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Stenciled hands at the Cueva de las Manos in Argentina. Left hands make up over 90% of the artwork, demonstrating the prevalence of right-handedness.[1]
A schoolgirl writing with her left hand

In human biology, handedness is an individual's preferential use of one hand, known as the dominant hand, due to and causing it to be stronger, faster or more dextrous. The other hand, comparatively often the weaker, less dextrous or simply less subjectively preferred, is called the non-dominant hand.[2][3][4] In a study from 1975 on 7,688 children in US grades 1–6, left handers comprised 9.6% of the sample, with 10.5% of male children and 8.7% of female children being left-handed.[5][6][7] Overall, around 90% of people are right-handed.[8] Handedness is often defined by one's writing hand. It is fairly common for people to prefer to do a particular task with a particular hand. Mixed-handed people change hand preference depending on the task.

Not to be confused with handedness, ambidexterity describes having equal ability in both hands. Those who learn it still tend to favor their originally dominant hand. Natural ambidexterity (equal preference of either hand) does exist, but it is rare—most people prefer using one hand for most purposes.

Most research suggests that left-handedness has an epigenetic marker—a combination of genetics, biology and the environment. In some cultures, the use of the left hand can be considered disrespectful. Because the vast majority of the population is right-handed, many devices are designed for use by right-handed people, making their use by left-handed people more difficult.[9] In many countries, left-handed people are or were required to write with their right hands. However, left-handed people have an advantage in sports that involve aiming at a target in an area of an opponent's control, as their opponents are more accustomed to the right-handed majority. As a result, they are over-represented in baseball, tennis, fencing,[10] cricket, boxing,[11][12] and mixed martial arts.[13]

Types

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  • Right-handedness is the most common type. Right-handed people are more skillful with their right hands. Approximately 90% of people are right-handed.[7][14]
  • Left-handedness is less common. Left-handed people are more skillful with their left hands. Studies suggest that approximately 10% of people are left-handed.[7][15]
  • Ambidexterity refers to having equal ability in both hands. Natural ambidexterity is uncommon, with about a 1% prevalence.[16]
  • Mixed-handedness or cross-dominance is the change of hand preference between different tasks. This is about as widespread as left-handedness.[17] This is highly associated with the person's childhood brain development.[18]

Measurement

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Handedness may be measured behaviourally (performance measures) or through questionnaires (preference measures). The Edinburgh Handedness Inventory has been used since 1971 but contains some dated questions and is hard to score. Revisions have been published by Veale[19] and by Williams.[20] The longer Waterloo Handedness Questionnaire is not widely accessible. More recently, the Flinders Handedness Survey (FLANDERS) has been developed.[21]

Evolution

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Handedness has been found in dozens of non-human vertebrates.[22] While data for fish is contentious, handedness has been found in amphibians, reptiles, birds, and various mammals.[22] Some non-human primates have a preferred hand for tasks, but they do not display a strong right-biased preference like modern humans, with individuals equally split between right-handed and left-handed preferences. When exactly a right handed preference developed in the human lineage is unknown, although it is known through various means that Neanderthals had a right-handedness bias like modern humans.[citation needed] Attempts to determine handedness of early humans by analysing the morphology of lithic artefacts have been found to be unreliable.[23]

Causes

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There are several theories of how handedness develops.

Genetic factors

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

Two theoretical single-gene models have been proposed to explain the patterns of inheritance of handedness, by Marian Annett[26] of the University of Leicester, and by Chris McManus[24] of UCL.

However, growing evidence from linkage and genome-wide association studies suggests that genetic variance in handedness cannot be explained by a single genetic locus.[27][28][29][30][31][32][33][excessive citations] From these studies, McManus et al. now conclude that handedness is polygenic and estimate that at least 40 loci contribute to the trait.[34]

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 handedness.[35] Brandler and Paracchini suggest 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 being a reflection of brain asymmetry for motor function).[36]

In 2019, Wiberg et al. performed a genome-wide association study and found that handedness was significantly associated with four loci, three of them in genes encoding proteins involved in brain development.[37]

Prenatal hormone exposure

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Four studies have indicated that individuals who have had in-utero exposure to diethylstilbestrol (a synthetic estrogen-based medication used between 1940 and 1971) 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".[38][39][40][41]

Ultrasound

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Another theory is that ultrasound may sometimes affect the brains of unborn children, causing higher rates of left-handedness in children whose mothers receive ultrasound during pregnancy. Research suggests there may be a weak association between ultrasound screening (sonography used to check the healthy development of the fetus and mother) and left-handedness.[42]

Epigenetic markers

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Twin studies indicate that genetic factors explain 25% of the variance in handedness, and environmental factors the remaining 75%.[43] While the molecular basis of handedness epigenetics is largely unclear, Ocklenburg et al. (2017) found that asymmetric methylation of CpG sites plays a key role for gene expression asymmetries related to handedness.[44][45]

Language dominance

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One common handedness theory is the brain hemisphere division of labor. In most people, the left side of the brain controls speaking. The theory suggests it is more efficient for the brain to divide major tasks between the hemispheres—thus most people may use the non-speaking (right) hemisphere for perception and gross motor skills. As speech is a very complex motor control task, the specialised fine motor areas controlling speech are most efficiently used to also control fine motor movement in the dominant hand. As the right hand is controlled by the left hemisphere (and the left hand is controlled by the right hemisphere) most people are, therefore right-handed. The theory depends on left-handed people having a reversed organisation.[46] However, the majority of left-handers have been found to have left-hemisphere language dominance—just like right-handers.[47][48] Only around 30% of left-handers are not left-hemisphere dominant for language. Some of those have reversed brain organisation, where the verbal processing takes place in the right-hemisphere and visuospatial processing is dominant to the left hemisphere.[49] Others have more ambiguous bilateral organisation, where both hemispheres do parts of typically lateralised functions. When tasks designed to investigate lateralisation (preference for handedness) are averaged across a group of left-handers, the overall effect is that left-handers show the same pattern of data as right-handers, but with a reduced asymmetry.[50] The majority of the evidence comes from literature assessing oral language production and comprehension. When it comes to writing, findings from recent studies were inconclusive for a difference in lateralization for writing between left-handers and right-handers.[51]

Developmental timeline

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Researchers studied fetuses in utero and determined that handedness in the womb was a very accurate predictor of handedness after birth.[52] In a 2013 study, 39% of infants (6 to 14 months) and 97% of toddlers (18 to 24 months) demonstrated a hand preference.[53]

It has been observed that infants fluctuate significantly when choosing which hand to use for grasping and object manipulation tasks, particularly when comparing one-handed and two-handed grasping. Between the ages of 36 and 48 months, there is a significant decline in variability in handedness for one-handed grasping, which can be observed earlier for two-handed manipulation. Children aged 18–36 months showed a stronger hand preference when performing bimanual tasks than when grasping with one hand.[54]

The decrease in handedness variability in children of 36–48 months may be attributable to preschool or kindergarten attendance due to increased single-hand activities such as writing and coloring.[54] Scharoun and Bryden noted that right-handed preference increases with age up to the teenage years.[6]

Correlation with other factors

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The modern turn in handedness research has been toward emphasizing degree rather than direction of handedness as a critical variable.[55]

Intelligence

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Further information: Handedness and mathematical ability and List of musicians who play left-handed

