Handedness

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. 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. Overall, around 90% of people are right-handed. 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. 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, cricket, boxing, and mixed martial arts.
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 and by Williams. The longer Waterloo Handedness Questionnaire is not widely accessible. More recently, the Flinders Handedness Survey (FLANDERS) has been developed.
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. Attempts to determine handedness of early humans by analysing the morphology of lithic artefacts have been found to be unreliable.
There are several theories of how handedness develops.
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) indicates that the heritability of handedness is roughly 24%.
Two theoretical single-gene models have been proposed to explain the patterns of inheritance of handedness, by Marian Annett of the University of Leicester, and by Chris McManus 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. From these studies, McManus et al. now conclude that handedness is polygenic and estimate that at least 40 loci contribute to the 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 handedness. 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).
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.
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".
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.
Twin studies indicate that genetic factors explain 25% of the variance in handedness, and environmental factors 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 related to handedness.
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. However, the majority of left-handers have been found to have left-hemisphere language dominance—just like right-handers. 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. 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. 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.
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.
Infants have been observed to fluctuate heavily when choosing a hand to lead in grasping and object manipulation tasks, especially in one- versus two-handed grasping. Between 36 and 48 months, there is a significant decline in variability between handedness in one-handed grasping; it can be seen earlier in two-handed manipulation. Children of 18–36 months showed more hand preference when performing bi-manipulation tasks than with simple grasping.
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. Scharoun and Bryden noted that right-handed preference increases with age up to the teenage years.
The modern turn in handedness research has been towards emphasizing degree rather than direction of handedness as a critical variable.
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.
Writing in Scientific American, he states:
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. Left-handers are nevertheless 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".
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. 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).
The prevalence of difficulties in left-right discrimination was investigated in a cohort of 2,720 adult members of Mensa and Intertel by Storfer. 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. The study also revealed an effect of age, with younger participants reporting more problems.
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 to 24 months. They discovered that when a child developed a consistent use of their 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), they were 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 the subjects.
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. 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.
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, and because many of these conditions could have presented a Darwinian fitness challenge in ancestral populations, this indicates left-handedness may have previously been rarer than it currently is, due to natural selection. However, on average, left-handers have been found to have an advantage in fighting and competitive, interactive sports, which could have increased their reproductive success in ancestral populations.
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, but among college-educated people, left-handers earned 10 to 15% more than their right-handed counterparts.
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.
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. In tennis, 40% of the seeded players are left-handed. The term southpaw is sometimes used to refer to a left-handed individual, especially in baseball and boxing. Some studies suggest that right handed male athletes tend to be statistically taller and heavier than left handed ones.
Other, sports-specific factors may increase or decrease the advantage left-handers usually hold in one-on-one situations:
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 that men are 23% more likely to be left-handed. For example, if the incidence of female left-handedness was 10%, then the incidence of male left-handedness would be approximately 12% (10% incidence of left-handedness among women multiplied by an odds ratio of 1:1.23 for women:men results in a 12.3% incidence of left-handedness among men).
Some 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 people assigned male at birth whose gender identity did not align with their assigned sex, 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 have 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.
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. 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. 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.
A high level of handedness (whether strongly favoring right or left) is associated with poorer episodic memory, and with poorer communication between brain hemispheres, which may give poorer emotional processing, although bilateral stimulation may reduce such effects.
A high level of handedness is associated with a smaller corpus callosum whereas low handedness with a larger one.
Left-handedness is associated with better divergent thinking.
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."
Many products for left-handed use are made by specialist producers, although not available from normal suppliers. Items as simple as a knife ground for use with the right hand are less convenient for left-handers. There is a multitude of 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. 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.
Writing from left to right as in many languages, in particular, with the left hand covers and tends to smear (depending upon ink drying) what was just written. Left-handed writers have developed various ways of holding a pen for best results. 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.
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. Writing left-handed with a dip pen, in particular, was prone to blots and smearing.
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 languages, including English, the word for the direction "right" also means "correct" or "proper". Throughout history, being left-handed was considered negative, or evil.
Black magic is sometimes referred to as the "left-hand path".
Before the development of fountain pens and other writing instruments, 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. Even with more modern pens, writing from left to right, as in many languages, with the left hand covers and can smear what was just written when moving across the line.
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, or had their left hands tied behind their backs to force them to write with their right hand. 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.
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.
In the Soviet school system, all left-handed children were forced to write with their right hand.
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."
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. 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.