Handedness

From Wikipedia, the free encyclopedia

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 less capable or less preferred hand is called the non-dominant hand. Men are somewhat more likely to express a strongly dominant left hand than women. It is estimated that between 70 and 95 percent of the world's population is right-handed.

Types

• Right-handedness is the most common type. Right-handed people are more skillful with their right hands when performing tasks. Studies suggest 70–95% of the world's population is right-handed.
• 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.
• 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.
• Ambidexterity, equal ability to use both hands, is exceptionally rare, although it can be learned. 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.

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

Division of labor

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

Verbal processing in right-handed individuals takes place mostly in the left hemisphere, whereas visuospatial processing is mostly done in the opposite hemisphere. Left-handed individuals have a heterogeneous brain organization in which their brain 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.

Genetic factors

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

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

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

Brandler et al. performed a genome-wide association study for a measure of relative hand skill and found that genes involved in the determination of left/right asymmetry in the body play a key role in determining handedness. These results 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%. 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.

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".

Prenatal vestibular asymmetry

Previc, after reviewing a large number of studies, found evidence that the position of the fetus in the final trimester and a baby's subsequent birth position can affect handedness. About two-thirds of fetuses present with their left occiput (back of the head) at birth. This partly explains why prematurity results in a decrease in right-handedness. Previc argues that asymmetric prenatal positioning creates asymmetric stimulation of the vestibular system, which is involved in the development of handedness. In fact, every major disorder in which patients show reduced right-handedness is associated with either vestibular abnormalities or delay, and asymmetry of the vestibular cortex is strongly correlated with the direction of handedness.

Ultrasound

Another theory is that ultrasound may affect the brains of unborn children, causing higher rates of left-handedness in children whose mothers received ultrasounds during pregnancy. Research on this topic suggests there may exist a weak association between ultrasound screening (sonography used to check on the healthy development of the fetus and mother during pregnancy) and left-handedness.

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. 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.

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.

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.

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. 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).

Early childhood intelligence

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

Health

Lower-birth-weight and complications at birth are positively correlated with left-handness.

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.

A 2012 study showed that nearly 40% of children with cerebral palsy were left-handed, 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. 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.

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

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.

Left-handers are more likely to suffer bone fractures.

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

If handedness is entirely genetic, these health problems mean left-handness could be eliminated through natural selection. However, left-handers enjoy an advantage in fighting and sports increasing their likelihood of reproduction.

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.

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

Left-handedness and sports

Interactive sports such as table tennis, badminton 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.

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).
• In baseball, a right-handed pitcher's curve ball will break away from a right-handed batter and towards a left-handed batter. Historical batting averages show that left-handed batters have a slight advantage over right-handed batters when facing right-handed pitchers. Because there are fewer left-handed pitchers than right-handed pitchers, left-handed batters have more opportunities to face right-handed pitchers than their right-handed counterparts have against left-handed pitchers. Fourteen of the top twenty career batting averages in Major League Baseball history have been posted by left-handed batters. Left-handed batters have a slightly shorter run from the batter's box to first base than right-handers. This gives left-handers a slight advantage in beating throws to first base on infield ground balls.
• Because a left-handed pitcher faces first base when he is in position to throw to the batter, whereas a right-handed pitcher has his back to first base, a left-handed pitcher has an advantage when attempting to pickoff baserunners at first base.
• Defensively in baseball, left-handedness is considered an advantage for first basemen because they are better suited to fielding balls hit in the gap between first and second base, and because they do not have to pivot their body around before throwing the ball to another infielder. For the same reason, the other infielder's positions are seen as being advantageous to right-handed throwers. Historically, there have been few left-handed catchers because of the perceived disadvantage a left-handed catcher would have in making the throw to third base, especially with a right-handed hitter at the plate. A left-handed catcher would have a potentially more dangerous time tagging out a baserunner trying to score. With the ball in the glove on the right hand, a left-handed catcher would have to turn his body to the left to tag a runner. In doing so, he can lose the opportunity to brace himself for an impending collision. On the other hand, the Encyclopedia of Baseball Catchers states:

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.
• 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.

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, though findings are mixed.

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).

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". Greater rates of left-handedness has also been documented among pedophiles.

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.

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."

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

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 word "sinister" (only when referring to the bearer's left of a coat of arms).

There are many negative connotations associated with the phrase "left-handed": clumsy, awkward, unlucky, insincere, sinister, malicious, and so on. A "left-handed compliment" is considered one that is unflattering or dismissive in meaning. In French, gauche means both "left" and "awkward" or "clumsy", while droit(e) (cognate to English direct and related to "adroit") means both "right" and "straight", as well as "law" and the legal sense of "right". The name "Dexter" derives from the Latin for "right", as does the word "dexterity" meaning manual skill. As these are all very old words, they would tend to support theories indicating that the predominance of right-handedness is an extremely old phenomenon. 

