In human biology, handedness is a better, faster, or more precise performance or individual preference for use of a hand, known as the dominant hand; the incapable, less capable or less preferred hand is called the non-dominant hand.^[1][2][3] Men are slightly more likely to express a strongly dominant left hand than women.^[4] Right-handedness is the most common form, and it is estimated that around 90% of the world’s population is right-handed.

Types

• Right-handedness is by far the most common type. Right-handed people are more skillful with their right hands when performing tasks. Studies suggest that approximately 90% of the world's population is right-handed.^[5]
• Left-handedness is far 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.^[6]
• Cross-dominance or mixed-handedness is the change of hand preference between tasks. This is very uncommon in the population with about a 1% prevalence.^[7]
• Ambidexterity, equal ability to use both hands, is exceptionally rare. A completely 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.^[1]

Causes

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.^[8] In a 2013 study, 39% of infants (6 to 14 months) and 97% of toddlers (18 to 24 months) demonstrated a hand preference.^[9]

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.^[10]

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 hemispheres are 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.^[10]

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.^[11] A large study of twins from 25,732 families by Medland et al. (2006) has indicated that the heritability of handedness is roughly 24%.^[12]

To date, two theoretical single gene models have been proposed to explain the patterns of inheritance of handedness, the first by Marian Annett^[13] of the University of Leicester and the second by Chris McManus^[11] 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.^{[14][15][16][17][18][19][20][21]} From these studies McManus et al. now conclude that handedness is polygenic and estimate that at least 40 loci contribute to determining this trait.^[22]

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.^[23] These results 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 is an outward reflection of brain asymmetry for motor function).

Epigenetic factors

Twin studies indicate that genetic factors explain 25% of the variance in handedness, while environmental factors explain the remaining 75%.^[24] 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 that have been related to handedness.^[25][26]

Prenatal hormone exposure

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".^{[27][28][29][30]}

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,^[31] and asymmetry of the vestibular cortex is strongly correlated with the direction of handedness.^[32]

Ultrasound

Another theory is that ultrasound may, but not always 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 left-handedness.^[33]

Handedness developmental timeline

Infants have been known to fluctuate heavily when choosing a hand to lead in grasping and object manipulation tasks. This is especially shown when observing hand dominance in one versus two-handed grasping tasks. Between 36 and 48 months, variability between handedness in one handed grasping begins to decline significantly. This difference can be seen earlier in bi-manual manipulation tasks. Children aged 18 to 36 months showed more hand preference when performing bi-manipulation tasks than simple grasping.^[34] The decrease in handedness variability for 36-to-48-month-old children could likely be attributed to preschool or kindergarten attendance. The increase in required single hand grasping activities such as writing or coloring can force children to develop a hand preference.^[34]

Correlation with other factors

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.^[35]

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.^[36]

Conversely, Joshua Goodman found evidence that left-handers were overrepresented amongst high end of the cognitive spectrum was weak due to methodological and sampling issues in conducted studies. Goodman also found that left-handers were overrepresented at the low end of the cognitive spectrum, with the mentally disabled being twice as likely to be left-handed compared to the general population, as well as generally lower cognitive and non-cognitive abilities amongst left-handed children.^[37] Moreover, Ntolka and Papadatou-Pastou in a systematic review and meta-analysis found that it is right-handers who have higher IQ scores, but this difference is negligible (about 1.5 points).^[38]

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.^[39] The researchers used Bayley scales of infant and toddler development to assess all the subjects.

Health

Left-handedness has been commonly associated with increased rates of a wide variety of different serious terminal and mental illnesses, and even earlier death.

