What causes some people to be left-handed, and why are fewer people left-handed than right-handed?

Answer 1

A person who is left-handed primarily uses their left hand, more so than their right hand; a left-hander will probably use the left hand for tasks such as personal care, cooking, and so on. Writing is not as precise an indicator of handedness as it might seem, because many people who are left-handed write with their right hand and use their left for everything else.

People who are left-handed are technically said to be sinistral, and left-handedness is sometimes referred to as sinistrality. Both words derive from sinister, the Latin word for “left”. This word in turn derives from sinus, the Latin word for “pocket” referring to the fact that the pocket in a Roman toga was on the left side, for the convenience of a right-handed wearer
Statistics of left-handedness
Approximately 10% of the population is left-handed. People who can use both hands equally well are ambidextrous. True ambidexterity is rare. See laterality.

Generally, there are about 10% more male left handers than female left handers (5.24% Male, 4.76% Female). Approximately one-third of identical twins are left-handed. [1] Statistically, the twin of a left-handed person has a 76% chance of being left-handed. [1]

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Causes of left-handedness
No one knows for certain why the human population is right-handed-dominant, but a number of theories have been proposed.

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Evolutionary theories
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The warrior and his shield
This theory attempts to explain why left-handedness is rare by the position of a warrior's shield and his heart. Basically, since the heart is on the left side of the body, a right-handed warrior (who holds his shield with his left hand to free the right hand for a weapon) would be better able to protect his heart and therefore more likely to survive.

There are a number of objections to this theory:

The heart is not that far off centre. Protecting it with a shield would result in a weak selective pressure.
There have not been enough generations since the Bronze Age to make a difference.
Analysis of ancient cave paintings [2] indicates that humanity was dominantly right-handed long before the Bronze Age.
Some believe it predicts that more men would be right-handed than women. However, data indicates that more males are left-handed than females. This objection also demonstrates a misunderstanding of heredity. The theory would predict that fewer left-handed males would survive but says absolutely nothing about ratios of male: female left-handedness after that time. Nor does it explain why there would be either right-or left-handedness to begin with.
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Advantage in fist-fighting
A variant of the above argument says that left-handed people have an advantage in fighting without weapons, because of the "surprise" factor. (This fact is well known to boxers and was employed to world-record effect on Nov. 4th 1947 when Mike Collins, a natural left-hander, emerged from his corner in a right-handed stance before suddenly shifting left and delivering the fight's first and last punch, knocking out opponent Pat Brownson in 4 seconds.)

A 2004 study by Charlotte Fauriet and Michel Raymond of the University of Montpellier II in France, published in the Proceedings of the Royal Society, argues that there is such a link. To prove their theory, Faurie and Raymond surveyed nine primitive societies in five separate continents. Through a mix of direct observation and existing data, they estimated the number of left-handed people within each population. They also looked at murder rates, thinking that those communities with higher murder rates might favor populations with more left-handed people. The more violence, the more chances lefties would have at issuing their unexpected left hook, or other such weapon, and come out on top.

Among these samples, they found strong support for the idea that, at least in primitive societies with higher levels of violence, lefties thrive. [3]

Like the previous theory, it too doesn't actually explain why handedness would arise in the first place. And moreover, its result has nothing to do with why right-handedness is more common.

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Brain hemisphere division of labour
This is the most commonly accepted theory of handedness. The premise of this theory is that since both speaking and handiwork require fine motor skills, having one hemisphere of the brain do both would be more efficient than having it divided up. And since in most people, the left side of the brain controls speaking, right-handedness would prevail. It also predicts that left-handed people would have a reversed brain division of labour. Lastly, since other primates do not have a spoken language (at least of the type we have) there would be no stimulus for right-handed preference among them, and that is true.

Objections:

It does not explain why the left hemisphere would always be the one controlling language. Why not 50% of the population left and 50% right?
While 95% of right-handers do indeed use the left side of the brain for speaking, it is more variable for left-handers. Some do use the right for linguistic skills, some use the left hemisphere, and others use both.
On the balance, it appears that this theory could well explain some left-handedness, but it has too many gaps to explain all left-handedness.

Several brain-scan studies have verified a fundamental difference between left-handed and right-handed brains. A right-handed person's brain is typically highly specialized, with a specific portion of the brain dedicated to each task. This same specialization is much less prevalent in left-handed brains. Left-handed stroke victims often rehabilitate more quickly than right-handed stroke victims, as the left-handed brain has a lesser tendency to compartmentalize and specialize its abilities.