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

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

International Left-Handers Day

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

In other animals

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

Our brains reveal our choices before we're even aware of them, study finds

by Lachlan Gilbert, University of New South Wales
Original link: https://medicalxpress.com/news/2019-03-brains-reveal-choices-aware.html?fbclid=IwAR26kamLC1X5fJZCIK5n_GD-2CmokH1Ovs8fC882wUOPcSH5SCn9mPXgM6o

A new UNSW study suggests we have less control over our personal choices than we think, and that unconscious brain activity determines our choices well before we are aware of them.

Published in Scientific Reports today, an experiment carried out in the Future Minds Lab at UNSW School of Psychology showed that free choices about what to think can be predicted from patterns of brain activity 11 seconds before people consciously chose what to think about.

The experiment consisted of asking people to freely choose between two visual patterns of red and green stripes – one of them running horizontally, the other vertically – before consciously imagining them while being observed in a functional magnetic resonance imaging (fMRI) machine.

The participants were also asked to rate how strongly they felt their visualisations of the patterns were after choosing them, again while researchers recorded their brain activity during the process.

Not only could the researchers predict which pattern they would choose, they could also predict how strongly the participants were to rate their visualisations. With the assistance of machine learning, the researchers were successful at making above-chance predictions of the participants' volitional choices at an average of 11 seconds before the thoughts became conscious.

The brain areas that revealed information about the future choices were located in executive areas of the brain – where our conscious decision-making is made – as well as visual and subcortical structures, suggesting an extended network of areas responsible for the birth of thoughts.

Lab director Professor Joel Pearson believes what could be happening in the brain is that we may have thoughts on 'standby' based on previous brain activity, which then influences the final decision without us being aware.

"We believe that when we are faced with the choice between two or more options of what to think about, non-conscious traces of the thoughts are there already, a bit like unconscious hallucinations," Professor Pearson says.

"As the decision of what to think about is made, executive areas of the brain choose the thought-trace which is stronger. In, other words, if any pre-existing brain activity matches one of your choices, then your brain will be more likely to pick that option as it gets boosted by the pre-existing brain activity.

"This would explain, for example, why thinking over and over about something leads to ever more thoughts about it, as it occurs in a positive feedback loop."

Interestingly, the subjective strength of the future thoughts was also dependent on activity housed in the early visual cortex, an area in the brain that receives visual information from the outside world. The researchers say this suggests that the current state of activity in perceptual areas (which are believed to change randomly) has an influence in how strongly we think about things.

These results raise questions about our sense of volition for our own private and personal mental visual images. This study is the first to capture the origins and content of involuntary visual thoughts and how they might bias subsequent voluntary conscious imagery.

The insight gained with this experiment may also have implications for mental disorders involving thought intrusions that use mental imagery, such as PTSD, the authors say.

However, the researchers caution against assuming that all choices are by nature predetermined by pre-existing brain activity.

"Our results cannot guarantee that all choices are preceded by involuntary images, but it shows that this mechanism exists, and it potentially biases our everyday choices," Professor Pearson says.

Bias against left-handed people

From Wikipedia, the free encyclopedia

Bias based on handedness is bias or design that is usually unfavorable against people who are left-handed. Part of this is due to design in the world which is often right-hand biased. Handwriting is one of the biggest sources of actual disadvantage for left-handed people, other than for those forced to work with certain machinery. About ten percent of the world's population is left-handed, yet many common articles are designed for efficient use by right-handed people, and may be inconvenient, painful, or even dangerous for left-handed people to use. These may include school desks, kitchen implements, and tools ranging from simple scissors to hazardous machinery such as power saws.

 

Beyond being inherently disadvantaged by a right-handed bias in the design of tools, left-handed people have been subjected to deliberate discrimination and discouragement. In certain societies, they may be considered unlucky or even malicious by the right-handed majority. Many languages still contain references to left-handedness to convey awkwardness, dishonesty, stupidity, or other undesirable qualities. Even in relatively advanced societies, left-handed people were historically (and in some cases still are) forced as children to use their right hands for tasks which they would naturally perform with the left, such as eating or writing.

Favorable perceptions

Lloque Yupanqui, the third Sapa Inca, whose name means "the glorified lefthander"

 

Among Incas left-handers were called (and now are called among the indigenous peoples of the Andes) lloq'e (Quechua: lluq'i) which has positive value. Peoples of the Andes consider left-handers to possess special spiritual abilities, including magic and healing. 

The Third Sapa Inca—Lloque Yupanqui—was left-handed. His name, when translated from Quechua, means "the glorified lefthander."

In the Chinese language, the character for "left", 左, depicts a left hand attending to its work. In contrast, the character for "right", 右 (yòu), depicts a right hand in relation to the mouth, suggesting the act of eating.

In tantra Buddhism, the left hand represents wisdom.

In early Roman times, the left side retained a positive connotation, as the Augures proceeded from the eastern side. The negative meaning was subsequently borrowed into Latin from Greek, and ever since in all Roman languages. 

