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Handedness is a better (faster or more precise) performance or individual preference for use of a hand, known as the dominant hand. Handedness is not a discrete variable (right or left), but a continuous one that can be expressed at levels between strong left and strong right.

There are four types of handedness: left-handedness, right-handedness, mixed-handedness, and ambidexterity. Left-handedness is somewhat more common among men than among women.

Types

 * Right-handedness is most common. Right-handed people are more skillful with their right hands when performing tasks. Studies suggest that 88–92% of the world population is right-handed.
 * Left-handedness is less common than right-handedness. Left-handed people are more skillful with their left hands when performing tasks. Studies suggest that approximately 10% of the world population is left-handed.
 * Cross-dominance or Mixed-handedness is the change of hand preference between tasks. This is common in the population with about a 1% prevalence.
 * Ambidexterity is exceptionally rare, although it can be learned. A truly ambidextrous person is able to do any task equally well with either hand. Those who learn it still tend to favor their originally dominant hand.
 * Ambilevous or ambisinister people demonstrate awkwardness with both hands. Ambisinistrous motor skills or a low level of dexterity may be the result of a debilitating physical condition, such as "dysgraphia".

Developmental theories of handedness
There are several theories of how handedness develops in individual humans. Occurrences during prenatal development may be important; researchers studied fetuses in utero and determined that handedness in the womb was a very accurate predictor of handedness after birth. In a 2013 study, 39% of infants (6 to 14 months) and 97% of toddlers (18 to 24 months) demonstrated a hand preference.

Division of labor
One common theory, as to how handedness affects the hemispheres, is the brain hemisphere division of labor. Since speaking and handiwork require fine motor skills, its presumption is that it would be more efficient to have one brain hemisphere do both, rather than having it divided up. Since in most people, the left side of the brain controls speaking, right-handedness predominates. This theory also predicts that left-handed people have a reversed brain division of labor.

Verbal processing in right-handed individuals takes place mostly in the left hemisphere, whereas visuospatial processing is mostly done in the opposite hemisphere. Left-handed individuals have a heterogeneous brain organization in which their brain hemisphere is either organized in the same way as right-handers (but with the hemispheres reversed) or even such that both hemispheres are used for verbal processing. When the average is taken across all types of left-handedness, it shows that left-handers are less lateralized.

Genetic factors
Handedness displays a complex inheritance pattern. For example, if both parents of a child are left-handed, there is a 26% chance of that child being left-handed. A large study of twins from 25,732 families by Medland et al. (2006) has indicated that the heritability of handedness is roughly 24%.

To date, two theoretical single gene models have been proposed to explain the patterns of inheritance of handedness, the first by Marian Annett of the University of Leicester and the second by Professor Chris McManus of UCL.

However, the growing weight of evidence from linkage and genome-wide association studies suggests that genetic variance in handedness cannot be explained by a single genetic locus. From these studies McManus et al. now conclude that handedness is polygenic and estimate that at least 40 loci contribute to determining this trait.

Brandler et al. performed a genome-wide association study for a measure of relative hand skill and found that genes involved in the determination of left/right asymmetry in the body play a key role in determining handedness. These results suggests the same mechanisms that determine left/right asymmetry in the body (e.g. Nodal signaling and ciliogenesis) also play a role in the development of brain asymmetry (handedness is an outward reflection of brain asymmetry for motor function).

Prenatal hormone exposure
A 2003 study endorsed by the Centers for Disease Control determine that males with in-utero exposure to diethylstilbestrol (a synthetic estrogen-based fertility drug) were more likely to be left-handed over the clinical control group. Diethylstilbestrol animal studies "suggest that estrogen affects the developing brain, including the part that governs sexual behavior and right and left dominance".

A 2005 meta-analysis found no evidence to suggest testosterone exposure during development effects handedness in humans.

Seasonal Anisotropy
There is an amount of evidence exemplified in a meta-analysis performed Jones and Martin 2008 that suggests the season in which an individual is born may have an effect on their handedness later in life. This effect was estimated to be somewhere between an increase of 8-11% in frequency of non-right-handedness in males. The study found specifically that males born in the spring months (March-July) had a higher frequency of NRH (non-right-handedness) The causal determination of this effect is controversial and, indeterminate when discussing whether Seasonal Anisotropy is calendric or climatic. It is assumed that whether or not the effect is climatic or calendric, that this effect is moderated by maternal metabolism and hormone exposure.