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Is left-handedness genetic?
Scientists previously believed handedness runs in families, but recent studies seem to run counter to what is increasingly looked upon as little more than unfounded lore. One study conducted in 1998 by researchers at James McDevitt University, Oklahoma, revealed that when both parents are left-handed, there is only a 26% chance of their child being left-handed (Only 10% of the human population are left-handed), an indication that genetics may not play the prominent role once thought in determining handedness. Thus, it is clear that genetics is not the only cause. Handedness must also be influenced by some of the other theories presented here.

Apparently, the Clan Kerr of Scotland built their castles with counter-clockwise staircases, so that a left-handed swordsman would be better able to defend it. However, a 1993 study found no statistically significant increase in left-handedness among people with the family name Kerr or Carr.

Many members of the British royal family are left-handed. Genetic factors are generally used to explain this. King George VI (who was the younger brother of Edward VIII, and therefore not expected to inherit the throne) was left-handed. During his boyhood and adolescence, his father George V required the prince to wear a long string tied to his left wrist; whenever he used his left hand, his father would tug the string violently, hoping to train him to become right-handed. As a result of this mistreatment, George VI developed a severe stammer. Nelson Rockefeller was also left-handed; his father used this same string technique in an equally unsuccessful attempt to switch him to right-handedness.

There is a profound stigma against left-handedness in Arabian cultures. This stigma dates back many centuries, to the pre-industrial period when paper was extremely rare and (in desert regions) water was too precious to be used for hand-washing. Because it was necessary to use one hand for wiping oneself after defecation, and because it was impossible to cleanse this hand thoroughly, the hand used for this task (traditionally, the left hand) was deemed unfit to be used for any other activity, especially as most Arabs of that time lacked eating utensils, and so they ate with their fingers (of the right hand) while keeping the left hand entirely concealed at mealtime. To expose the left hand during a public meal is still considered profoundly offensive in many Arab cultures, especially in desert regions.

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Biological theories
There is strong evidence that prenatal testosterone contributes to brain organization. One theory is that high levels of prenatal testosterone results in a higher incidence of left-handedness. See Geschwind-Galaburda Hypothesis.

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Environmental theories
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Birth stress
Its basic premise is that left-handedness is due to brain damage during the birth process. Some statistics support this theory.

Difficult or stressful births happen far more commonly among babies who grow up to be left-handed or ambidextrous. Birth stress is also associated with a number of birth defects and complications, including cerebral palsy and autism.

There are objections to the birth stress theory:

Throughout history and throughout the world, the level of medicine and technology to assist with childbirth has improved. In spite of that, the proportion of left-handed people has not decreased.
It does not explain why humans are right-handed by default, with only birth stress making them left-handed. It could, however, explain left-handedness in combination with some of the other theories presented here.
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Ultrasound
Scientists in Sweden say they have found evidence that ultrasound scans may cause brain changes in unborn babies, after they found men whose mothers had tests were more likely to be left-handed. The study suggested that scanning produced an extra three left-handed babies per one-hundred births. This research has come under some criticism, since it implies that left-handed people are in some way "brain damaged."

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Social stigma and repression of left-handedness
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Linguistic suggestion
Some left-handed people consider themselves oppressed, even to the point of prejudice. Etymology often lends weight to the argument:

In many European languages, "right" stands for authority and justice: German and Dutch, "recht", French, "droit" (from Latin 'directus'[4] ); in most Slavic languages the root "prav" is used in words "right", "correct", "justice" etc. Being right-handed has also historically been thought of as being skillful: the Latin word for right-handed is "dexter," as in dexterity; the Spanish "derecha" (also from Latin 'directus'[5]) also means right.

On the other hand, the English word "sinister" comes from Latin and it originally meant "left" but took on meanings of 'evil, unlucky' by the Classical Latin era. "Sinister" comes from the Latin word "sinus" meaning "pocket": a traditional Roman toga had only one pocket, located on the left side for the convenience of a right-handed wearer. The modern-Italian "sinistra" has both meanings of sinister and left. The Spanish "siniestra" has both, too, although the 'left' meaning is less common and is usually expressed by 'izquierda'[2], a Basque word. A left-hander was supposed to be not only unlucky, but also awkward and clumsy, as shown in the French "gauche" and the German "links" and "linkisch." As these are all very old words, they support theories indicating that the predominance of right-handedness is an extremely old phenomenon. 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 ancient China, the left has been the "bad" side. The adjective "left" (左 Mandarin: zuo) means "improper," "out of accord." For instance, the phrase "left path" (左道 mandarin: zuodao) stands for illegal or immoral means.

In some parts of China, some adults can still remember suffering for the "crime" (with suitable traumatic punishments) of not learning to be right-handed in both primary and secondary schools, as well as in some "Keeping-good-face" families.

