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A person who is left-handed primarily uses his or her left hand, more so than the 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 some people who are left-handed write with their right hand and use their left for everything else.
Approximately 8–15% of the adult population is left-handed. Studies indicate that left-handedness is more common in males than females, and more in homosexuals than in heterosexuals. Left-handedness, in comparison to the general population, also appears to occur more frequently in identical twins,and several groups of neurologically disordered individuals (epilepsy, Down's Syndrome, autism, mental retardation, etc.) Statistically, the identical twin of a left-handed person has a 76% chance of being left-handed. 
Causes of left-handedness
- see main article at handedness
- Warrior and his shield theory: Warriors hold their shield with their left hand to protect the heart which is slightly on the left-side of the body. This leave the right hand free for weapon use.
- Brain hemisphere division of labor: 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.
- Advantage in Hand-to-Hand Combat: Left-handers have a 'surprise' factor in combat, since the majority of the population is right-handed.
- Testosterone: According to one theory, exposure to higher rates of testosterone before birth can lead to a left-handed child.
- Birth stress theory: Left-handedness may be due to stress at birth.
- Ultrasound theory: Ultrasound scans may affect the brain of unborn children, causing higher rates of left-handedness in mothers who have ultrasound scans compared to those who do not.
Social stigma and repression of left-handedness
Common names for left-handedness
There are many colloquial terms used to refer to a left-handed person. Some are just slang or jargon words, while others may be offensive either in context or in origin. In more technical contexts, 'sinistral' may be used in place of 'left-handed' and 'sinistrality' in place of 'left-handedness'. Both of these technical terms derive from sinister, a Latin word meaning 'left'.
In Britain, the term "cack-handed" derives from a coarse Dutch term for excrement.
A left-handed individual may be known as a southpaw, particularly within sports in the United States. It is widely accepted that the term originates in baseball. Ballparks are often designed so that the batter is facing east, in order that the afternoon or evening sun does not shine in his eyes. This means that left-handed pitchers are throwing from the south side. The first use of the term 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, just three years after the first organized baseball game.
In boxing (not just in the United States) someone who boxes left-handed is usually referred to as southpaw. They are often considered trickier opponents 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. (In the film Rocky, Rocky Balboa says the term came from a boxer named Paul, whose left arm always faced south to New Jersey.)
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", Spanish, "derecha", (from Latin 'directus'); 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 "diestro" means both "skillful" and "right-handed".
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,' 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.
In Norway, the expression "venstrehåndsarbeid" (left hand work) means "something that is done in a sloppy or insatisfactory way".
Even the word "ambidexterity" reflects the bias. Its intended meaning is, "skillful at both sides." However, since it keeps the Latin root "dexter," which means "right," it ends up conveying the idea of being "right-handed at both sides."
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 arranged so that the line being cut along can be seen by a right-handed user, but is not visible to a left-handed user. Furthermore, the handles are often 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 also very frequently shaped to fit the right hand. Rulers as well are difficult to use, resulting in upside down measurements. Writing is difficult to learn for a left handed child if, as is usually the case, the writing teacher is right handed. This is because, when properly done, left handed writing is a mirror image to that of the right hander, making the learning process confusing for the left handed student. The result is that the majority of left handed people write with their hand curled around the pen so that it can meet the paper at the same angle as the right hander, rather than simply tilt the paper the opposite way. Once this habit is formed, it is difficult to break.
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. Many examples can be found in the Christian-Greek scriptures in which the wicked or evil sit at the left hand of God, while the righteous sit at the right hand of God, during Judgement. The Inuit 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 interfered 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.
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) from communal dishes, while keeping the left hand entirely concealed at mealtime. To expose the left hand during a public meal is still considered grievously offensive in some Arab cultures, particularly in desert regions [How to reference and link to summary or text]. Not all Arabs consider the left hand to be unclean. Some believe that all are born with an angel on each shoulder: the right shoulder angel records your achievements while the left shoulder angel records your faults [How to reference and link to summary or text]. This group also feels that only eating should be constrained to the right hand. Everything else may be done with the left.