In his book Right-Hand, Left-Hand, Chris McManus of University College London argues that the proportion of left-handers is increasing, and that an above-average quota of high achievers have been left-handed. He says that left-handers' brains are structured in a way that increases their range of abilities, and that the genes that determine left-handedness also govern development of the brain's language centers.[56]

Writing in Scientific American, he states:

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.[57]

Left-handers are overrepresented among those with lower cognitive skills and mental impairments, with those with intellectual disability being roughly twice as likely to be left-handed, as well as generally lower cognitive and non-cognitive abilities amongst left-handed children.[58] Conversely, left-handers are also overrepresented in high IQ societies, such as Mensa. A 2005 study found that "approximately 20% of the members of Mensa are lefthanded, double the proportion in most general populations".[59]

Ghayas & Adil (2007) found that left-handers were significantly more likely to perform better on intelligence tests than right-handers and that right-handers also took more time to complete the tests.[60] In a systematic review and meta-analysis, Ntolka & Papadatou-Pastou (2018) found that right-handers had higher IQ scores, but that difference was negligible (about 1.5 points).[61]

The prevalence of difficulties in left-right discrimination was investigated in a cohort of 2,720 adult members of Mensa and Intertel by Storfer.[62] According to the study, 7.2% of the men and 18.8% of the women evaluated their left-right directional sense as poor or below average; moreover participants who were relatively ambidextrous experienced problems more frequently than did those who were more strongly left- or right-handed.[62] The study also revealed an effect of age, with younger participants reporting more problems.[62]

Early childhood intelligence

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Nelson, Campbell and Michel investigated whether the development of handedness in infants was correlated with language abilities in toddlers. In their article, they assessed 38 infants, following them from birth to 12 months, and then again from 18 to 24 months. They found that children who developed a consistent preference for using either their right or left hand during infancy (e.g. putting the pacifier back in with the right hand or grasping random objects with the left hand) were more likely to have superior language skills as toddlers. Children who did not become lateral until after infancy (i.e., when they were toddlers) showed normal language development and had typical language scores. The researchers used Bayley scales of infant and toddler development to assess the subjects.[63]

Music

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In two studies, Diana Deutsch found that left-handers, particularly those with mixed-hand preference, performed significantly better than right-handers in musical memory tasks.[64][65] There are also handedness differences in perception of musical patterns. Left-handers as a group differ from right-handers, and are more heterogeneous than right-handers, in perception of certain stereo illusions, such as the octave illusion, the scale illusion, and the glissando illusion.[66]

Health

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Studies have found a positive correlation between left-handedness and several specific physical and mental disorders and health problems, including:

As handedness is a highly heritable trait associated with various medical conditions that could present a challenge to Darwinian fitness challenge in ancestral populations, it is possible that left-handedness was rarer in the distant past due to natural selection. However, left-handers have on average been found to have an advantage in combat and competitive sports, which could have increased their reproductive success in ancestral populations.[79]

Income

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In 2006, researchers from Lafayette College and Johns Hopkins University concluded that there was no statistically significant correlation between handedness and earnings for the general population. However, among people who had attended college, left-handers earned 10 - 15% more than their right-handed counterparts.[80]

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.[58]

Sports

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See also: Southpaw stance
Michael Vick, a left-handed American football quarterback, winds up to throw the ball to his teammate.

Interactive sports such as table tennis, badminton and cricket 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.[81] In tennis, 40% of the seeded players are left-handed.[82] The term southpaw is sometimes used to refer to a left-handed individual, especially in baseball and boxing.[83] Some studies suggest that right handed male athletes tend to be statistically taller and heavier than left handed ones.[84]

Other, sports-specific factors may increase or decrease the advantage left-handers usually hold in one-on-one situations:

  • In baseball, a right-handed pitcher's curve ball will break away from a right-handed batter and towards a left-handed batter (batting left or right does not indicate left or right handedness). While studies of handedness show that only 10% of the general population is left-handed, the proportion of left-handed MLB players is closer to 39% of hitters and 28% of pitchers, according to 2012 data.[85] Historical batting averages show that left-handed batters have a slight advantage over right-handed batters when facing right-handed pitchers.[86] 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.[87] Fifteen of the top twenty career batting average leaders in Major League Baseball history have been posted by left-handed batters.[88]
    • 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 pick off baserunners at first base.[89]
    • 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.[90] For the same reason, the other infielders' 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.[91] A left-handed catcher would have a potentially more dangerous time tagging out a baserunner trying to score.[91] 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.[91] On the other hand, the Encyclopedia of Baseball Catchers states:[91]
  • One advantage is a left-handed catcher's ability to frame a right-handed pitcher's breaking balls. A right-handed catcher catches a right-hander's breaking ball across his body, with his glove moving out of the strike zone. A left-handed catcher would be able to catch the pitch moving into the strike zone and create a better target for the umpire.
  • 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 handball, left-handed players have an advantage on the right side of the field when attacking, getting a better angle, and that defenders might be unused to them. Since few people are left-handed, there is a demand for such players.
  • 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. Left-handed drivers are usually on the right side of the field, because they can get better angles to pass the ball or shoot for goal.
  • Ice hockey typically uses a strategy in which a defence pairing includes one left-handed and one right-handed defender. A disproportionately large number of ice hockey players of all positions, 62 percent, shoot left, although this does not necessarily indicate left-handedness.[92]
  • In American football, the handedness of a quarterback affects blocking patterns on the offensive line. Tight ends, when only one is used, typically line up on the same side as the throwing hand of the quarterback, while the offensive tackle on the opposite hand, which protects the quarterback's "blind side", is typically the most valued member of the offensive line. Receivers also have to adapt to the opposite spin.[93] While uncommon, there have been several notable left-handed quarterbacks.
  • In bowling, the oil pattern used on the bowling lane breaks down faster the more times a ball is rolled down the lane. Bowlers must continually adjust their shots to compensate for the ball's change in rotation as the game or series is played and the oil is altered from its original pattern. A left-handed bowler competes on the opposite side of the lane from the right-handed bowler and therefore deals with less breakdown of the original oil placement. This means left-handed bowlers have to adjust their shot less frequently than right-handed bowlers in team events or qualifying rounds where there are possibly 4-10 people per set of two lanes. This can allow them to stay more consistent. However, this advantage is not present in bracket rounds and tournament finals where matches are 1v1 on a pair of lanes.