Lower-birth-weight and complications at birth are positively correlated with left-handness.^[40]

A variety of neuropsychiatric and developmental disorders like autism spectrum disorders, depression, bipolar disorder, anxiety disorders, schizophrenia, and alcoholism has been associated with left- and mixed-handedness.^[41][42]

A 2012 study showed that nearly 40% of children with cerebral palsy were left-handed,^[43] while another study demonstrated that left-handedness was associated with a 62 percent increased risk of Parkinson's disease in women, but not in men.^[44] Another study suggests that the risk of developing multiple sclerosis increases for left-handed women, but the effect is unknown for men at this point.^[45]

Left-handed women have a higher risk of breast cancer than right-handed women and the effect is greater in post-menopausal women.^[46]

At least one study maintains that left-handers are more likely to suffer from heart disease, and in a cardiovascular context, are more likely to have reduced longevity.^[47]

Left-handers are more likely to suffer bone fractures.^[48]

One systematic review concluded: "Left-handers showed no systematic tendency to suffer from disorders of the immune system".^[49]

As handedness is a highly heritable trait, these health problems would have presented a fitness challenge indicating left-handness would previously have been eliminated through natural selection. However, left-handers enjoy an advantage in fighting and competitive, interactive sports, increasing their likelihood of reproduction.^[50]

Income

In a 2006 U.S. study, researchers from Lafayette College and Johns Hopkins University concluded that there was no statistically 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.^[51]

However, 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.^[37]

Left-handedness and sports

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.^[52]

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

• In cricket, the overall advantage of a bowler's left-handedness exceeds that resulting from experience alone: even disregarding the experience factor (i.e., even for a 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).^{[citation needed]}
• In baseball, a right-handed pitcher's curve ball will break away from a right-handed batter and towards a left-handed batter. While studies of handedness show that only 10% of the general population are left-handed, the proportion of left-handed MLB players is closer to 39% of hitters and 28% of pitchers, according to 2012 data.^[53] Historical batting averages show that left-handed batters have a slight advantage over right-handed batters when facing right-handed pitchers.^[54] 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.^[55] Fourteen of the top twenty career batting averages in Major League Baseball history have been posted by left-handed batters.^[56] 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.^[57]
• 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.^[57]
• 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.^[58] 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.^[59] A left-handed catcher would have a potentially more dangerous time tagging out a baserunner trying to score.^[59] 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.^[59] On the other hand, the Encyclopedia of Baseball Catchers states:^[59]

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 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.
• 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, though this does not necessarily indicate left-handedness.^[60]
• 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. While uncommon, there have been several notable left-handed quarterbacks.

Gender

According to a meta-analysis of 144 studies, totaling 1,787,629 participants, the best estimate for the male to female odds ratio was 1.23, indicating 23% more men are left-handed. 11% of men and 9% of women would be approximately 10% overall, at a 1.22 male to female odds ratio.^{[4][clarification needed]}

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 left-handedness,^[61] though findings are mixed.^[62][63][64]

A 2001 study also found that children who are genetically male but have different gender identities were more than twice as likely to be left-handed than a clinical control group (19.5% vs. 8.3%, respectively).^[65]

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".^[66] Greater rates of left-handedness has also been documented among pedophiles.^{[67][68][69][70]}

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.^[71]

In culture

Many tools and procedures are designed to facilitate use by right-handed people, often without realizing the difficulties incurred by the left-handed. John W. Santrock has written, "For centuries, left-handers have suffered unfair discrimination in a world designed for right-handers."^[72]

As a child 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.^[73] McManus noted 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 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.^[74]

Negative appeal

Moreover, apart from inconvenience, left-handed people have historically 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 words "sinister" (meaning both 'evil' and 'on the bearer's left on a coat of arms') and "ambisinister" meaning 'awkward or clumsy with both or either hand'.

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 one that has two meanings, one of which is unflattering to the recipient. 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, Japan and both North and South Korea), left-handed people were 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.^[75] 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.^{[citation needed]} 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.^[76][77]

In the Soviet Union, all left-handed kids were forced to write with their right hand in the soviet school system.^[78][79]

International Left-Handers Day

International Left-Handers Day is held annually every August 13.^[80] It was founded by the Left-Handers Club in 1992, with the club itself having been founded in 1990.^[80] 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."^[80] 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!"^[80]

In other animals

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.^[81] Studies of dogs, horses, and domestic cats have shown that females of those species tend to be right-handed, while males are left-handed.^[82]

See also

General