In Russian, "levsha" (lefty, lefthander) became a common noun for skilled craftsman, after the title character from "The Tale of Cross-eyed Lefty from Tula and the Steel Flea" written in 1881 by Nikolai Leskov.

Unfavorable perceptions

The unfavorable associations and connotations of the use of the left hand among cultures are varied. In some areas, in order to preserve cleanliness where sanitation was an issue, the right hand, as the dominant hand of most individuals, was used for eating, handling food, and social interactions. The left hand would then be used for personal hygiene, specifically after urination and defecation. These rules were imposed on all, no matter their dominant hand. Through these practices, the left hand became known as the "unclean" hand. Currently, amongst Muslims and in some societies including Nepal and India it is still customary to use the left hand for cleaning oneself with water after defecating. The right hand is commonly known in contradistinction from the left, as the hand used for eating. In the film Lawrence of Arabia there is no insult taken when the guide offers his food to Lawrence and he takes it to his mouth with his left hand.

In many religions, including Christianity, the right hand of God is the favored hand. For example, Jesus sits at God's right side. God's left hand, however, is the hand of judgement. The Archangel Gabriel is sometimes called "God's left hand", sits at God's left side, and is one of six angels of death. Those who fall from favor with God are sent to left, as described in Matthew 25: 32–33, in which sheep represent the righteous and goats represent the fallen: "And he shall separate them one from another, as a shepherd divideth his sheep from the goats. And he shall set the sheep on his right, but the goats on his left." In 19th-century Europe, homosexuals were referred to as "left-handed". In Protestant-majority parts of the United Kingdom, Catholics were called "left-footers", and vice versa in Catholic-majority parts of Ireland and Irish America. Black magic is sometimes referred to as the "left-hand path", which is strongly associated with Satanism.

Various innocuous activities and experiences become rude or even signs of bad luck when the left hand becomes involved. In some parts of Scotland, it is considered bad luck to meet a left-handed person at the start of a journey. In Ghana, pointing, gesturing, giving or receiving items with the left hand is considered taboo or rude. A person giving directions will put their left hand behind them and even physically strain to point with their right hand if necessary. In some Asian countries, holding eating utensils in the left hand is considered impolite.

Forced use of the right hand

Due to cultural and social pressures, many left-handed children were encouraged or forced to write and perform other activities with their right hands. This conversion can cause multiple problems in the developing left-handed child, including learning disorders, dyslexia, stuttering and other speech disorders. Shifts from left- to right-handed are more likely to be successful than right to left, though neither have a high success rate to begin with. Successful shifters are more likely to become ambihanded than unsuccessful ones. Conversions can be successful with consistent daily practice in a variety of manual activities, but though activity in the non-dominant left-hemisphere of the brain will increase during tasks, so too will activity in the dominant right-hemisphere. Consistent left-handers have no higher activity in these task centers than converted left-handers, so it may be inferred that "attempts to switch handedness by educational training far from weakening the functional expression of lefthandedness in higher-order motor areas of the (dominant) right hemisphere in fact enhance it."

Many Asian countries encourage or force their children to become right-handed due to cultural perceptions of bad luck associated with the left hand. In India and Indonesia, it is considered rude to eat with the left hand. In a 2007 study in Taiwan, about 59.3% of children studied had been forced to convert from left-handedness to right-handedness. The study took into account economic status of the children's families and found that children whose parents had less education were more likely to be forced to convert. Even among children whose parents had higher levels of education, the conversion rate was 45.7%. Among naturally left-handed Japanese senior high school students, only 0.7% and 1.7% of individuals used their left hand for writing and eating, respectively, though young Japanese are more likely to convert to using chopsticks right-handed than forks or spoons (29.3% to 4.6%). The proportion of females subjected to forced conversion is significantly higher compared to males (95.1% to 81.0%).

Malawians cite their views that "the left hand is less skilled and less powerful than the right one" as main reasons for forcing left-handers to convert. Among students, teachers and parents, 75% said the left hand should not be used to perform habitual activities, and 87.6% of these believed left-handers should be forced to switch dominant hands. Parents and close relatives are most responsible for impressing these beliefs upon their children.

