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. Globally, roughly 12% of men and 10% of women are left-handed.
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.
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 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 roughly 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).
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 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.
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.
Writing in Scientific American, McManus states that,
Studies in the U.K., U.S. and Australia have revealed that left-handed people differ from right-handers by only one IQ point, which is not noteworthy ... Left-handers’ brains are structured differently from right-handers’ in ways that can allow them to process language, spatial relations and emotions in more diverse and potentially creative ways. Also, a slightly larger number of left-handers than right-handers are especially gifted in music and math. A study of musicians in professional orchestras found a significantly greater proportion of talented left-handers, even among those who played instruments that seem designed for right-handers, such as violins. Similarly, studies of adolescents who took tests to assess mathematical giftedness found many more left-handers in the population.
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. 0.3 ms), 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.[better source needed]
International Left-Handers Day is held annually every August 13th. 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 [meaning left-handedness] and increase public awareness of the advantages and disadvantages of being left-handed." 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!"
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