This article is about left- and right-handedness in humans.
For other uses, see Handedness (disambiguation).
"Left-hander" redirects here.
For other uses, see Left-Hander (disambiguation).
"Leftie" redirects here.
For the political orientation, see Left-wing politics.
In human biology, handedness is the 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.
Right-handedness is most common; about 90% of people are right-handed.
Handedness is often defined by one's writing hand, as it is fairly common for people to prefer to do some tasks with each hand.
There are examples of true ambidexterity (equal use of either hand), but it is rare—most people prefer one hand for most purposes.
Most of the current research suggests that left-handedness has an epigenetic marker—a combination of genetics, biology and the environment.
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.
Left-handed people are also more prone to certain health problems.
However, left-handed people have an advantage in sports that involve aiming at a target, as their opponents are more accustomed to the right-handed majority.
- Right-handedness is by far the most common type. Right-handed people are more skillful with their right hands. Studies suggest that approximately 90% of people are right-handed.
- Left-handedness is far less common than right-handedness. Studies suggest that approximately 10% of people are left-handed.
- Cross-dominance or mixed-handedness is the change of hand preference between tasks. This is very uncommon, with about a 1% prevalence.
- Ambidexterity , equal ability in both hands, is rare. Those who learn it still tend to favor their originally dominant hand. This is very uncommon, with about a 1% prevalence.
Handedness may be measured behaviourally (performance measures) or through questionnaires (preference measures).
The Edinburgh Handedness Inventory has been used since 1971 but contains many dated questions and is hard to score.
The longer Waterloo Handedness Questionnaire is not widely accessible.
More recently, the Flinders Handedness Survey (FLANDERS) has been developed.
There are several theories of how handedness develops.
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 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 work requiring fine motor skills, facilitating the use of speech while working, and resulting in right-handedness.
The theory implies that left-handed people have a reversed brain division of labor.
Verbal processing in right-handed people occurs mostly in the left hemisphere, whereas visuospatial processing is done mostly in the left.
Left-handed people have a heterogeneous brain organization where the hemispheres are either organized as in right-handers, but with hemispheres reversed; or such that both hemispheres are used for verbal processing.
An average taken across all types of left-handedness shows that left-handers are less lateralized.
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%.
From these studies, McManus et al.
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.
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.
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.
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.
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.
Correlation with other factors
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:
Conversely, Joshua Goodman found that evidence for left-handers was overrepresented amongst those with higher cognitive skills, such as Mensa members and higher-performing takers of SAT and MCAT tests, due to methodological and sampling issues in studies.
He also found that left-handers were overrepresented among those with lower cognitive skills and mental impairments, with those with Intellectual disability (ID) being roughly twice as likely to be left-handed, as well as generally lower cognitive and non-cognitive abilities amongst left-handed children.
In a systematic review and meta-analysis, Ntolka and Papadatou-Pastou found that right-handers had higher IQ scores, but that difference was 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 to 24 months.
What they discovered was 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 all 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.
Left-handed people are much more likely to have several specific physical and mental disorders and health problems.
Lower-birth-weight and complications at birth are positively correlated with left-handedness.
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 in post-menopausal women.
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".
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 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.
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.
Left-handedness and sports
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
Further information: Handedness and sexual orientation
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.
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.
Mortality rates in combat
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.
Episodic memory etc
A high level of handedness 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.
Main article: Bias against left-handed people
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."
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.
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.
Negative connotations and discrimination
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, or evil; even into the 20th century, left-handed children were beaten by schoolteachers for writing with their left hand.
The Latin adjective sinister or sinistra (as applied to male or female nouns — Latin nouns are gender specific) 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.
Until very recently in Taiwan (and still in Mainland China, Japan and both North and South Korea), left-handed people were forced 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.
In the Soviet Union, all left-handed children were forced to write with their right hand in the Soviet school system.
International Left-Handers Day
Main article: 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
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.
- Cardinal direction
- Clockwise, which also discusses counterclockwise/anticlockwise, the two terms for the opposite sense of rotation
- Dexter and sinister
- Left- and right-hand traffic
- Ocular dominance (eyedness)
- Proper right and proper left
- Relative direction
- Edinburgh Handedness Inventory
- Geschwind–Galaburda hypothesis
- Neuroanatomy of handedness
- Situs inversus
- Twins and handedness
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Handedness.