Prenatal vestibular asymmetry
Previc, after reviewing a large number of studies, found evidence that the position of the fetus in the final trimester and a baby's subsequent birth position can affect handedness. About two-thirds of fetuses present with their left occiput (back of the head) 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 involved in the development of handedness. In fact, every major disorder in which patients show 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
Another 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.

Life expectancy
Although scientific papers published in 1989 and 1991 claimed that the life expectancy of left-handed people was nine years less than that of right-handed people, these findings were quickly discredited. The authors had examined the handedness and age of the recently deceased in California, and failed to allow for the fact that older people there were less likely than younger people to identify as left-handed.

Intelligence
In his book Right-Hand, Left-Hand, Chris McManus of University College London argues that the proportion of left-handers is increasing and left-handed people as a group have historically produced an above-average quota of high achievers. He says that left-handers' brains are structured differently (in a way that increases their range of abilities) and the genes that determine left-handedness also govern development of the language centers of the brain.

Writing in Scientific American, McManus states that,"Studies in the U.K., U.S. and Australia have revealed that left-handed people differ from right-handers by only one IQ point, which is not noteworthy ... Left-handers’ brains are structured differently from right-handers’ in ways that can allow them to process language, spatial relations and emotions in more diverse and potentially creative ways. Also, a slightly larger number of left-handers than right-handers are especially gifted in music and math. A study of musicians in professional orchestras found a significantly greater proportion of talented left-handers, even among those who played instruments that seem designed for right-handers, such as violins. Similarly, studies of adolescents who took tests to assess mathematical giftedness found many more left-handers in the population."According to a 2014 study published in the Harvard Gazette, right-handed people have higher cognitive skills, fewer behavioral and speech problems, less often have learning disabilities, and more often graduate their school. This effect is further corroborated by findings from a meta-analysis 16,076 individuals, which specifically found that individuals with intellectual disabilities had signifigantly higher chances of non-right-handedness. Researchers further found that in intellectually gifted individuals, there was no signifigant association with non-right-handedness.

Early childhood intelligence
Nelson, Campbell, and Michel studied infants and whether developing handedness during infancy correlated with language abilities in toddlers. In the article they assessed 38 infants and followed them through to 12 months and then again once they became toddlers from 18–24 months. What they discovered was that when a child developed a consistent use of its right or left hand during infancy (such as using the right hand to put the pacifier back in, or grasping random objects with the left hand), it was more likely to have superior language skills as a toddler. Children who became lateral later than infancy (i.e., when they were toddlers) showed normal development of language and had typical language scores. The researchers used Bayley scales of infant and toddler development to assess all the subjects.

Birth-Weight
Lower-birth-weight and complications at birth are positively correlated with left-handness. Pre-Term births also were found to correlate with non-right-handedness. Researchers found through a 30-year meta-analysis that pre-term births resulted in a two-fold increase in the odds of the child being non-right-handed.

Immune System Function
One systematic review concluded: "Left-handers showed no systematic tendency to suffer from disorders of the immune system".

Deafness
A 2016 meta-analysis of deaf individuals found that deafness was associated with having non-right-hand dominance. Specifically, deaf individuals were found to have a incidence of non-right-handedness 2.61 times higher than the average individual.

Autism
A 2014 study on lateralization (specifically inferior frontal gyrus measures) in individuals with autism found a "moderate but non-signifigant effect size" in reference to autistic individuals tendency towards non-normative brain lateralization and further non-right-handedness.

Schizophrenia
An excess of non-right-handedness in schizophrenia is demonstrated as an empirical effect attributed to the abnormal brain lateralization and not as an artefact of gender. This efffect was found in multiple meta-analyses. Lack of lateralization was found more frequently in individuals with schizophrenia, however; schizophrenic patients who presented with auditory hallucinations were found even more frequently to have abnormal lateralization than other patients.

Dyslexia
A 1994 meta-analysis found a increased frequency of non-right-handed individuals in populations of individuals with dyslexia.