Sex

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According to a meta-analysis of 144 studies totalling 1,787,629 participants, the best estimate of the male to female odds ratio was 1.23, indicating that men are 23% more likely to be left-handed. For instance, if the prevalence of female left-handedness was 10%, the prevalence of male left-handedness would be approximately 12% (10% prevalence of left-handedness among women multiplied by an odds ratio of 1:1.23 for women: men results in a 12.3% prevalence of left-handedness among men).[94][clarification needed]

Sexuality and gender identity

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Further information: Handedness and sexual orientation

Some studies examining the relationship between handedness and sexual orientation have reported that a disproportionately large fraction of homosexual people exhibit non-right-handedness,[95] though findings are mixed.[96][97][98]

A 2001 study found that individuals assigned male at birth, whose gender identity did not align with their assigned sex, were more than twice as likely to be left-handed as a clinical control group (19.5% vs. 8.3%, respectively).[99]

Paraphilias (atypical sexual interests) have also been linked to higher rates of left-handedness. A 2008 study analyzing the sexual fantasies of 200 males found "elevated paraphilic interests were correlated with elevated non-right handedness".[100] Greater rates of left-handedness have also been documented among pedophiles.[101][102][103][104]

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 left-handed than their heterosexual counterparts.[105]

Mortality rates in combat

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A study at Durham University—which examined mortality data for cricketers whose handedness was a matter of public record—found that left-handed men were almost twice as likely to die in war as their right-handed contemporaries.[106] The study theorised that this was because weapons and other equipment was designed for the right-handed. "I can sympathise with all those left-handed cricketers who have gone to an early grave trying desperately to shoot straight with a right-handed Lee Enfield .303", wrote a journalist reviewing the study in the cricket press.[107] The findings echo those of previous American studies, which found that left-handed US sailors were 34% more likely to have a serious accident than their right-handed counterparts.[108]

Episodic memory

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A high level of handedness (whether strongly favoring right or left) is associated with poorer episodic memory,[109][110] and with poorer communication between brain hemispheres,[111] which may give poorer emotional processing, although bilateral stimulation may reduce such effects.[112][113]

Corpus callosum

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A high level of handedness is associated with a smaller corpus callosum whereas low handedness with a larger one.[114]

Divergent thinking

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Left-handedness is associated with better divergent thinking.[115]

Products for left-handed use

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Many tools and procedures are designed to facilitate use by right-handed people, with the difficulties faced by left-handed people often being overlooked. John W. Santrock has written, "For centuries, left-handers have suffered unfair discrimination in a world designed for right-handers."[9]

Although many products for left-handed use are made by specialist producers, they are not available from normal suppliers. Even simple items such as knives designed for right-handed use are less convenient for left-handers. There are many examples: kitchen tools such as knives, corkscrews and scissors, garden tools, and so on. While not requiring a purpose-designed product, there are more appropriate ways for left-handers to tie shoelaces.[116] There are companies that supply products designed specifically for left-handed use. One such is Anything Left-Handed, which in 1967 opened a shop in Soho, London; the shop closed in 2006, but the company continues to supply left-handed products worldwide by mail order.[117]

Stringed instruments such as the guitar and electric bass have been adapted for left-handed musicians. However, some guitarists such as Jimi Hendrix simply reversed the strings and played their instrument "upside down," though others such as Paul McCartney use left-hand guitar and bass models.

Writing from left to right, as in many languages, with the left hand tends to smear what has just been written, depending on how long it takes for the ink to dry. Left-handed writers have developed various ways of holding a pen to achieve the best results.[118] For using a fountain pen, preferred by many left-handers, nibs ground to optimise left-handed use (pushing rather than pulling across the paper) without scratching are available.

Bias against left-handers

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Main article: Bias against left-handed people

McManus observed that, as the Industrial Revolution spread across Western Europe and the United States in the 19th century, workers needed to operate complex machines that were designed with right-handers in mind. This would have made left-handers more visible, while also making them appear less capable and more clumsy. For example, writing with a dip pen was particularly prone to blots and smearing.

Negative connotations and discrimination

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Apart from the inconveniences they face, left-handed people have historically been considered unlucky or even malicious because of their difference by the right-handed majority. In many languages, including English, the word for the direction "right" also means "correct" or "proper". Throughout history, being left-handed was considered negative, or evil.[119]

Black magic is sometimes referred to as the "left-hand path".[120]

Discrimination in education

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Before the development of fountain pens and other modern writing instruments, children were taught to write with a dip pen. While a right-hander could smoothly drag the pen across the paper from left to right, it was difficult for a left-hander to push the dip pen across the paper without digging into it and creating blots and stains.[121] Even with more modern pens, writing from left to right with the left hand can smear what has just been written when moving across the line, as is the case in many languages.

Even into the 20th and even the 21st century, left-handed children in Uganda were beaten by schoolteachers or parents for writing with their left hand,[122] or had their left hands tied behind their backs to force them to write with their right hand.[123] As a child, the future British king George VI (1895–1952) was naturally left-handed. He was forced to write with his right hand, as was common practice at the time. He was not expected to become king, so that was not a factor.[124]

Depending on the position and inclination of the writing paper, and the writing method, left-handed writers can write as neatly and efficiently, or as messily and slowly, as right-handed writers. Left-handed children usually need to be taught how to write correctly with their left hand, as discovering a comfortable left-handed writing method independently can be difficult.[125][126]

In the Soviet school system, all left-handed children were forced to write with their right hand.[127][128]

International Left-Handers Day

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Main article: International Left-Handers Day

International Left-Handers Day is held annually every August 13.[129] It was founded by the Left-Handers Club in 1992, with the club itself having been founded in 1990.[129] International Left-Handers Day is, according to the club, "an annual event when left-handers everywhere can celebrate their sinistrality (left-handedness) and increase public awareness of the advantages and disadvantages of being left-handed".[129] 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."[129]

In other animals

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Kangaroos and other macropod marsupials show a left-hand preference for everyday tasks in the wild. 'True' handedness is unexpected in marsupials however, 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.[130] Studies of dogs, horses, and domestic cats have shown that females of those species tend to be right-handed, while males tend to be left-handed.[131]

See also

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General

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Handedness

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References

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Further resources

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Handedness

View on Grokipedia
from Grokipedia
Handedness is the preferential use of one hand over the other for unimanual tasks requiring dexterity, with right-handedness predominant in approximately 90% of humans and left-handedness in roughly 10%. This manifests early in development and persists lifelong, influencing tool use, writing, and performance, where left-handers may hold advantages in interactive contexts due to opponents' adaptation to right-hand dominance. , the capacity to use both hands equally, occurs in fewer than 1% of individuals. Handedness arises from interactions between genetic predispositions and environmental factors, including prenatal influences such as exposure and birth stress, with heritability estimates around 25%. It reflects cerebral lateralization, where right-handers typically show stronger left-hemisphere dominance for and motor functions, whereas left-handers exhibit reduced lateralization and greater bilateral activation._-_DRAFT_for_Review/15:_Language_and_the_Brain/15.04:_Handedness_Language_and_Brain_Lateralization) Genome-wide studies identify variants in genes related to development influencing this trait, though no single locus accounts for the majority of variance.

Definition and Classification

Types of handedness

Handedness refers to the preferential use of one hand over the other for performing unimanual tasks, with classifications generally encompassing right-handedness, left-handedness, mixed-handedness, and , though it exists on a continuum rather than strict categories. Right-handedness involves a consistent preference for the right hand in activities such as writing, throwing, and tool use, observed in approximately 90% of individuals across populations. Left-handedness, conversely, features a preference for the left hand in these tasks and occurs in about 10% of people, often linked to distinct patterns of cerebral lateralization. Mixed-handedness, also termed , describes inconsistent hand preference across tasks, where an individual may favor the right hand for writing but the left for sports like throwing or batting. This type, distinct from true , reflects task-specific dominance rather than equal proficiency and is estimated to affect a of the population beyond the binary right-left divide, potentially influenced by environmental or developmental factors. involves comparable skill and preference for either hand, affecting roughly 1% of individuals who exhibit no marked dominance. True is rare and often requires training, as opposed to innate mixed preferences. Beyond these, handedness can be further subdivided by strength of , such as strong right-handedness (consistent use across nearly all tasks) versus inconsistent or weak right-handedness, with similar gradations for left-handers. Assessments like hand inventories reveal this , where dichotomous labels overlook nuances; for instance, some classifications distinguish ambisinistrality, characterized by clumsiness with both hands, as a maladaptive extreme opposing skilled . Empirical studies using objective skill measures alongside subjective underscore that hand skill and do not always align, complicating rigid categorization.