Even the word "ambidexterity" reflects the bias. Its intended meaning is, "skillful at both sides." However, since it keeps the Latin root "dext," which means "right," it ends up conveying the idea of being "right-handed at both sides."

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Daily suppression
Left handed people are placed at a constant disadvantage by society. Nearly all tools and devices are designed to be comfortably used with the right hand. For example, scissors, a very common tool, are molded in a way that is very difficult for a left hander to hold, and extensive use causes severe discomfort and pain. Computer mice are very frequently shaped to fit the right hand, and it is nearly impossible to find ones that are left handed. Even writing is awkward, as the left hand is dragged over the text, resulting in smearing, and must be pushed to form writing as opposed to pulled using the right hand, especially in cursive writing. The hand must also be held at an extreme angle to prevent it from covering up what was just written, causing cramping. However, this difficulty is only present in cultures that write left to right.

In many parts of the world, such as Indonesia, it is considered impolite to eat and accept gifts with the left hand. The reason to this is a person who uses his left hand to eat would often cause trouble with the person to the left of him. Another stated reason for this is that the left hand is used in some countries, like Indonesia, during a bathroom visit. Until the latter part of the twentieth century, Roman Catholic nuns in American elementary schools (and possibly elsewhere, for example Dutch and German primary schools) would punish children for using their left hand to write, typically by slapping their left hand with a ruler if they attempted to pick up a pen with it. An example of such treatment involves baseball players Lou Gehrig and Babe Ruth, who both hit and threw left-handed and wrote right-handed after enduring left-handed suppression during their formative years.

Left-handedness was often interpreted as a sign of Satanic influence, and thus prohibited. The Eskimos also believed that every left-handed person was a sorcerer.

The Romans also frowned upon left-handedness. A left-handed boy who was training to be in a Roman legion would have his hand bound to his side, and would be forced to use the gladius with his right hand. This was done out of necessity, as a left-handed Roman would have interferred with the cohesion of the Roman legions.

The use of left hand was also frowned upon in Asia. Allegedly, though there were few examples of its happening, a Japanese man could once divorce his wife if he discovered that she was left-handed.

Until very recently, in Chinese societies, left-handed people were strongly encouraged to switch to being right-handed. However, this may be in part because, while Latin characters are equally easy to write with either hand, it is more difficult to write legible Chinese characters with the left hand. The prescribed direction of writing each line of a Chinese character is designed for the movements of the right hand, and some shapes tend to feel awkward to follow with the left hand's fingers. It results in a less soft writing than it would be with the right hand.

Despite the suppression, there have been, however, many famous left-handed people, and the associated right brain hemisphere that is said to be more active in left-handed people has been found in some circumstances to be associated with genius and is correlated with artistic and visual skill. As visual thinking is much promoted nowadays, left-handers cannot help but begin to gain more and more respect. As well, in certain fields, left-handedness is advantageous; for example, in baseball, it is commonly known that a left-handed batter is more successful against right-handed pitchers than a right-handed batter. A left-handed pitcher is more successful against left-handed batters than a right-handed pitcher. In soccer, left-handed players are often more skilled at playing with the left foot (though being left-handed doesn't necessarily imply being left-footed), which makes them valuable as they can play better on the left side of the field than right-handed players.

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Common names for left-handedness
There are many colloquial terms by which a left-handed person can be referred. Some are just slang or jargon words, while others may be offensive either in context or in origin.

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Southpaw
Southpaw is a term used to describe left-handed people, particularly within sports and in the United States.

The generally accepted origins of the term is from baseball. Ballparks are often designed so that the batter is facing east to avoid the afternoon or evening sun shining in his eyes. This means that left-handed pitchers are throwing from the south side. The first usage is credited to Finley Peter Dunne. However, the Oxford English Dictionary lists a non-baseball citation for "south paw", meaning a punch with the left hand, as early as 1848[3], just three years after the first organized baseball game.

In boxing a left handed boxer is usually referred to as southpaw. They are often considered a more tricky opponent than the more common right-hander. The term is also used to refer to a stance in which the boxer places his right foot in front of his left.

The official mascot of the Chicago White Sox is named Southpaw due to the team's stadium location on the south side of the city.[4]

Ciotog

Ciotog is an Irish language word used to describe left-handed people. It derives from the Irish word Ciotog which means 'Strange Person'. [6]

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Left-handedness and intelligence
A common belief suggests that left-handed people are more intelligent or creative than right-handed people, and this theory is supported by some evidence. Evidence such as numerous studies that prove left-handed children learn to come over obstacles at an earlier age, being left-handed in a primarily right-handed world.