Laterality of Parkinson's Disease Symptoms
Parkinson's Disease presents, usually, with a asymmetry of symptoms. A 2011 meta-analysis by van der Hoon Et. Al. found that handedness has an effect on the directionality of the asymmetry. Authors found that in right-handed Parkinson's patients, '59.5% had right-dominant PD symptoms, and 40.5 had left-dominant PD symptoms. For Left-handed PD patients, this was effect was reversed with 40.8% having right-dominant symptoms and 59.2 having left-dominant symptoms.

Persistence of Left-handedness as a Function of Fitness
These health problems mean left-handness should be eliminated through natural selection. However, left-handers enjoy an advantage in fighting and sports increasing their likelihood of reproduction.

In Twins

 * A 1999 meta-analysis found that twins had a higher frequency of non-right-handedness. There was no effect between monozygotic and dizygotic twins in handedness. Between twins, there was found to be a concordance of hand dominance.

Income
In a 2006 U.S. study, researchers from Lafayette College and Johns Hopkins University concluded that there was no scientifically significant correlation between handedness and earnings for the general population, but among college-educated people, left-handers earned 10 to 15% more than their right-handed counterparts.

More recently, in a 2014 study published by the National Bureau of Economic Research, Harvard economist Joshua Goodman finds that left-handed people earn 10 to 12 percent less over the course of their lives than right-handed people. Goodman attributes this disparity to higher rates of emotional and behavioral problems in left-handed people.

Left-handedness and sports
Interactive sports such as table tennis, badminton, cricket, and tennis have an overrepresentation of left-handedness, while non-interactive sports such as swimming show no overrepresentation. Smaller physical distance between participants increases the overrepresentation. In fencing, about half the participants are left-handed.

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: 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 or swing of a right- or left-handed player but not the other's. Even so, the "favoritism" on any given course is probably 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 the perspective of his or her handedness, 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—is likely to ensure game balance by adding handedness-specific obstacles in equal numbers and in places of similar tactical importance.
 * 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 batsman 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 a right-handed bowler does, 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, a right-handed pitcher's curve ball will break away from a right-handed batter and towards a left-handed batter. Historical batting averages show that left-handed batters have a slight advantage over right-handed batters when facing right-handed pitchers. Because there are fewer left-handed pitchers than right-handed pitchers, left-handed batters have more opportunities to face right-handed pitchers than their right-handed counterparts have against left-handed pitchers. Fourteen of the top twenty career batting averages in Major League Baseball history have been posted by left-handed batters. Left-handed batters have a slightly shorter run from the batter's box to first base than right-handers. This gives left-handers a slight advantage in beating throws to first base on infield ground balls.
 * Because a left-handed pitcher faces first base when he is in position to throw to the batter, whereas a right-handed pitcher has his back to first base, a left-handed pitcher has an advantage when attempting to pickoff baserunners at first base.
 * Defensively in baseball, left-handedness is considered an advantage for first basemen because they are better suited to fielding balls hit in the gap between first and second base, and because they do not have to pivot their body around before throwing the ball to another infielder. For the same reason, the other infielder's positions are seen as being advantageous to right-handed throwers. Historically, there have been few left-handed catchers because of the perceived disadvantage a left-handed catcher would have in making the throw to third base, especially with a right-handed hitter at the plate. A left-handed catcher would have a potentially more dangerous time tagging out a baserunner trying to score. With the ball in the glove on the right hand, a left-handed catcher would have to turn his body to the left to tag a runner. In doing so, he can lose the opportunity to brace himself for an impending collision. The lack of left-handed catchers might be due to traditions.
 * In four wall handball, typical strategy is to play along the left wall forcing the opponent to use their left hand to counter the attack and playing into the strength of a left handed competitor.
 * In water polo the centre forward position has an advantage in turning to shoot on net when rotating the reverse direction as expected by the centre of the opposition defence and gain an improved position to score.

Gender Ratio and Handedness
Left-handedness is more common among men than women. It is estimated that left-handedness is around 23% more common among men than women. A 2001 meta-analysis of sex differences in laterality suggests that increased hemispheric specialization may lead to differences in laterality by sex.

Sexuality and gender identity
A number of studies examining the relationship between handedness and sexual orientation have reported that a disproportionate minority of homosexual people exhibit non-right-handedness, though finds are mixed.

A 2001 study also found that children who were born male but have non-traditional gender identities were more than twice as likely to be left-handed than a clinical control group (19.5% vs. 8.3%, respectively). A 2000 meta-analysis of 16,423 individuals (14,808 men and 1,615 women) found that homosexual individuals had aroud a 39% increase of frequency of non-right-handedness.