Prevalence across populations

Approximately 90% of the human population exhibits right-handedness, with left-handedness occurring in about 10%, a ratio that has remained broadly stable across historical records and modern large-scale analyses spanning diverse ethnicities and cultures. A of over 2.3 million individuals estimated the global prevalence of left-handedness at 10.6%, with a range of 9.3% to 18.1% across studies, reflecting methodological variations but converging on this figure when cultural pressures are minimized. This distribution holds across continents, from to and , suggesting a biological basis resilient to superficial environmental shifts, though observed rates can deviate due to societal enforcement of right-hand use. Sex-based differences are consistent worldwide, with males showing higher left-handedness rates than females—typically 11-13% in males versus 8-11% in females—potentially linked to prenatal testosterone exposure influencing cerebral lateralization. In a nationwide survey of over 30,000 adults, childhood left-handedness was reported at 8.7% for males and 6.8% for females, with adult rates slightly lower due to partial switching. Age cohorts reveal a decline in reported left-handedness among older populations, from around 12% in those born after 1950 to under 5% in those born before 1910, attributable to historical stigma and forced retraining in schools and households rather than innate change. Geographical and ethnic variations primarily stem from cultural attitudes rather than , as innate handedness appears uniform when suppression is absent. Western nations like the (13.23%), (13.1%), and (12.8%) report higher rates, correlating with reduced historical coercion, while East Asian countries such as (2.64%) and (3.39%) show artificially low figures due to persistent right-hand training in and daily norms. Among indigenous groups, such as Amazonian tribes with minimal external influence, rates approach the global 10% benchmark, underscoring that deviations in high-pressure societies reflect behavioral adaptation over biological prevalence. Ethnic comparisons within multicultural studies indicate marginally higher left-handedness among , Asian, and groups compared to others, but these differences are small and overshadowed by and age effects.

Assessment Methods

Standardized measures

The Edinburgh Handedness Inventory (EHI), developed by Richard Oldfield in 1971, is the most widely cited self-report questionnaire for assessing handedness in adults, consisting of 10 items evaluating hand preference for activities such as writing, throwing, and using . Participants indicate preferences on a scale, yielding a quotient from -100 (strong left-handedness) to +100 (strong right-handedness), with values near zero indicating mixed or ambidextrous tendencies; test-retest reliability exceeds 0.97 in multiple studies, and it correlates well with performance-based measures. The Annett Hand Preference (AHPQ), introduced by Marian Annett in 1967 and modified in subsequent versions, uses 12 unimanual tasks (e.g., writing, striking a ) to classify individuals into categories such as consistent right, consistent left, or mixed handedness based on the number of right-hand preferences reported. It emphasizes simplicity for large-scale studies and has demonstrated retest reliability comparable to the EHI, though it provides categorical rather than continuous scoring, potentially underestimating degrees of . The Waterloo Handedness Questionnaire (WHQ), revised in 1989 by Steenhuis and Bryden, expands to 36 items assessing both preference and relative skill on a 5-point for tasks like throwing a or using a hammer, allowing differentiation between hand use in everyday versus skilled activities; factorial analysis supports its multidimensional structure, with high internal consistency ( >0.90) and validity against observational data. These questionnaires generally outperform single-item assessments (e.g., "which hand do you write with?") by capturing nuanced profiles, though self-reports can be influenced by cultural norms or , necessitating with proficiency tests for robust .

Challenges in classification

Classifying handedness presents several methodological and conceptual difficulties, primarily because it exists on a continuum rather than as discrete categories of left, right, or ambidextrous preference. Traditional dichotomous or trichotomous schemes overlook this gradation, where individuals exhibit varying degrees of lateral preference across tasks, leading to arbitrary cutoffs in assessment tools. For instance, Marian Annett's right-shift theory posits that handedness strength varies continuously from strong left to strong right, rendering binary oversimplistic and prone to misallocation of individuals with moderate preferences. Self-report questionnaires, such as the Edinburgh Handedness Inventory, are widely used but suffer from subjectivity and inconsistency, often diverging from objective performance measures like pegboard tasks or tests. These inventories rely on recalled preferences for activities like writing or throwing, which can be influenced by social desirability or memory bias, yielding lower reliability in detecting subtle asymmetries compared to behavioral observations. Performance-based assessments provide more objective data by quantifying speed and accuracy differences between hands, yet they too vary by task demands—handedness for fine motor skills like writing may not align with gross motor actions like hammering—complicating a unified classification. Cultural and historical pressures exacerbate classification errors, as left-handed individuals have frequently been coerced into right-hand use, inflating apparent right-handedness rates in self-reports from older cohorts or conservative societies. Studies show that birth year and location correlate with reported left-handedness prevalence, with declines in earlier generations attributable to such interventions rather than innate shifts. This suppression distorts inventories, as converted individuals may report right-hand dominance despite underlying left preferences, evidenced by of persistent neural asymmetries in former left-handers. Mixed-handedness, encompassing inconsistent preferences across tasks, further blurs boundaries, often conflated with rare true , which affects under 1% of the population and may reflect atypical lateralization rather than equal proficiency.

Biological Mechanisms

Genetic factors

Handedness exhibits moderate , with twin and family studies estimating that genetic factors account for approximately 23-28% of the variance in hand preference. A of handedness in twins confirmed that additive genetic influences explain up to 25% of variability, while shared and non-shared environmental factors contribute the remainder. These estimates derive from comparisons between monozygotic and dizygotic twins, where monozygotic pairs show higher concordance for non-right-handedness (around 20-30%) than dizygotic pairs (10-15%), indicating polygenic rather than complete genetic determination. Genome-wide association studies (GWAS) have identified handedness as a highly polygenic trait involving numerous common variants of small effect, with no single exerting dominant control. A 2020 GWAS of over 1.7 million individuals detected 48 genetic loci associated with relative hand skill, many linked to development pathways such as microtubule transport and neuronal migration. Subsequent analyses pinpointed 41 loci specifically for left-handedness, with polygenic scores explaining 1-6% of variance and correlating with asymmetries in language-related regions. Estimates suggest up to 40 genes may contribute, underscoring a liability-threshold model where cumulative genetic risk thresholds influence left-handed outcomes. Rare protein-altering variants also play a role, particularly in left-handedness, as revealed by exome-sequencing studies implicating genes in microtubule dynamics and ciliogenesis. For instance, variants in tubulin genes like TUBB and microtubule-associated proteins such as MAPT and MAP2 show associations with non-right-handedness, potentially disrupting asymmetric neuronal processes during brain lateralization. These findings highlight pathways involved in embryonic left-right patterning, though effect sizes remain modest and do not fully account for population prevalence. Genetic influences on handedness interact with prenatal and environmental factors, explaining why does not predict perfect familial transmission; for example, left-handed parents have about a 10-20% chance of left-handed offspring, far below Mendelian expectations. Recent polygenic models emphasize that while predispose , stochastic developmental events and external pressures modulate expression, with no evidence for a simple deterministic model. Ongoing prioritizes large-scale genomic to refine these estimates, but current evidence rejects monogenic theories in favor of multifaceted, low-penetrance contributions.