Rise in acceptance of left-handedness

On March 8, 1971, The Florence Times—Tri-Cities Daily reported that left-handed people "are becoming increasingly accepted and enabled to find their right (or left) place in the world." The Florence Times—Tri-Cities Daily also wrote "we still have a long way to go before the last vestiges of discrimination against left-handedness are uprooted, however." The frequency of left-handed writing in the United States, which was only 2.1 percent in 1932, had risen to over 11 percent by 1972. According to an article by The Washington Post from August 13, 1979, a psychologist from University of Chicago named Jerre Levy said: "In 1939, 2 percent of the population wrote with the left hand. By 1946, it was up to 7 1/2 percent. In 1968, 9 percent. By 1972, 12 percent. It's leveling off, and I expect the real number of left-handers will turn out to be about 14 percent." According to the article by The Washington Post from August 13, 1979, "a University of Michigan study points out that left-handers may not be taking over the world but...7 percent of the men and 6 percent of the women over 40 who were interviewed were lefties, but the percentages jumped to well above 10 percent in the 18-to-39 age group." According to the article by The Washington Post from August 13, 1979, Dr. Bernard McKenna of the National Education Association said: "There was recognition by medical authorities that left-handedness was normal and that tying the hand up in a child often caused stuttering." In Japan, Tokyo psychiatrist Soicki Hakozaki coped with such deep-seated discrimination against left-handed people that he wrote The World of Left-Handers. Hakozaki reported finding situations in which women were afraid their husbands would divorce them for being left-handed. According to the article by The Washington Post from August 13, 1979, an official at the Japanese Embassy said: "Before the war, there was discrimination against left-handers," said the official at the Japanese Embassy. "Children were not trained to use their left hand while eating or writing. I used to throw a baseball left-handed, but my grandparents wanted me to throw right-handed. I can throw either way. Today, in some local areas, discrimination may still remain, but on the whole, it seems to be over. There are many left-handers in Japan." In an article by The Washington Post from December 11, 1988, Richard M. Restak wrote that left-handedness became more accepted and people decided to leave southpaws alone and decided to quit working against left-handedness. In an article by The Gadsden Times from October 3, 1993, the newspaper mentioned a 5-year-old named Daniel, writing: "the advantage that little Daniel does have of going to school in the '90s is that he will be allowed to be left-hander. That wasn't always the case in years past." In a 1998 survey, 24 percent of younger-generation left-handed people reported some attempts to switch their handedness.

Equipment

Left-handed (left) and right-handed scissors

 

Kitchen knives: (1) symmetrical, (2) right-handed, (3) left-handed. Cross sections seen from the handle.

 

Because the vast majority of the world population is right-handed, most everyday items are mass-produced for ease of use with the right hand. Tools, game equipment, musical instruments and other items must be specially ordered for left-handed use, if they are even produced and are usually more expensive than their right-handed counterparts.

Household items

Right-handed tools can be difficult, uncomfortable, or dangerous for left-handed people to use.

Scissors

For example, (right-handed) scissors are arranged so that, in the right hand, fingers and thumb push the blades together laterally, creating the shearing action essential to scissors' utility. In the left hand, however, fingers and thumb tend to force right-handed blades apart, so that, rather than being sheared, the work-material is merely hacked, as by a knife, or slips between the blades uncut. Left-handers using right-handed scissors will often try to compensate by forcing the handles apart laterally, causing discomfort or injury to the first knuckle of the thumb.

In addition, a right-handed person using right-handed scissors can easily see the cutting line, but for a left-handed user the upper blade obscures the view.

Many scissors are offered as "ambidextrous" or "suitable for right- or left-handed use." Typically, these are merely right-handed scissors with modified handles to permit use in the left hand with less discomfort, but because the blades are still arranged for right-handed use, they still won't perform quite as well in the left hand.

Computer input devices

Input devices for computers can present obstacles to left-handed operation if intended for right-handed use. Some computer set-ups have the mouse placed on the right side of the keyboard and unable to be repositioned to the left. The mouse itself is also sometimes shaped to fit the right hand better. The functions of mouse buttons, even on mice shaped to fit both hands, default to right-hand use. On two-button mice, the left button —under the index finger of a right-handed person— is the primary button, while the right button performs secondary functions. The on-screen pointers themselves are also configured for right-handed people. Most desktop operating systems allow a user to reverse the functionality of mouse buttons to accommodate left-handed use, but left-handed cursors sometimes need to be specially downloaded. Trackballs and trackpads are often shaped for right-handed use. Even with the ability to change the functionality of buttons, these devices may be difficult for left-handed use. For a left handed person there are computer mice designed for left handed use, but they are a much smaller segment of the marketplace.

Video game controllers often have the action buttons on the right with the directional controls on the left. In first-person shooters, many games default to the right pointer-finger being used to fire, simulating a right-handed person pulling the trigger of the weapon. Certain systems' layouts, such as Nintendo's Wii U and 3DS, have games in which right-handed stylus use is assumed and can cause difficulty for left-handed players.

A right-handed trackball is difficult to use with the left hand.

Knives

While European-style kitchen knives are usually symmetrical, Japanese kitchen knives have the cutting edge ground asymmetrically, having the cutting edge closer to the user's body with ratios ranging from 70–30 for the average chef's knife, to 90–10 for professional sushi chef knives; left-handed models are rare, and usually must be specially ordered or custom made. On the other hand, the majority of "flat ground" general-purpose knives typically have the cutting edge on the right, as with a left-handed knife, for aesthetic reasons rather than practical.