Famous left-handed people
- Main article: List of famous left-handed people
Despite the suppression, there have been 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.
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.
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, where right-handed pitchers greatly outnumber lefties, it is commonly known that a left-handed batter is more successful against right-handed pitchers than a right-handed batter. 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. Interestingly, in the sport of ice hockey, there are many more left-handed shooters, and the majority of goaltenders catch with their left hand (forcing many of them to shoot left-handed, as well).
Left-handedness and intelligence
A common belief suggests that left-handed people are more intelligent or creative than right-handed people. While there is an unresolved debate within the scientific community on how to operationalize both intelligence and creativity, some studies have demonstrated a small positive correlation between left-handedness and creativity/intelligence.
In his book Right-Hand, Left-Hand, 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.  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.
Statistics show that older people are less likely to be left-handed than their younger counterparts — the percentages of left-handed people 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.
Another theory is that some lefties switch hands later in life, due to conformist pressures, or a "biological imperative." It has also been suggested that the percentage of children born left-handed may have been increasing over time.
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.[How to reference and link to summary or text]
Also, in field hockey, right-handed play is effectively required (though not explicitly so) because one rule states that the ball cannot be played with the back of the stick while another specifies that the stick be flat on its left side, which would be the "natural" side for a right-handed player. Having all players play with the same handedness is essential to keeping hockey a non-contact sport: a left-hander and a right-hander competing for the ball would tend to collide. All-left-hander matches are possible, but rare.
Being left-handed can be an advantage in many sports. For example, in fencing, a right-handed fencer is more accustomed to facing another right-handed fencer simply because being right-handed is more common. A left-handed fencer is also more accustomed to facing a right-handed opponent for the same reasons. Therefore, when a right-handed fencer faces a left-handed opponent, the righty is not as used to fighting a lefty as the lefty is used to fighting a righty, causing a slight yet noticeable advantage. The same advantage may be present for most one-on-one or face-to-face sporting events. Baseball is particularly suited to left-handed hitters for three reasons: lefties are already a step or two closer to first base in their batter's box before they even hit the ball so are more likely to beat out close plays; many baseball parks have shorter right field fences which gives left-handed sluggers a few more home runs that would otherwise be outs; and finally, most pitchers are right-handed which gives the left-handed hitter a better angle to see the ball. That is why a good switch hitter is considered valuable.
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."
Effects in Humans on Thinking
There are many theories on how being left handed affects the way a person thinks. One theory divides left- and right-handed thinkers into two camps: Linear Sequential vs. Visual Simultaneous    
Right-handed persons are thought to process information using a "linear sequential" method in which one thread must complete its processing before the next thread can be started.
Left-handed persons are thought to process information using a "visual simultaneous" method in which several threads can be processed simultaneously. Another way to view this is such: Suppose there were a thousand pieces of popcorn and one of them was colored pink. The right-handed person - using the linear sequential processing style - would look at the popcorn one at a time until they encountered the pink one. The left-handed person would spread out the pieces of popcorn and visually look at all of them to find the one that was pink. A side effect of these differing styles of processing is that right handed persons need to complete one task before they can start the next. Left-handed people, by contrast, are capable and comfortable switching between tasks. This makes them appear (to the right-handed majority) as if they do not finish anything. Alternately, left-handed people have an excellent ability to multi-task. Perhaps the anecdotal evidence that suggests they are more creative stems from this ability to multi-task.
Right-handed people process information using "analysis", which is the method of solving a problem by breaking it down to its pieces and analyzing the pieces one at a time. By contrast, left-handed people process information using "synthesis", which is the method of solving a problem by looking at the whole and trying to use pattern-matching to solve the problem. Ultimately, being left handed is not an all or nothing situation. The processing styles operate on a continuum where some folks are more visual-simultaneous and others are more linear-sequential. Also, it is very difficult to ascertain the way a human being behaves within the black box that is the brain. Whether this information about processing styles is valid or not will be borne out by future experimentation.