Prenatal and environmental influences

Prenatal factors contribute to the establishment of handedness through influences on fetal and motor development. , for instance, correlates with increased rates of non-right-handedness, as evidenced in a study of triplets where it explained substantial variation in handedness independent of genetic factors, likely via impacts on early and neural lateralization. Fetal exposure has been associated in observational studies with elevated non-right-handedness, particularly among males; a 2011 meta-analysis of 15 studies reported a weak but statistically significant of 1.13 for non-right-handedness following routine prenatal screening. However, randomized controlled trials, including Cochrane reviews, have not replicated this in intention-to-treat analyses, suggesting potential by selective scanning of at-risk pregnancies rather than causation. Hormonal influences during show mixed empirical support. Prenatal testosterone levels, often hypothesized to disrupt typical rightward bias via effects on organization, yielded inconsistent results across studies; a 2022 longitudinal analysis of samples found no substantial predictive role for testosterone or in later handedness outcomes. Similarly, prenatal stress exposure during the third trimester lacked significant association with mixed-handedness in cohort data, though earlier periods warrant further scrutiny. Other elements, such as vertex presentation and restricted womb space in multiples, may subtly favor right-hand dominance through biomechanical constraints on fetal thumb-sucking and arm positioning, but these effects are modest and not fully disentangled from . Postnatal environmental factors primarily modulate expressed handedness rather than innate preference, which solidifies early in childhood. Cultural norms suppressing left-hand use—prevalent in many societies—can induce behavioral shifts, resulting in higher reported right-handedness rates; cross-cultural comparisons indicate that while training increases , underlying neural asymmetries persist, with innate left-preferring individuals comprising approximately 10% globally regardless of enforcement. Early life experiences, including differential tool access and parental modeling, account for up to 75% of variance in twin studies, predominantly through unique non-shared effects like random motor practice rather than systematic family influences. These modifications do not reverse prenatal biases but can mask them, as longitudinal tracking reveals stability in core lateralization post-infancy.

Neural lateralization and brain connectivity

Neural lateralization refers to the tendency for certain cognitive and motor functions to be predominantly processed in one cerebral hemisphere, with handedness serving as a primary behavioral marker of this asymmetry. In right-handed individuals, who comprise approximately 90% of the population, motor control for the preferred hand is typically lateralized to the contralateral (left) hemisphere, as evidenced by functional magnetic resonance imaging (fMRI) studies showing stronger activation asymmetry during unimanual tasks. Left-handed individuals exhibit reduced hemispheric dominance, often displaying more bilateral activation patterns in motor and language-related regions, which correlates with weaker overall lateralization. This difference persists even in the absence of structural volume disparities between hemispheres, suggesting that handedness influences the laterality of functional connectivity rather than gross anatomy alone. Language processing provides a clear example of handedness-related lateralization variance. Among right-handers, 96% demonstrate left-hemispheric dominance for tasks via fMRI, compared to only about 70-80% in left-handers, with the remainder showing bilateral or right-hemispheric patterns. Genetic factors contributing to left-handedness are associated with reduced asymmetry in -related cortical regions, such as the and , implying a heritable component to atypical lateralization. These patterns hold in healthy individuals without , indicating that the link between handedness and dominance is a natural neurodevelopmental feature rather than an artifact of injury. Brain connectivity, particularly interhemispheric pathways, shows nuanced differences by handedness. Functional analyses reveal that left-handers exhibit distinct patterns in regions involved in , hand control, and visuospatial processing, with less pronounced compared to right-handers. Regarding structural connectivity, early studies reported a larger (by approximately 11%) in left-handers, potentially facilitating greater interhemispheric communication to compensate for reduced unilateral specialization. However, a 2023 meta-analysis of 23 studies found no significant differences in total size or subsections between handedness groups, challenging prior claims and attributing inconsistencies to methodological variations like sample size and imaging techniques. (EEG) studies further support connectivity divergences, with left- and right-handers activating different cortical networks during motor tasks, particularly in alpha and beta frequency bands. Overall, while handedness modulates the degree of lateralization and connectivity efficiency, these effects are probabilistic rather than deterministic, with individual variability influenced by genetic and experiential factors.

Evolutionary Perspectives

Origin theories

Archaeological evidence indicates that a population-level right-hand bias existed in early hominins. Tooth wear patterns on fossils of Homo habilis from approximately 1.8 million years ago show grooves consistent with right-handed stone tool use held against the teeth. Neanderthal remains and East African tool assemblages from around 500,000 years ago further support a similar ~90% right-hand preference, comparable to modern humans. Hand stencils in Paleolithic caves, such as those dating to 13,000 years ago in Argentina, predominantly depict left hands, implying artists used their right hand for pigment application. Several evolutionary theories attempt to explain the origin of this asymmetry. The manipulation hypothesis proposes that right-handedness arose from selection pressures favoring precise visuomotor control in bipedal reaching and , potentially inherited from a common with great apes through early tool-use behaviors. This is supported by observations of right-hand biases in non-human for similar tasks, suggesting a deep evolutionary root predating Homo sapiens. Alternative models emphasize social and communicative factors. The gestural origins theory links right-hand dominance to the of , positing that left-hemisphere specialization for sequential extended from proto-gestures to speech, enforcing a right-hand for emphatic signaling. The fighting hypothesis, conversely, argues that right-handers gained a advantage by striking over the opponent's left arm, with the stable maintained by cultural transmission despite left-handers' rarity providing tactical surprises in fights. Developmental and genetic perspectives frame handedness as emerging from left-right body asymmetry pathways conserved across vertebrates, with population bias potentially amplified by prenatal hormonal influences or random developmental biases fixed by selection. However, the low heritability (~25%) and persistence of left-handedness challenge purely adaptive explanations, suggesting roles for , , or fluctuating selection in maintaining polymorphism. Empirical data from prehistoric records refute purely cultural origins, as the right-hand bias predates evidence of complex imitation or enforcement.

Population-level biases and selection pressures

Approximately 90% of modern human populations exhibit right-handedness, with this bias observed consistently across diverse ethnic groups, including Europeans, Asians, Africans, and Indigenous populations, based on self-report and performance measures. Fossil evidence from Neanderthals and early Homo sapiens indicates a similar ~90% right-handed , as inferred from tooth wear patterns and tool-use asymmetries dating back over 500,000 years, suggesting the bias predates modern cultural influences. This population-level skew toward right-handedness is exceptionally strong compared to other , where individual handedness exists but population biases are rare or absent. Evolutionary models propose that selection pressures favoring right-handed conformity arose from advantages in social coordination and tool use, where population-level consistency facilitates shared manufacturing standards, gesture communication, and cooperative tasks like or crafting, reducing errors in group settings. For instance, right-dominant tool kits in archaeological records from the era imply selective retention of right-preferring individuals in lineages reliant on standardized implements, potentially amplifying the bias through cultural transmission reinforced by genetic predispositions. Prenatal neural lateralization, evident from fetal thumb-sucking asymmetries around 15 weeks , may represent an early substrate under for right bias, as deviations could impair survival in kin-based groups emphasizing predictable motor patterns. The persistence of left-handedness at ~10%, despite apparent selection against it, is explained by negative , particularly via the fighting hypothesis, where rare left-handers gain combat advantages against right-dominant opponents due to unfamiliarity, as demonstrated in modern sports like and where lefties win 10-15% more bouts than their frequency predicts. This rarity confers a fitness benefit in adversarial encounters, balancing losses from potential coordination disadvantages or higher risks in right-biased environments, such as accidents with right-handed machinery. Genetic analyses support multilocus models where right-handedness alleles predominate but rare variants or stochastic prenatal factors maintain left-handed polymorphism, preventing fixation under weak . studies show no significant deviation in bias rates over millennia, indicating equilibrium where left-handed underrepresentation reflects cumulative pressures without eradicating the minority.