Cameras

One of the few cameras ever produced for left-handers was the half-frame Yashica Samurai. Cameras predominantly have the hand grip, shutter release, film wind lever and commonly used selection buttons, switches and dials controlled by the right hand. Lens controls (where present) tend to be accessible by either hand. When an unskilled left-handed person uses a right-handed camera the hand control can be less steady and hence produce camera shake leading to poorer pictures at low shutter speeds.

Musical instruments

Left-handed adaptations have even bridged the world of music. Left-handed string instruments are produced, including guitars and violins. Inverted trumpets are made, too. Although the trumpet's valves are normally designed to be operated with the right hand, the prevailing belief is that left-handed trumpeters are not at a significant disadvantage. The French horn, for example, is played with the left hand, yet most horn players are right-handed. Left handed drummers also set up drum kits the exact opposite to conventional right-handed setup (i.e. hi-hat on the right, bass pedal under left foot, ride cymbal to the drummer's left, etc.).

Sports

A left-handed individual may be known as a southpaw, particularly in a sports context. It is widely accepted that the term originated in the United States, in the game of baseball. Ballparks are often designed so that batters are facing east, so that the afternoon or evening sun does not shine in their eyes. This means that left-handed pitchers are throwing with their south-side arm. The Oxford English Dictionary lists a non-baseball citation for "south paw", meaning a punch with the left hand, as early as 1848, just three years after the first organized baseball game, with the note "(orig. U.S., in Baseball)." A left-handed advantage in sports can be significant and even decisive, but this advantage usually results from a left-handed competitor's unshared familiarity with opposite-handed opponents. Baseball is an exception since batters, pitchers, and fielders in certain scenarios are physically advantaged or disadvantaged by their handedness. 

In baseball, due to the direction in which curveballs and sliders break, it is generally accepted that the pitcher has an advantage when his handedness is the same as the batter's, and the batter has an advantage when they are opposite. For this reason, many baseball teams include a left-handed specialist pitcher, who is brought into the game specifically to pitch to dangerous left-handed batters in crucial situations. A left-handed first baseman uses a more fluid motion to tag out a baserunner returning to first base during a pickoff attempt by the pitcher and has less difficulty avoiding baserunners while presenting their mitt as a target for other fielders to throw to. It is very uncommon to see a left-handed player playing any infield position other than pitcher or first basemen due to the counterclockwise flow of the game when throwing the ball around the bases. A fielder's handedness is either a physical advantage or hindrance for similar reasons throughout the infield positions, and left-handedness is not always the more desirable dexterousness. 

Left-handed bowlers are usually at an advantage during ten-pin bowling competition when hooking the ball into the pins from the left side of the lane. As there are fewer left-handed players, the lane's left side is not used as much, and thus the applied oil pattern does not change as quickly as it does for right-handed bowlers.

In boxing, someone who boxes left-handed is frequently referred to as southpaw. The term is also used to refer to a stance in which the boxer places the right foot in front of the left, so it is possible for a right-handed boxer to box with a southpaw stance. Most boxers, southpaw or otherwise, tend to train with sparring partners who adopt an orthodox stance which gives southpaws an advantage. Manny Pacquiao is an example of a southpaw (although he writes with his right hand). In the popular boxing film series Rocky, the main character Rocky Balboa is a southpaw. Southpaw is also a term in professional wrestling, often giving them the advantage. 

Fencing weapons feature left- and right-handed grips, and a left-handed fencer's stance is opposite that of a right-handed opponent. Although commonly asserted that left-handed fencers have an advantage over right-handed opponents because the line of defence favors their sword arm, this assertion describes both fencers in a mixed-handed duel, so neither competitor has a unique physical advantage. Thus the left-hander's advantage in fencing stems mostly from a right-hander's unfamiliarity with a left-handed stance.

The game of golf is most commonly played right-handed, and left-handed players typically must provide their own special golf clubs. The game can be played with both hands, provided the player has both left- and right-handed clubs, giving an advantage over one-handed players. Professional golfer Phil Mickelson plays left-handed though he is naturally right-handed.

In tennis, southpaws hold the racket in their left hand. Because of this, their grip of the handle is supposedly adjusted in a slightly different style from right-handed players. Some world champion left-handed tennis players include Jimmy Connors, Guillermo Vilas, John McEnroe, Martina Navratilova, Marcelo Ríos, Goran Ivanišević and Rafael Nadal. 

When playing volleyball, left-handed players are at an advantage when placed on the right wing to spike. This is because the ball does not travel over their head in order for them to hit the ball, and is therefore easier for a lefty to hit the ball. However, the reverse is true for the left wing. Right handed players are at an advantage on the left wing because the ball does not pass over their head while they are jumping to hit, therefore making it easier to contact the ball. 

Often the playing cards have their pips printed only in the upper left corner assuming holding them with right hand. Such design may be uncomfortable for left-handed people who may prefer all four corners of the card to be used.

