; Corry, M. (2008). Evidence-Based Maternity Care: What It Is and What It Can Achieve. Milbank Memorial Fund. http://www.childbirthconnection.org/pdfs/evidence-based-maternity-care.pdf
  4. Enkin, M.; Keirse, M.; Neilson, J.; Crowther, C.; Duley, L.; Hodnett, E; Hofmeyr, J. (2000). A Guide to Effective Care in Pregnancy and Childbirth. Oxford University Press. http://www.childbirthconnection.org/article.asp?ClickedLink=194&ck=10218&area=2
  5. Francks et al. Molecular Psychiatry (2007) 12: 1129-1139
  6. Gene for left-handedness is found , http://news.bbc.co.uk/2/hi/health/6923577.stm, BBC, 31 July 2007
  7. BBC - Science & Nature - Sex ID - Handedness
  8. AJ Giannini, J Daood, MC Giannini, R Boniface, PG Rhodes. Intellect versus intuitiion--a dichotomy in the reception of nonverbal communication. Journal of General Psychology (1978). 99:19-24,1978.
  9. AJ Giannini,ME Barringer,MC Giannini, RH Loiselle. Lack of relationship between handedness and intuitive and intellectual (retionalistic) modes of information processing. Journal of General Psychology(1984). 111:31-37.
  10. Santrock, John W.(2008). Motor, Sensory, and Perceptual Development. Mike Ryan [Ed.], A Topical Approach to Life-Span Development(pgs.172-205). Boston, MA: McGraw-Hill Higher Education.


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