Handedness is a vague term that does not have a fixed or agreed definition. Although the terms left and right are enough to define handedness in an ordinary disclosure, they do not suffice for scientific research.  The scientific definition of handedness can be based on two theoretical assumptions. It can be defined as the hand that performs faster or more precisely on tasks or the hand that one prefers to use, regardless of performance.  In a scientific study, it should be recognized that handedness is not a discrete variable (right or left), but a continuous one that can be expressed at various levels between strong left and strong right. There are four different types of handedness that include: left-handedness, right-handedness, mixed-handedness, and ambidexterity. 
- Right-handedness is most common. Right-handed people are more dexterous with their right hands when performing tasks. A variety of studies suggest that 70–90% of the world population is right-handed.
- Left-handedness is less common than right-handedness. Left-handed people are more dexterous with their left hands when performing tasks. A variety of studies suggest that approximately 10% of the world population is left-handed.
- Mixed-Handedness is the change of hand preference between different tasks. This is common in the population with about a 30% prevalence.
- Ambidexterity is exceptionally rare, although it can be learned. A truly ambidextrous person is able to do any task equally well with either hand. Those who learn it still tend to favor their originally dominant hand. 
Emergence in humans 
Division of labor 
One common theory, as to how handedness affects the hemispheres, is the brain hemisphere division of labor. Since speaking and handiwork require fine motor skills, its presumption is that it would be more efficient to have one brain hemisphere do both, rather than having it divided up. Since in most people, the left side of the brain controls speaking, right-handedness predominates. This theory also predicts that left-handed people have a reversed brain division of labor.
Verbal processing in right-handed individuals takes place mostly in the left hemisphere, whereas visuospatial processing is mostly done in the opposite hemisphere. Left-handed individuals have a heterogeneous brain organization in which their brain hemisphere is either organized in the same way as right handers, the opposite way, or even such that both hemispheres are used for verbal processing. When the average is taken across all types of left-handedness, it shows that left-handers are less lateralized. 
Prenatal vestibular asymmetry 
Previc, after reviewing a large number of studies, found evidence that the position of the fetus 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.
A according to whom?] is that ultrasound may affect the brains of unborn children, causing higher rates of left-handedness in children whose mothers received ultrasounds during pregnancy. Research on this topic suggests there may exist a weak association between ultrasound screening (sonography used to check on the healthy development of the fetus and mother during pregnancy) and non-right-handedness.[
Twins theory 
This theory postulates that left-handed individuals were originally part of an identical twin pair, with the right-handed twin fetus failing to develop early in development. Although Australian researchers claimed to have debunked the related vanishing twin theory, it is re-examined in Rik Smits's book "The Puzzle of Left-Handedness (2012)", which considers the fact that twin children have a high frequency of left-handedness / right-handedness in the pair.
Genetic factors 
Handedness displays a complex inheritance pattern. For example, if both parents of a child are left-handed, there is only a 26% chance of that child also being left-handed. A large study of twins from 25,732 families by Medland et al. (2006) has indicated that the heritability of handedness is 24%. This leaves about three quarters of the effect to be explained by environmental factors.
To date, two theoretical single gene models have been proposed to explain the patterns of inheritance of handedness, the first by Dr. Marian Annett of the University of Leicester and the second by Professor Chris McManus of UCL.
Both models propose that there is a variant in a single gene that has two alleles. Carriers of one allele are more likely to be right-handed, and the other allele does not specify the direction of handedness, instead leaving it to chance. They differ on the precise effect of the 'right-shift' allele, but both models provide similar fits to data on the inheritance of handedness. Oxford University psychiatrist Professor Tim Crow has taken the single-gene model one step further, and proposed that mutations in the gene PCDH11X were responsible for the evolution of handedness, cerebral asymmetry, language, susceptibility to schizophrenia, and was the speciation event that created Homo sapiens.
Although single-gene models can be fitted to the data, a number of genetic linkage studies have been performed, all of which have provided evidence for different regions of the genome contributing to variation in handedness.  Only one of these studies has led to the identification of a specific gene that is proposed to contribute to variation in handedness. 