Weapons

A US Navy SEALs left-handed shooter

 

The vast majority of firearms are designed for right-handed shooters, with the operating handle, magazine release, or safety mechanisms set up for manipulation by the right hand, and fired cartridge cases ejected to the right. Also, scopes and sights may be mounted in such a way as to require the shooter to place the rifle against the right shoulder. A left-handed shooter must either purchase a left-handed or ambidextrous firearm (which are manufactured in smaller numbers and are generally more expensive and/or harder to obtain), shoot a right-handed gun left-handed (which presents certain difficulties, such as the controls being improperly located for the left hand or hot shell cases being ejected towards the shooter's body, especially the eyes or down the collar or right sleeve), or learn to shoot right-handed (which may be less comfortable or "natural"). A related issue is ocular dominance, due to which left-handed people may wish to shoot right-handed, and vice versa. Ocular dominance plays more of a rule in shooting accuracy than does left versus right handiness. Therefore, ocular dominance will cause a preference on how a shooter chooses to hold a gun.

Some modern firearms are ambidextrous (e.g. the FN P90 and Heckler & Koch P7), or can be converted between right- and left-handed operation (e.g. the Heckler & Koch G36 and Steyr AUG). Bullpup rifles are particularly problematic for left-handers unless they can be reconfigured, since empty shells would be ejected straight into the shooter's face and cheek potentially causing injury, or otherwise designed from the ground up for ambidextrous use, often by way of complex ejection systems as seen on the FN F2000 and the Kel-Tec RFB. The British L85 Assault Rifle must be fired right-handed, placing left-handed soldiers at a disadvantage. In contrast, the Steyr AUG is of a modular design and the ejection port and extractor can be switched/replaced to suit the handedness of the soldier operating it. The M-16 and its variants have a fixed ejection port, but being a conventional (i.e. not bullpup) design the ejection port is forward of the operator and hence able to be fired either-handed. Circa 1985, with the introduction of the M16A2 version, a case deflector was incorporated adjacent to the ejector port to direct discarded shells in a more forward direction, making the rifle even more left hand operator friendly. The deflector is not always effective, however, as the ejected shell still stands a chance of hitting the left-handed shooter's cheek, especially in windy conditions. 

Lever action and pump action firearms present fewer difficulties for left-handers than bolt action weapons do. Many weapons with adjustable sights allow for left-handed use, but for a right eye dominant shooter it is necessary to adjust. In fact, most weapons adjust well enough that a weapon will not eject shells into a left-hander's eye.

Machinery

Power tools, machinery and other potentially dangerous equipment is typically manufactured with the right-handed user in mind. Common problems faced by left-handed operators include the inability to keep materials steady, and difficulty reaching the on/off switch, especially in emergency situations. Table saws, whose blades protrude from the top of a table and pose the risk of losing fingers or hands, have their cutting area on the right side. This makes it difficult for a left-handed operator to guide the material being cut, as it must be pushed to the right side to align with the fence to avoid kickback. On bandsaws, the blade teeth are on the left side of the blade, necessitating the material being cut to be pushed from the left side of the machine. However, at this angle, the casing of the machine containing the rest of the blade is on the operator's left side, making it extremely difficult to guide the wood with their left hand. 

Handheld circular saws are made almost exclusively for right-handers, with the motor and grip on the right side. If held in the left hand, it is impossible for the operator to see what they are cutting. Tool manufacturer Porter-Cable produces a left-handed circular saw that can be purchased relatively inexpensively.

• 

  Left-handed Remington 700 rifle action—note that the bolt handle is not visible from this side
  
• 

  A table saw. Notice how the cutting surface, fence, etc. are on the right side
  

Language

Historically, the left side, and subsequently left-handedness, was considered negative. The word "left" itself derives from the Anglo-Saxon word lyft, "weak". In Ancient Greek both words meaning "left" were euphemisms: the word ἀριστερός, aristerós (the standard word in Modern Greek as well) is derived from ἂριστος, áristos, "best", and the word εὺώνυμος, euōnymos, "of good name", is another euphemism used in lieu of "ill-named". The Latin adjective sinister/sinistra/sinistrum originally meant "left" but took on meanings of "evil" or "unlucky" by the Classical Latin era, and this double meaning survives in European derivatives of Latin, and in the English word "sinister". Alternatively, sinister comes from the Latin word sinus meaning "pocket": a traditional Roman toga had only one pocket, located on the left side. The right hand has historically been associated with skill: the Latin word for right-handed is dexter, as in "dexterity", meaning manual skill. Even the word "ambidexterity" reflects the bias. Its intended meaning is "skillful on both sides". However, since it keeps the Latin root dexter, which means "right", it ends up conveying the idea of being "right-handed on both sides". This bias is also apparent in the lesser-known antonym "ambisinistrous", which means "left-handed [i.e., clumsy] on both sides". In more technical contexts, "sinistral" may be used in place of "left-handed" and "sinistrality" in place of "left-handedness". In both Ancient Greek and Roman religion, auspices (usually the flight paths of birds, as observed by a bird-diviner, or augur) were thought to be unfavorable if appearing on the diviner's left-hand side and favorable if on the right: an ancient custom mentioned in Homer's Iliad and of apparently Middle Eastern origin (as attested in the Amarna correspondence, in which a king of Alashiya, i.e. Cyprus, requests an eagle-diviner from the Pharaoh of Egypt). 