In his book Right-Hand, Left-Hand[7], Chris McManus of University College London, argues that the proportion of left-handers is rising 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 widens their range of abilities, and the genes that determine left-handedness also govern development of the language centres of the brain.

In Britain, around 11% of men and women aged 15-24 are now left-handed, compared to just 3% in the 55-64 age category. [8] McManus suggests a number of factors that may be driving this increase:

Left-handers were severely discriminated against during the 18th and 19th centuries and it was often "beaten out" of people
In adulthood, left-handers were often shunned by society, resulting in fewer marrying and reproducing
As discrimination was reduced in the 20th century, the number of natural left-handers who stayed left-handed increased
The rising age of motherhood contributed as, statistically, older mothers are more likely to give birth to left-handed children
McManus says that the increase could produce a corresponding intellectual advance and a leap in the number of mathematical, sporting or artistic geniuses.

Unfortunately, they tend to be over-represented at both ends of the intellectual scale, and as well as geniuses, the group also produces a disproportionately high number of those with learning handicaps. There have been suggestions of links between left-handedness and dyslexia, stuttering, and child autism, among other disabilities.

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"Disappearing" Left-Handers
Statistics show that older people are less likely to be left-handed than their younger counterparts — the percentages of lefties sharply drop off with increased age. In America, 12% of 20 year olds are left-handed, while only 5% of 50 year olds and less than 1% of people over 80 are. These numbers are surprisingly divergent — how can they be explained?

A study (no longer deemed credible) published in 1991 claimed that these statistics indicate that left handed peoples' lifespans are shorter than those of their right-handed counterparts by as much as 9 years. They explained this gap by asserting that left-handed people are more likely to die in accidents as a result of their "affliction," which renders them clumsier and ill-equipped to survive in a right-handed world.

Researchers now attribute most of the difference between the age groups to the fact that older people would be more likely to have experienced pressure to switch hands, a factor not affecting the younger generations. This is supported by the fact that more women than men switched hands, and women live longer than men. However, this reasoning cannot explain all variation, and "the case of the disappearing southpaws" remains a mystery.

Other contribution factors that have been suggested include that some lefties eventually succumb to conformist pressures, and that a "biological imperative" grips them late in life and they lose their left-handedness. Also, it has been suggested that the percentage of left-handed children born may have been increasing over time.

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Left-handers in sport
There are many left-handers in sports, however, a written rule in Polo states that one must not hold a stick in their left hand. There are only three or four left-handed professionals in Polo, who are each required to use their right hand.[citation needed]

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Left-handedness and developmental disorders
Studies have shown that left-handedness is more common in those suffering from pervasive developmental disorders (such as autism or mental retardation) than among the unaffected population.

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Left-handedness and sexual offences
Studies have shown that there is a correlation between committing sexual crimes against children and being left-handed. Cantor, et. al in their article Handedness in pedophilia and hebephilia (Archives of Sexual Behavior, August 2005, vol 34 n. 4, p447ff), based on a sample of 404 adult men under treatment for pedophilia or ephebophilia, found that:

Under phallometric testing, those more aroused by stimuli of depictions of prepubescent children were more left-handed, while those more aroused by adult stimuli were found to be more right handed
However, there was no observable relationship between left-handedness and the number of pre-pubertal victims (a previous study by some of the same authors found that left-handed pedophiles tended to have more child victims than right-handed ones)
Men who offended against prepubescent children were twice as likely to be left-handed than men whose offences were primarily against adults. Once men with incest offences are excluded, men who offend against prepubescent children were three times as likely to be left-handed as men who offended against adults.
It is unclear why this is so. One possibility is that pedophilia and sexual offending is caused by a neurological condition in the brain, a condition that could also lead to left-handedness.

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Left-sidedness
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In humans
Studies show that left-handedness does not necessarily correspond with "left-sidedness." (Using your left foot to kick with, for example.) The same thing holds with "eyedness."

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In animals
Most primates also exhibit a preference for using one hand over the other although their populations are not right-hand preferential.

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See also
Handedness
List of famous left-handed people
List of fictional left-handed characters
The Tale of Cross-eyed Lefty from Tula and the Steel Flea
Geschwind-Galaburda cluster
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Sources
Habib, Marlene (2000). http://www.canoe.ca/Health0007/06_hands.html
http://www.sciencedaily.com/releases/2000/07/000710071931.htm
http://news.bbc.co.uk/1/hi/health/1699905.stm
↑ Word detective research
↑ Chicago White Sox mascot Southpaw
↑ David Wolman's book explores the mystery and meaning of all things southpaw
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External links
Lefties Have The Advantage In Adversarial Situations, ScienceDaily, April 14, 2006.
Science Creative Quarterly's overview of some of the genetic underpinnings of left-handedness
Quirks & Quarks June 10, 2006 (CBC radio documentary on left-handedness including interviews with four scientists holding different views on the determinants of handedness)

Answer 2

I dont know what causes people to become left-handed.
But here is the logic anyway for most of us being right-handed.