Medland et al. found a gene that is positively correlated with left-handedness in females, and negatively correlated in males. This may help to explain why there are more left-handed men than women (around 12% in men versus 10% in women globally).
In his book Right-Hand, Left-Hand, Chris McManus of University College London argues that the proportion of left-handers is increasing and left-handed people as a group have historically produced an above-average quota of high achievers. He says that left-handers' brains are structured differently (in a way that increases their range of abilities) and the genes that determine left-handedness also govern development of the language centers of the brain, although many scientists agree that there is no real difference in intelligence between the two.
In a 2006 U.S. study, researchers from Lafayette College and Johns Hopkins University concluded that there was no scientifically 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.
Athletes have a larger distribution of left-handedness, not because of skill, but due to the fact that the time taken for the brain to send a signal to the left hand is slightly shorter than the time for the brain to send a signal to the right hand (approx. 0.3ms), and as such, the reaction times of left handed athletes are slightly faster.  Interactive sports such as table tennis, badminton, cricket, and tennis have an over-representation of left-handedness, while non-interactive sports such as swimming show no over-representation. Smaller physical distance between participants increases the over-representation.
Several studies have found that lower APGAR scores are strongly associated with left- handedness. Overall, left-handed people are two times more likely than right-handed people to have gone through a stressful birth. Also, mothers who smoke during pregnancy tend to give birth to left-handed children due to hypoxia. Observation of data shows that in the five different types of life threatening accidents (work, sports, home, tools, and driving) left-handed people are 1.2-1.8 times as likely as right-handed people to suffer a fatal accident. 
See also 
- In general
- Edinburgh Handedness Inventory
- Geschwind–Galaburda hypothesis
- Handedness and mathematical ability
- Musicians who play left-handed
- Situs inversus
- Southpaw stance (boxing)
- Handedness and sexual orientation
- Annett, Marian (2002). Handedness and Brain Asymmetry.
- Holder, M. K. (1997). "Why are more people right-handed?". Sciam.com. Scientific American Inc. Retrieved 2008-04-14.
- Psychology for A-level second edition, page 309
- Hardyck C, Petrinovich LF (1977). "Left-handedness". Psychol Bull 84 (3): 385–404. doi:10.1037/0033-2909.84.3.385. PMID 859955.
- Banich, Marie (1997). Neuropsychology: The Neural Bases of Mental Function.
- Nonright-handedness, central nervous system and related pathology, and its lateralization: A reformulation and synthesis.
- Dieterich, M.; Bense, S.; Lutz, S.; Drzezga, A.; Stephan, T.; Bartenstein, P.; Brandt, T. (2003). "Dominance for vestibular cortical function in the non-dominant hemisphere". Cerebral Cortex 13 (9): 994–1007. doi:10.1093/cercor/13.9.994. PMID 12902399.
- Salvesen, K. Å. (1 September 2011). "Ultrasound in pregnancy and non-right handedness: meta-analysis of randomized trials". Ultrasound in Obstetrics & Gynecology 38 (3): 267–271. doi:10.1002/uog.9055.
- Vanishing twin theory debunked
- How New Humans Are Made by Charles E. Boklage
- McManus, Chris. Right Hand, Left Hand. Phoenix Paperbacks, 2003. Print.
- Medland et al. Neuropsychologia (2008) 47:330-337
- Annett, M. (2009). 5. In Sommer, Iris E. C; Kahn, René S. "The Genetic Basis of Lateralization". British Language lateralization and psychosis: 73–86. doi:10.1017/CBO9780511576744.006. ISBN 9780511576744. Unknown parameter
- Crow, TJ (2008 Jul). "The 'big bang' theory of the origin of psychosis and the faculty of language". Schizophrenia research 102 (1–3): 31–52. doi:10.1016/j.schres.2008.03.010. PMID 18502103.