Meanings gradually developed from use of these terms in the ancient languages. In many modern European languages, including English, the word for the direction "right" also means "correct" or "proper", and also stands for authority and justice. 

In Sanskrit, the word "वाम" (waama) stands for both "left" and "wicked." 

In most Slavic languages the root prav (right) is used in words carrying meanings of correctness or justice. In colloquial Russian the word левый (levyĭ) "left" means unofficial, counterfeit, strange. In Polish, the word prawo means "right" as well as "law", prawy means: lawful; the word lewy means "left" (opposite of right), and colloquially "illegal" (opposite of legal).

In French, droit(e) (cognate to English direct) means both "right" and "straight", as well as "law" and the legal sense of "right", while gauche means "left" and is also a synonym of maladroit, literally "not right", meaning "clumsy". Spanish, Italian, Portuguese and German have similar constructs. The Spanish term diestro and the Italian term destro mean both "right-handed" and "skillful". The contemporary Italian word sinistra has both meanings of sinister and left (the masculine adjective for sinister being sinistro), and maldestro means "clumsy". The Spanish siniestra has both, too, although the "left" meaning is less common and is usually expressed by izquierda, a Basque word that made its way into Portuguese as well. In some Spanish-speaking countries, to do something por izquierda means to engage in corrupt conduct or employ illegitimate means, whereas por derecha or a derechas means to do it the right (legitimate) way. Also, in Spanish, to tell someone "Eres tan zurdo" means that they are being clumsy, though the literal meaning is "You're so lefty." In Portuguese, the most common word for left-handed person, canhoto, was once used to identify the devil, and canhestro, a related word, means "clumsy"

.

In Romanian drept/dreaptă (coming from Latin directus) means both "right" and "straight". The word for "left" is stâng/stângă coming from Latin stancus (= stanticus) meaning "tired".

In German, recht means "right" in both the adjectival sense (correct) and the nominal (legal entitlement). The word for "left" is links, and is closely related to both link (underhand, questionable), and linkisch (clumsy).

The Dutch words for "left" (links, linker) and "right" (recht, rechts, rechter) have much the same meanings and connotations as in English. The adjective link means "cunning, shifty" or "risky". A linkerd is a "crafty devil". To look at someone over the left shoulder (iemand over de linkerschouder aanzien) is to regard him or her as insignificant.

In Irish, deas means "right side" and "nice". Ciotóg is the left hand and is related to ciotach meaning "awkward"; ciotógach (kyut-OH-goch) is the term for left-handed. In Welsh, the word chwith means "left", but can also mean "strange", "awkward", or "wrong". The Scots term for left-handedness is corrie fistit. The term can be used to convey clumsiness. 

In Finnish, the word oikea means both "right" (okay, correct) and "right" (the opposite of left). 

In Swedish, att göra något med vänsterhanden (literally "to do something with your left hand") means "to do something badly". In Swedish, vänster means "left". The term vänsterprassel means "infidelity", "adultery" and "cheating". From this term the verb vänstra is derived. 

In Hungarian, the word for right is jobb, which also means "better". The word for left is bal, which also means "bad". In Estonian, the word pahem stands for both "left" and "worse" and the word parem stands for both "right" and "better".

In Turkish, the word for right is sağ, which means "alive". The word for left is sol, which means "discolor", "die", "ill". 

In Chinese culture, the adjective "left" (Chinese character: 左, Mandarin: zuǒ) sometimes means "improper" or "out of accord". For instance, the phrase "left path" (左道, zuǒdào) stands for unorthodox or immoral means. 

In Korean, the word for right is oreun (오른), to be compared to the word meaning morally proper, orheun (옳은) which shares the same pronunciation.

In Hebrew, as well as in other ancient Semitic and Mesopotamian languages, the term "left" was a symbol of power or custody. There were also examples of left-handed assassins in the Old Testament (Ehud killing the Moabite king). The left hand symbolized the power to shame society, and was used as a metaphor for misfortune, natural evil, or punishment from the gods. This metaphor survived ancient culture and was integrated into mainstream Christianity by early Catholic theologians, such as Ambrose of Milan, to modern Protestant theologians, such as Karl Barth, to attribute natural evil to God in explaining God's omnipotence over the universe.