Developmental Aspects

Timeline from prenatal to adulthood

Hand preference emerges prenatally, with lateralized behaviors detectable as early as gestational weeks (GW) 7–8 through spontaneous fetal movements, and a full repertoire of arm extensions by GW 14. Kinematic analysis of self-directed (e.g., hand-to-eye or hand-to-mouth) and outer-directed (e.g., hand-to-uterine wall) movements reveals that right-handed fetuses exhibit faster right-hand performance for precise targets from GW 18 onward, predicting postnatal handedness with 89–100% accuracy based on movement time metrics. Ultrasound observations of thumb sucking show a strong right-hand bias, with approximately 94.6% of fetuses preferring the right thumb, aligning with the population-level ~90% right-handedness in adults. In infancy, hand preference for object acquisition manifests before 6 months, becoming prominent between 6 and 12 months before declining as bimanual coordination increases. Approximately 32% of infants display consistent right-hand preference and 12% left-hand preference across this period for reaching tasks, with 26% trending rightward and 30% showing no clear bias. Unimanual manipulation preferences emerge around 10–11 months, while role-differentiated bimanual manipulation (e.g., one hand stabilizing, the other manipulating) strengthens from 13–14 months and stabilizes by 18 months in about 80% of cases. Early consistent preferences, particularly right-handed ones, correlate with advanced skills at 24 months, accounting for up to 25% of variance in expressive and receptive abilities. During toddlerhood and early childhood, handedness preferences solidify, with many children showing a clear hand bias for tasks like grasping toys between 18 months and 3 years, though full dominance typically establishes between 3 and 6 years. Bimanual handedness onset occurs between 10 and 18 months in most infants, with 23 of 24 longitudinally studied cases (96%) exhibiting stable preferences by this window. Preterm infants often display less consistent or delayed handedness compared to full-term peers, potentially linked to lower birth weight and disrupted prenatal development. By late childhood and into hood, handedness achieves high stability, with early preferences reliably predicting adult dominance in the majority of cases, though minor shifts can occur due to environmental pressures like forced . Longitudinal data indicate that consistent handedness from infancy facilitates cognitive milestones like tool use and object , with stability evident by age 5–6 in population studies. left-handedness rates (~10%) trace back to prenatal biases rather than postnatal shifts in most instances, underscoring the trajectory's continuity despite some plasticity in mixed-handers.

Plasticity and cultural modification

Handedness exhibits developmental plasticity, particularly during , where initial preferences can be influenced by environmental factors before stabilizing in later years. Longitudinal studies tracking prehension from 6 to 14 months show fluctuating hand use that consolidates into consistent preferences, with the degree of lateralization increasing more rapidly in left-handers than right-handers by age 6. This plasticity diminishes post-critical periods, as evidenced by limited success in reversing handedness direction through training, with behavioral and neural analyses indicating a effect on shifts in preference strength. Cultural practices have demonstrably modified handedness expression by suppressing left-hand use, elevating apparent right-handedness rates in populations under such pressures. Historical evidence from diverse societies, including Mesopotamian records associating left-handedness with and widespread ceremonial right-hand mandates, correlates with reduced left-handed , estimated at 3% in ancient times versus 10-12% today. Forced right-hand conversion in childhood, common until the mid-20th century, induces lasting structural changes in brain regions like the and functional differences in handwriting-related , persisting into adulthood despite behavioral adaptation. In modern contexts, reduced cultural stigma and targeted training, such as in sports, can foster greater without full reversal. Expert players display decreased one-hand bias and improved inter-manual performance due to repetitive bilateral demands, highlighting environmental modulation of motor lateralization. However, such interventions carry potential costs, including associations with speech impediments in converted individuals, underscoring limits to plasticity beyond innate predispositions. Cross-cultural variations persist, with ongoing religious or hygienic taboos in some communities influencing self-reported handedness assessments.

Empirical Correlations

Cognitive and psychological traits

Left-handedness represents a natural aspect of human neurological diversity, akin to variations observed in neurodevelopmental conditions such as autism spectrum disorders and ADHD. These associations underscore brain variations rather than inherent deficits, often linked to atypical patterns of cerebral lateralization during development. Meta-analyses show that individuals on the autism spectrum are approximately 2.49 times more likely to be left-handed than neurotypical individuals, with prevalence rates around 28% compared to 10% in the general population. Similarly, non-right-handedness (left- or mixed-handed) is elevated in ADHD, occurring in 27.3% of cases versus 18.1% in controls. Regarding potential cognitive differences, some research suggests that the varied lateralization in left-handers may support holistic processing or creative thinking; however, meta-analyses indicate mixed or weak empirical support for enhanced creativity, revealing no substantial advantage over right-handers in divergent thinking tasks. Non-right-handed individuals, encompassing left- and mixed-handers, exhibit elevated rates of certain neurodevelopmental and psychiatric conditions compared to strong right-handers. Meta-analyses indicate that is robustly associated with increased prevalence of non-right-handedness, particularly mixed-handedness, with odds ratios suggesting a 1.5- to 2-fold higher likelihood. Similarly, mixed-handedness correlates with higher reporting of psychotic-like experiences in non-clinical populations and is overrepresented in disorders such as autism spectrum conditions and . These associations persist across large-scale studies but do not imply causation; they may reflect underlying disruptions in cerebral lateralization during prenatal brain development, as non-right-handedness often co-occurs with brain asymmetry patterns. Regarding general cognitive abilities, systematic reviews and meta-analyses find no substantial IQ advantage for left-handers over right-handers; right-handers show a negligible but statistically significant edge of approximately 1-2 IQ points on average. Conflicting smaller studies claiming left-handed superiority in lack replication in broader syntheses, potentially due to sampling biases or measurement inconsistencies. In specific domains, right-handers demonstrate a small advantage in , with effect sizes around d=0.1-0.2 in meta-analytic data, possibly linked to more consistent hemispheric specialization for visuospatial processing. Longitudinal evidence also suggests faster cognitive decline in left-handers during aging, with steeper performance drops in executive function and tasks relative to right-handers. Psychological traits show subtler patterns, with mixed-handedness tied to greater symptom severity in affective disorders and , independent of diagnosis alone. studies link non-right-handedness to atypical cognitive profiles, including both vulnerabilities (e.g., higher risk) and potential strengths in , though the latter remains debated without strong meta-analytic support. Overall, while population-level data highlight risks for in non-right-handers—potentially doubling rates for some conditions like —most individuals remain unaffected, underscoring that handedness serves as a marker rather than a determinant of cognitive or psychological outcomes.