Paraphilias (atypical sexual interests) have also been linked to higher rates of non-right-handedness. A 2008 study analyzing the sexual fantasies of 200 males found "elevated paraphilic interests were correlated with elevated non-right handedness." Greater rates of non-right-handedness has also been documented among individuals who have pedophilia.

A 2014 study attempting to analyze the biological markers of asexuality asserts that non-sexual men and women were 2.4 and 2.5 times, respectively, more likely to be non-right-handed than their heterosexual counterparts.

Social stigma and repression of left-handedness
Many tools and procedures are designed to facilitate use by right-handed people, often without even realizing difficulties placed on the left-handed. John W. Santrock has written, "For centuries, left-handers have suffered unfair discrimination in a world designed for right-handers."

McManus noted that, beginning at the time of the Industrial Revolution, workers needed to operate complex machines that were almost certainly designed with right-handers in mind. This would have made left-handers more visible and at the same time appear less capable and more clumsy. During this era, children were taught to write with a dip pen. While a right-hander could smoothly drag the pen across paper from left to right, a dip pen could not easily be pushed across by the left hand without digging into the paper and making blots and stains.

Moreover, apart from 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 adjective sinister means "left" as well as "unlucky", and this double meaning survives in European derivatives of Latin, including the English word "sinister" (only when referring to the bearer's left of a coat of arms).

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 and related to "adroit") 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 (and still in Mainland China), left-handed people were strongly encouraged to switch to being right-handed, or at least switch to writing with the right hand. Due to the importance of stroke order, developed for the comfortable use of right-handed people, 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 writer. Because writing when moving one's hand away from its side towards the other side of the body can cause smudging if the outward side of the hand is allowed to drag across the writing, writing in the Latin alphabet might possibly be less feasible with the left hand than the right under certain circumstances. Conversely, right-to-left alphabets, such as the Arabic and Hebrew, are generally considered easier to write with the left hand in general. Depending on the position and inclination of the writing paper, and the writing method, the left-handed writer can write as neatly and efficiently or as messily and slowly as right-handed writers. Usually the left-handed child needs to be taught how to write correctly with the left hand, since discovering a comfortable left-handed writing method on one's own may not be straightforward.

International Left-Handers Day
International Left-Handers Day is held annually every August 13. It was founded by the Left-Handers Club in 1992, with the club itself having been founded in 1990. International Left-Handers Day is, according to the club, "an annual event when left-handers everywhere can celebrate their sinistrality [meaning left-handedness] and increase public awareness of the advantages and disadvantages of being left-handed." It celebrates their uniqueness and differences, who are from seven to ten percent of the world's population. Thousands of left-handed people in today's society have to adapt to use right handed tools and objects. Again according to the club, "in the U.K. alone there were over 20 regional events to mark the day in 2001- including left-v-right sports matches, a left-handed tea party, pubs using left-handed corkscrews where patrons drank and played pub games with the left hand only, and nationwide 'Lefty Zones' where left-handers' creativity, adaptability and sporting prowess were celebrated, whilst right-handers were encouraged to try out everyday left-handed objects to see just how awkward it can feel using the wrong equipment!"

In animals
Kangaroos and other macropod marsupials have a left-hand preference for everyday tasks in the wild. ‘True’ handedness is unexpected in marsupials because, unlike placental mammals, they lack a corpus callosum. Left-handedness was particularly apparent in the red kangaroo (Macropus rufus) and the eastern gray kangaroo (Macropus giganteus). Red-necked (Bennett’s) wallabies (Macropus rufogriseus) preferentially use their left hand for behaviours that involve fine manipulation, but the right for behaviours that require more physical strength. There was less evidence for handedness in arboreal species. Studies of dogs, horses, and domestic cats have shown that females of those species tend to be right handed, while males are lefties.

Great Apes
A 2012 meta-analysis of observational and behavioral data on handedness in Great Apes found that through observational data, there was a signifigant level of right-handedness in Bonobos, Chimpanzees, and Gorillas. Through experimental analysis, however; right-handedness persisted only in the Chimpanzee group. Other data has suggested that handed-ness in Great Apes is associated with maternal familial lines.