Expressions and colloquialisms

The left side is often associated with awkwardness and clumsiness. The Spanish expression "tener dos pies izquierdos", in English, the expression "to have two left feet", refers to clumsiness in the domains of football or dancing. A "left-handed compliment" is considered one that is unflattering or dismissive in meaning. The Polish expression "mieć dwie lewe ręce", Dutch "twee linkerhanden hebben", German "zwei linke Hände haben", the Bulgarian expression "dve levi ratse", French "avoir deux mains gauches", Hungarian kétbalkezes and Czech "Mít obě ruce levé" all mean "to have two left hands"—that one is clumsy or is a very poor handyman; the English equivalent is "to be all thumbs". Moreover, the German idiom "mit dem linken Fuß aufgestanden sein", the Spanish expression "levantarse con el pie izquierdo", the French expression "s'être levé du pied gauche" and the Hungarian expression "bal lábbal kelt fel" (literally, to get up with the left foot) mean to have a bad day and do everything wrong or unsuccessfully, related to the English expression "to get up on the wrong side of the bed". The Welsh phrase "tu chwith allan" (left side out) refers to an object being inside-out. In Russian, the use of the term nalyevo means "on the left", but can also connote taking bribes or "sneaky" behavior. Balszerencse (lit. "left luck") is Hungarian for "bad luck". 

There are many colloquial terms used to refer to a left-handed person, e.g. "southpaw" (USA). Some are just slang or jargon words, while other references may be offensive or demeaning, either in context or in origin. In some parts of the English-speaking world, "cack-handed" is slang for left-handed, and is also used to mean clumsy. The origin of this term is disputed, but some suggest it is derived from the Latin cacare, in reference to the habit of performing ablutions with the left hand, leaving the right hand "clean". However, other sources suggest that it is derived from the Old Norse word keikr, meaning "bent backwards". Australians frequently use "cacky-handed". A less common Australian slang word for a left-handed individual is the term Molly-Dooker, whose origins cannot be determined with certainty.

Handwriting

Left-handed people who speak Arabic, Persian, Urdu, Hebrew or any other right-to-left script do not have the same difficulties with writing. The right-to-left nature of these languages prevents left-handers from running their hand on the ink as happens with left-to-right languages. Because writing when moving one's hand away from its side of the body can cause smudging if the outward side of the hand is allowed to drag across the writing, it is considered easier to write the Latin alphabet with the right hand than with the left. Furthermore, it is considered more difficult to write legible Chinese characters with the left hand than it is to write Latin letters, though difficulty is subjective and depends on the person in question.

Left-to-right alphabets can be written smudge-free and in proper "forward slant" with the left hand if the paper is turned 1/4 turn clockwise (90 degrees to the right), and the left hand is drawn toward the body on forward strokes, and left to right on upward strokes (as expressed in directionality of the text). It is also possible to do calligraphy in this posture with the left hand, but using right-handed pen nibs. Otherwise, left-handed pen nibs are required in order to get the thick-to-thin stroke shapes correct for most type faces, and the left-handed calligrapher is very likely to smudge the text. Left-handed pen nibs are not generally easy to find, and strokes may have to be done backwards from traditional right-handed calligraphic work rules to avoid nib jamming and splatter. Left-handed people have an advantage in learning 19th-century copperplate hands, which control line-width by pressure on the point.

Civic life

Schooling

As the majority of students in public schools are right-handed, most commonly used implements are designed for the right-handed, from pencil sharpeners to scissors to desks. The consequences to left-handed students can vary from decreased academic performance or physical ailments to nothing at all.

In many classrooms and lecture halls, desks are designed so that the writing surface is attached to the chair instead of separate from it. In this design, the desk is attached on the right side, offering an armrest for right-handed people to use while writing. In some of these desks, the writing surface does not extend fully to the left, necessitating a left-handed user to turn their body in order to write properly, sometimes causing back, neck and shoulder problems. This contorted posture may also cause the student to be accused of cheating. In some cases, however, large lecture halls will use left-handed desks on the left-most column of each section so that left-handed people can comfortably write without bumping against a neighbor. 

This right-handed bias is reflected in standardized tests as well. Multiple-choice tests tend to put the question on the left side of the page and the answers on the right side. If the answers must be filled in in a separate booklet, it can be awkward and uncomfortable for a left-handed student to line up the booklet with the answers. The time it takes to find a comfortable, convenient position cuts into test-taking time, resulting in rushed answers and unchecked work.

In Vietnam, schools officially require students to write with their right hands, and some teachers of Grade 1 (when writing is taught) would implement that rule by deducting points from tests written by the left hand. The Civic Education textbook for grade 6 mentions a student thanking her old teacher for helping her write with her right hand, and stopping her from using her left hand to write.

Employment

In research done on the relations of handedness and employment, researchers may start their experiments believing left-handers earn lower wages than their right-handed counterparts, due to effects like difficulty using right-handed tools and increased rate of illness among left-handed people. However, their findings are more complex. In studies in the United States and United Kingdom, it was found that left-handed men earn more than right-handed men; about 5% more in the UK. Conversely, left-handed women earn about 7.5% less than right-handed women.