Health, longevity, and mortality

Studies have investigated potential associations between handedness and longevity, with early cross-sectional analyses suggesting left-handers experience reduced lifespan. A 1991 analysis of death records and obituaries reported that left-handers died approximately nine years earlier on average than right-handers, attributing this to higher vulnerability to accidents, infections, and other environmental risks in a right-handed dominant society. However, this finding has faced substantial scrutiny, as subsequent longitudinal cohort studies, including a six-year follow-up of over 3,700 older adults, found no significant mortality difference between left- and right-handers after adjusting for age and confounders. Similarly, a prospective study of elderly women showed no elevated mortality risk for left-handers ( 1.10, 95% CI 0.70-1.72). The apparent longevity disparity in older is largely explained by cohort effects from historical suppression of left-handedness, such as forced conversion to right-handed use in and culture, which reduced reported left-handedness in pre-1950s birth cohorts. Modeling studies indicate that without such suppression, left-handers would show equivalent or slightly longer lifespans, as modern cohorts exhibit stable left-handedness rates around 10% into , with no . Niche populations, like players, have shown shorter survival for left-handers, potentially due to sport-specific injury risks, but this does not generalize to the broader population. Beyond , non-right-handedness correlates with elevated risks for certain health conditions, though causation remains unestablished and effect sizes are typically small. Left-handers exhibit higher prevalence of immune-related disorders, including allergies and autoimmune conditions, possibly linked to atypical lateralization affecting immune regulation. Associations exist with neurodevelopmental issues like (odds ratio approximately 1.5-2.0 in meta-analyses) and learning disabilities, as well as physical ailments such as and , though these may reflect shared genetic or prenatal factors rather than handedness per se. difficulties, including anxiety and ADHD, are more common in non-right-handers during childhood and , independent of language lateralization atypicality. Potential cardiovascular risks, such as increased heart disease incidence, have been hypothesized via EEG asymmetry patterns, but require further validation. Overall, while left-handers may face marginally higher morbidity in specific domains due to mismatched environmental designs (e.g., accidents from right-handed tools), population-level health outcomes do not indicate systematic disadvantage when accounting for confounding historical and methodological factors.

Athletic and motor skills

![Michael Vick throwing left-handed][float-right] Left-handed individuals are overrepresented among elite athletes in interactive sports, comprising approximately 15% of sporting elites compared to 10% in the general population, according to a meta-analysis by Papadatou-Pastou et al. This disparity is particularly pronounced in antagonistic or one-on-one disciplines such as , , , and , where left-handers exhibit a tactical advantage due to opponents' relative unfamiliarity with facing left-sided actions. The "fighting hypothesis" posits that this stems from evolutionary pressures favoring left-handedness in combat scenarios, conferring a benefit when rare. In sports like and squash, left-handers achieve higher rankings and win rates against right-handers, as modeled in economic analyses of one-on-one interactions, where the rarity of left-handed opponents disrupts practiced responses. Similarly, in shooting, left-handed players demonstrate superior scoring percentages (63.08% vs. 57.86% for right-handers) in certain contexts, though overall performance varies by sport demands. Handedness influences not only offensive but also defensive proficiency; for instance, right-handed athletes in interactive sports show performance asymmetries favoring their dominant side, but left-handers maintain competitive edges in high-pressure, time-constrained environments like or . Regarding general motor skills, right-handers typically exhibit faster skill acquisition and proficiency with their dominant hand in tasks requiring precision, such as fine , due to lateralized neural efficiencies. Left-handers, however, often display superior bilateral fine motor skills across both hands compared to right-handers, potentially aiding adaptability in sports demanding . Interlimb transfer of motor skills shows variability by handedness and task complexity, with left-handers experiencing differential effects in complex skilled practices. In gross motor acquisition, some indicates left-handed children may lag in certain novel tasks, though elite athletic contexts override this through specialized training. Overall, handedness modulates but does not inherently predict prediction accuracy in action outcomes, emphasizing practice over innate in non-interactive skills.

Socioeconomic and behavioral outcomes

Studies indicate mixed associations between left-handedness and socioeconomic outcomes. Analysis of British Household Panel Survey data from 1991 to 2005 revealed that left-handed males experienced approximately 5% higher compared to right-handed males, after controlling for factors such as age, , and region, though no significant effect was observed for females. Conversely, U.S. National Longitudinal Survey of Youth data suggested that left-handed individuals may face earnings penalties linked to cognitive or disadvantages, though these effects diminish when accounting for ability measures. Occupational distributions show elevated left-handedness among high-income professions, particularly among males, with rates exceeding population averages in fields requiring spatial skills or interactivity. In , left-handed children demonstrate poorer performance across cognitive, motor, and socioemotional domains. Using data from the National Longitudinal Survey of Youth Child Sample (1986–1994), left-handers were 4–6 percentage points more likely to be rated as "less competent" by teachers and exhibited delays in vocabulary, math skills, and externalizing behaviors compared to right-handers. These developmental disparities may contribute to long-term gaps, though adult outcomes often converge or favor left-handers in select domains due to potential advantages in or adversity adaptation. Behavioral outcomes linked to handedness include variations in and traits. Strong right-handers tend to exhibit lower sensation-seeking and higher , with consistent handedness correlating to increased sensitivity and reduced novelty-seeking behaviors. Left- and mixed-handers, by contrast, show heightened interhemispheric interaction, potentially fostering greater in perceptual tasks but elevated engagement in health risk behaviors such as substance use and in adulthood. Evidence on criminality is inconsistent across contexts. Among , left-handers scored lower on potential measures than right-handers, suggesting reduced propensity for severe . However, analyses of traditional societies found a positive between left-handedness (up to 27% in high- groups) and rates, attributed to combat advantages under negative where rarity confers surprise benefits in physical confrontations. Modern studies associate left-handedness with modestly higher rates of addictive and antisocial behaviors, potentially mediated by neurodevelopmental factors, though remains unestablished.

Societal and Cultural Impacts

Historical biases and discrimination

In ancient civilizations such as , , , and , the left hand was often reserved for unclean or impure tasks, fostering early cultural prejudices against left-handedness that equated it with inferiority or misfortune. These biases stemmed from practical asymmetries in tools and writing systems designed for right-hand users, reinforced by omens and superstitions where left-sided actions portended evil.60854-4/fulltext) During the in , left-handed individuals faced heightened stigma, with the left hand associated with the and , leading to accusations of demonic possession or curses. Religious doctrines, particularly in Catholicism, portrayed the right hand as godly while deeming the left sinister, a view echoed in biblical references and teachings that influenced societal norms. Educational practices amplified discrimination through forced conversion to right-handedness, common in Victorian-era schools and persisting into the mid-20th century, where teachers tied left hands or administered to enforce right-hand writing.60854-4/fulltext) In the United States during the 1940s and 1950s, pseudoscientific theories linked left-handedness to and behavioral deviance, prompting widespread "retraining" via immobilization, humiliation, or aversion techniques in schools and clinics. This pathologization of left-handedness as a developmental abnormality requiring correction parallels historical societal attitudes toward neurodivergent traits, such as autism and ADHD, where minority brain variations were similarly treated as disorders necessitating intervention to enforce conformity to neurotypical norms. Such interventions for left-handedness, often justified by flawed psychological models, caused lasting motor difficulties and psychological distress without empirical support for benefits. Globally, similar enforcements occurred in religious institutions, including Catholic schools where nuns beat or reprimanded left-handed children into compliance, viewing it as correcting a or developmental flaw until the late . These practices reflected broader societal norms that pathologized atypical neurological variations, akin to past treatments of autism and ADHD as moral or behavioral failings rather than natural diversity. They declined post-1970s with growing awareness of the neurological innateness of handedness, though residual biases lingered in conservative or traditional settings. Historical thus reflected a confluence of , religious , and unexamined right-hand dominance rather than evidence-based concerns.