If u are Right-handed then that is your Master hand which u will use it to handle a weapon, example a Sword.
That leaves with your Left-Hand to protect your Heart !
Imagine it the other way! So after many countless battles in the ancient times, we humans have came to except the Right hand as the Master Hand and the Left Hand to protect the heart.

Answer 3

The left side of the brain is more dominant than the right side. The left hemisphere controls the right side of the body. And the right hemisphere controls the left side of the body. I know that's odd but that is how we work. With the left side being more dominant, most people are right handed. Left handedness occurs when the right side becomes more dominant in development. And the reasons for this are as vast as human imagination.

Answer 4

It's a recessive gene while right-handed is a dominant gene. If a RH gene is together with a LH gene, the RH gene takes over so the person is RH. The only way for a person to be LH is if 2 LH genes match up. Hence, 25% of people are LH. Your mom and dad both give you 2 genes b/c that's what they have. Depending on how they match up will determine if you are LH or RH.

Answer 5

well the fact is, there's no cause of having a left handed. It because, we have two hands it's up to an individual person which hands they comfortable to be used.

Answer 6

well...I think it's how they were brought up. Some people learned to write lefthandedly while others righthandedly or it may also be based on their parents' decision whether or not their child would be left or right handed

Answer 7

one theory...

Clare Porac, a professor of psychology at Pennsylvania State University who studies handedness, explains.
Researchers who study human hand preference agree that the side of the preferred hand (right versus left) is produced by biological and, most likely, genetic causes. The two most widely published genetic theories of human hand preference argue that evolutionary natural selection produced a majority of individuals with speech and language control in the left hemisphere of the brain. Because the left hemisphere also controls the movements of the right hand--and notably the movements needed to produce written language--millennia of evolutionary development resulted in a population of humans that is biased genetically toward individuals with left hemisphere speech/language and right-hand preference. Approximately 85 percent of people are right-handed. These theories also try to explain the persistent and continuing presence of a left-handed minority (about 15 percent of humans).

The genetic proposal to explain hand preference states that there are two alleles, or two manifestations of a gene at the same genetic location, that are associated with handedness. One of these alleles is a D gene (for dextral, meaning “right”) and the other allele is a C gene (for “chance”). The D gene is more frequent in the population and is more likely to occur as part of the genetic heritage of an individual. It is the D gene that promotes right-hand preference in the majority of humans. The C gene is less likely to occur within the gene pool, but when it is present, the hand preference of the individual with the C gene is determined randomly. Individuals with the C gene will have a 50 percent chance of being right-handed and a 50 percent chance of being left-handed.

These theories of hand preference causation are intriguing because they can account for the fact that the side of hand preference of individuals with the C gene (most left-handers and some right-handers) can be influenced by external cultural and societal pressures, a phenomenon that researchers have documented. These theories can also explain the presence of right-handed children in families with left-handed parents and the presence of left-handed children in families with right-handed parents. If the familial genetic pool contains C genes, then hand preference becomes amenable to chance influences, including the pressures of familial training and other environmental interventions that favor the use of one hand over the other. The proposed genetic locus that determines hand preference contains an allele from each parent, and the various possible genetic combinations are DD individuals who are strongly right-handed, DC individuals who are also mostly right-handed, and CC individuals who are either right-handed or left-handed. These genetic combinations leave us with an overwhelming majority of human right-handers and a small, but persistently occurring, minority of left-handers.

other causes?
Others studies suggest that men are 1.5% to 4% more likely to be left-handed than women. This difference could be linked to the male hormone testosterone. According to the alt.lefthanders Frequently Asked Questions page, some scientists speculate that exposure to excess levels of testosterone during pregnancy may cause babies to be left-handed. Adult females normally produce a small amount of testosterone, as do male fetuses -- these two influences could lead to high testosterone levels. But, much like the genetic link, the testosterone link to left-handedness hasn't been proven convincingly.

A recent Swedish study connects ultrasound scans during pregnancy with an increase in left-handed babies. The study examined men born in the 1970s and suggested a 30% increase in left-handedness among those who were exposed to ultrasound (which was less sophisticated at that time) in the womb. The study seems to indicate that prenatal trauma is one possible cause of left-handedness

Answer 8

When I was young my . . . cousin told me to write with my left hand.

Answer 9

it was during the changing of the moon