- Francks, C; DeLisi, LE; Fisher, SE; Laval, SH; Rue, JE; Stein, JF; Monaco, AP (2003 Feb). "Confirmatory evidence for linkage of relative hand skill to 2p12-q11". American Journal of Human Genetics 72 (2): 499–502. doi:10.1086/367548. PMC 379245. PMID 12596796.
- Van Agtmael, T; Forrest, SM; Williamson, R (2002 Oct). "Parametric and non-parametric linkage analysis of several candidate regions for genes for human handedness". European journal of human genetics : EJHG 10 (10): 623–30. doi:10.1038/sj.ejhg.5200851. PMID 12357333.
- Warren, Diane M.; Stern, Michael; Duggirala, Ravindranath; Dyer, Thomas D.; Almasy, Laura (1 November 2006). "Heritability and linkage analysis of hand, foot, and eye preference in Mexican Americans". Laterality: Asymmetries of Body, Brain and Cognition 11 (6): 508–524. doi:10.1080/13576500600761056.
- Laval, SH; Dann, JC; Butler, RJ; Loftus, J; Rue, J; Leask, SJ; Bass, N; Comazzi, M; Vita, A; Nanko, S; Shaw, S; Peterson, P; Shields, G; Smith, AB; Stewart, J; DeLisi, LE; Crow, TJ (1998 September 7). "Evidence for linkage to psychosis and cerebral asymmetry (relative hand skill) on the X chromosome". American journal of medical genetics 81 (5): 420–7. doi:10.1002/(SICI)1096-8628(19980907)81:5<420::AID-AJMG11>3.0.CO;2-E. PMID 9754628.
- Francks et al. Molecular Psychiatry (2007) 12:1129-1139
- Gene for left-handedness is found, http://news.bbc.co.uk/2/hi/health/6923577.stm, BBC, 31 July 2007
- Medland, Sarah E.; Duffy, David L.; Spurdle, Amanda B.; Wright, Margaret J.; Geffen, Gina M.; Montgomery, Grant W.; Martin, Nicholas G. (1 November 2005). "Opposite Effects of Androgen Receptor CAG Repeat Length on Increased Risk of Left-Handedness in Males and Females". Behavior Genetics 35 (6): 735–744. doi:10.1007/s10519-005-6187-3. PMID 16273319.
- Papadatou-Pastou, M; Martin, M; Munafò, MR; Jones, GV (2008 Sep). "Sex differences in left-handedness: a meta-analysis of 144 studies". Psychological Bulletin 134 (5): 677–99. doi:10.1037/a0012814. PMID 18729568.
- Right-Hand, Left-Hand official website Accessed June 2006.
- Waldfogel, Joel (August 16, 2006). "Sinister and Rich: The evidence that lefties earn more". Slate.
|Look up handedness in Wiktionary, the free dictionary.|
|Wikimedia Commons has media related to: Left-handedness|
- 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
- A left-handed senior citizen recalls the emotional torment he faced at a New York public school in the 1920s. (Audio slideshow)
- Woznicki, Katrina (2005). "Breast Cancer Risk Doubles for Southpaw Women", MedPage Today, 26 September.
- Hansard (1998) ‘Left-handed Children’, Debate contribution by the Rt Hon. Mr. Peter Luff (MP for Mid-Worcestershire), House of Commons, 22 July.
- Is your Child Left-Handed? Why, according to psychological tests, left-handed people ought to remain so. Popular Science. December 1918. p. 22.
- Handedness and Earnings / Higher paychecks: a left-handed compliment?
- Handedness & earnings, published in Journal of Human Resources 2007[dead link]
|In cognitive abilities||Geschwind–Galaburda hypothesis|
|In brain||Brain asymmetry · Dual brain theory · Bicameralism|
|In eyes||Ocular dominance|
|Handedness in boxing||Southpaw stance||Orthodox stance|
|Handedness in people||Musicians · US presidents|
|Handedness related to||Sex · Maths|
|Handedness measurement||Edinburgh Handedness Inventory|
|In major viscera||Situs solitus||Situs ambiguus||Situs inversus|