Adaptations, products, and modern awareness

, observed annually on since its inception in 1992 by the Left-Handers Club, serves to raise awareness of the challenges faced by left-handed individuals and to promote greater accommodation in daily life. This event highlights that left-handers comprise approximately 10.6% of the global population, emphasizing the need for to mitigate disadvantages in a predominantly right-handed world. Modern awareness has shifted from historical stigma—where left-handers were often forced to convert to right-hand use—to recognition of handedness as a innate trait influenced by genetic and neurological factors, reducing such coercive practices in and society. Products tailored for left-handers have proliferated in response to this awareness, addressing ergonomic mismatches in tools originally designed for right-hand dominance. Common items include left-handed scissors, which feature reversed blade orientation to prevent straining the left wrist; can openers with handles suited for leftward rotation; and spiral-bound notebooks with rings on the right side to avoid interference during writing. Specialized retailers, such as Lefty's the Left Hand Store, have offered such adaptations since 1967, expanding to include kitchen utensils like measuring cups with left-side spouts and rulers marked to read from right to left. The niche market reflects demand, with the global left-handed scissors sector alone valued at USD 305 million in 2024, driven by consumer recognition of usability benefits. Societal adaptations extend to institutional settings, where left-handed desks in classrooms—angled for natural left-arm positioning—and adjustable workstations in offices accommodate motor preferences without compromising productivity. In professional environments, such as , left-handed variants of power tools and peelers minimize risks from ill-suited grips. These developments stem from empirical observations of higher rates and discomfort among left-handers using standard right-oriented , fostering a broader push for ambidextrous or modular designs in product . Overall, heightened awareness has correlated with decreased and improved , though left-handers still navigate a landscape where right-handed norms predominate in approximately 90% of tools and interfaces.

Comparative Handedness in Animals

Evidence in non-human species

Individual motor lateralization, manifested as consistent preference for one limb over the other in tasks like reaching or manipulation, occurs in numerous non-human species, with approximately 75% of studied species exhibiting such preferences at the individual level across 172 taxa. However, population-level asymmetries—where a of individuals favor the same side, akin to the ~90% right-handedness in humans—are rare and typically weaker or task-specific. This pattern holds across vertebrates, suggesting that while neural lateralization supports specialized hemispheric functions (e.g., right-hemisphere processing), directional biases in limb use may arise from ecological pressures rather than universal . In non-human , evidence for handedness derives primarily from bimanual tasks like the "tube-reaching" paradigm, where subjects extract food from a narrow tube using one hand to hold and the other to probe. Chimpanzees (Pan troglodytes) display right-hand biases in over 50% of studies for this task, throwing, and tool use such as termite fishing or nut-cracking, with population rightward shifts in coordinated actions but left biases in simpler reaching. (Gorilla gorilla) show right-handedness in bimanual feeding, while siamangs (Symphalangus syndactylus) prefer the left for complex water-drinking tasks; however, meta-analyses of 38 anthropoid species (n=1786 individuals) reveal significant population biases in only 2 species (chimpanzees and ), with no consistent directional trend across taxa. Arboreal tend toward stronger individual preferences than terrestrial ones, potentially due to demands for precise, unilateral grasping in branching environments, though this does not predict uniform population directionality. Among , paw preferences are robust at the individual level but balanced at the population level. A of mice (Mus musculus) found 81% of individuals exhibiting left- or right-paw bias in reaching tasks, while 84% of rats (Rattus norvegicus) showed similar preferences, yet neither species displayed net population asymmetry. Domestic dogs (Canis familiaris) often demonstrate individual pawedness (68% biased), with some studies reporting a slight right-paw in reaching or manipulation, potentially influenced by human handling or selection for cooperative tasks. Cats (Felis catus) and giant pandas (Ailuropoda melanoleuca) likewise show individual but not consistent population preferences, with sex differences in pandas (males right-biased during bamboo feeding). Beyond mammals, lateralization extends to non-limb behaviors with occasional population biases: parrots favor the right foot for manipulation, toads (Bombina orientalis) show rightward turning preferences under predation stress, and fish like guppies display asymmetric use in schooling. These findings indicate that while individual limb preferences enhance efficiency in asymmetric brains—evident in over 80% of studied—population-level handedness lacks the human-like uniformity, implying distinct selective forces in hominin , such as gestural communication or tool complexity, rather than a conserved vertebrate trait.

Implications for human evolution

The pronounced population-level right-handedness observed in modern humans, affecting approximately 90% of individuals, contrasts sharply with patterns in non-human , where hand preferences are typically individual-level, task-specific, and lack a consistent species-wide bias toward the right hand. Studies of great apes, such as chimpanzees and bonobos, reveal right-hand biases in certain coordinated bimanual actions like tube-feeding tasks, with rates around 50-67% right-handedness, but these do not approach human extremes and vary by species— showing some right bias while orangutans exhibit left bias. This comparative evidence implies that individual handedness likely predates the human-chimpanzee divergence around 5-7 million years ago, inherited from a , but the strong, population-level rightward shift in humans represents a derived evolutionary rather than a conserved trait. Such divergence suggests selective pressures unique to hominin evolution amplified existing preferences, potentially linked to advancements in tool manufacture and use, which archaeological dates to at least 2.6 million years ago in early species. In non-human , handedness correlates with manipulative skills and ecological niches—terrestrial species tending toward right preferences in some tasks, arboreal ones left—but without the cultural reinforcement seen in humans, where social learning and imitation could propagate right-hand norms for efficient group coordination. This human-specific escalation may have causal ties to cerebral lateralization, including left-hemisphere dominance for and sequential motor planning, facilitating evolutionary advantages in cooperative hunting, gestural communication precursors to speech, and division of labor; lacking population bias underscore that these pressures, absent or weaker in other species, drove the fixation of right-handedness as a polymorphism in . Alternative hypotheses, such as the fighting advantage theory positing right-hand dominance for efficacy in social groups, find limited support from data, where aggressive interactions do not consistently predict population biases, but gain traction from fossil records showing right-oriented on teeth and lesions from 1.8 million years ago. Genetic models indicate a polygenic basis with possible maintaining left-handed minorities (around 10%) for strategic advantages like surprise in adversarial contexts, a dynamic less evident in animals with balanced or absent biases. Overall, comparative animal handedness highlights human right-handedness as an adaptive specialization, evolving post-divergence to underpin cognitive and technological leaps, rather than a primitive feature shared broadly across mammals.

References

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