Most primates also exhibit a preference for using one hand over the other although their populations are not right-hand preferential. It is a common legend that most polar bears are left-handed.[How to reference and link to summary or text]
- ↑ Hardyck, C., & Petrinovich, L. F. (1977). "Left-handedness," Psychological Bulletin, 84, 385–404.
- ↑ Raymond, M.; Pontier, D.; Dufour, A.; and Pape, M. (1996). |Frequency-dependent maintenance of left-handedness in humans," Proceedings of the Royal Society of London, B, 263, 1627-1633.
- ↑ Homosexuality and Neoteny: correlation between homosexualty and left-handedness 
- ↑ Twinning Facts - National Organization of Mothers of Twins Clubs, Inc.. Accessed June 2006.
- ↑ Schachter, S. C.; Boulton, A.; Manoach, D.; O'Connor, M.; Weintraub, S.; Blume, H.; & Schomer D. L. (1995). "Handedness in patients with intractable epilepsy: Correlations with side of temporal lobectomy and gender," Journal of Epilepsy, 8, 190–192.
- ↑ Batheja, M., & McManus, I. C. (1985). "Handedness in the mentally handicapped," Developmental Medicine and Child Neurology, 27, 63–68.
- ↑ Cornish, K. M., & McManus, I. C. (1996). "Hand preference and hand skill in children with autism," Journal of Autism and Developmental Disorders, 26, 597–609.
- ↑ Grouios, G.; Sakadami, N.; Poderi, A.; & Alevriadou, A. (1999). "Excess of non-right handedness among individuals with intellectual disability: Experimental evidence and possible explanations," Journal of Intellectual Disability Research, 43, 306–313.
- ↑ Ask Yahoo!: Why am I right-handed, but my brother is left-handed? Accessed June 2006.
- ↑ yourDictionary Word of the Day: sinistral. Accessed June 2006.
- ↑ . Accessed August 2006.
- ↑ Morris, Evan (1995). Word detective research. Accessed June 2006.
- ↑ "My Left Foot," The Kingdom, 24 July, 2003. Accessed June 2006.
- ↑ Appendix I: Indo-European Roots. The American Heritage Dictionary of the English Language: Fourth Edition, 2000.
- ↑ Etimología de izquierda, deChile.com. Accessed June 2006. (Spanish)
- ↑ Right-Hand, Left-hand official website. Accessed June 2006.
- ↑ Steele, James & Mays, Simon (c1995). New findings on the frequency of left- and right-handedness in mediaeval Britain.
- ↑ Rule A.1(c), The International Rules for Polo, Federation of International Polo, 2002. (PDF) Accessed July 7, 2006.
- ↑ Rule 9.5, Rules of Hockey Including Explanations, The International Hockey Federation, 2006. (PDF) Accessed July 7, 2006.
- ↑ Rule 4.6, Rules of Hockey Including Explanations, The International Hockey Federation, 2006. (PDF) Accessed July 7, 2006.
- ↑ http://www.anthonyhempell.com/papers/tetrad/visual.html
- ↑ http://www.giftedservices.com.au/visualthinking.html
- ↑ http://www.arty4ever.com/right/brain.htm
- ↑ http://painting.about.com/library/blpaint/blrightbrain.htm
- Hardyck, C. and Petrinovitch, L.F. (1977) Left-handedness, Psychological Bulletin 84(3): 385-404.
- Schwartz, Alyssa (2005). "Lefties face increased breast cancer risk," C-Health News, 30 September.
- "Canadian Scientists Find More Homosexuals Left-Handed,", ScienceDaily, 2005. Accessed June 2006.
- Scans may 'cause brain changes', BBC News.
- 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)
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