Handedness
Most of the current research suggests that left-handedness has an epigenetic marker — a combination of genetics, biology and the environment.
As a result, they are over-represented in baseball, tennis, fencing,[10] cricket, boxing,[11][12] and mixed martial arts.
Attempts to determine handedness of early humans by analysing the morphology of lithic artefacts have been found to be unreliable.
[23] A large study of twins from 25,732 families by Medland et al. (2006) indicates that the heritability of handedness is roughly 24%.
[24] Two theoretical single-gene models have been proposed to explain the patterns of inheritance of handedness, by Marian Annett[25] of the University of Leicester, and by Chris McManus[23] of UCL.
[26][27][28][29][30][31][32][33][excessive citations] From these studies, McManus et al. now conclude that handedness is polygenic and estimate that at least 40 loci contribute to the trait.
[34] 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.
[37] 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".
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.
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 perception and gross motor skills.
[54] 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.
[56] Writing in Scientific American, he states: 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.
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.
[60] In a systematic review and meta-analysis, Ntolka & Papadatou-Pastou (2018) found that right-handers had higher IQ scores, but that difference was negligible (about 1.5 points).
[61] The prevalence of difficulties in left-right discrimination was investigated in a cohort of 2,720 adult members of Mensa and Intertel by Storfer.
[62] According to the study, 7.2% of the men and 18.8% of the women evaluated their left-right directional sense as poor or below average; moreover participants who were relatively ambidextrous experienced problems more frequently than did those who were more strongly left- or right-handed.
[62] Nelson, Campbell, and Michel studied infants and whether developing handedness during infancy correlated with language abilities in toddlers.
[63] 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.
[66] Studies have found a positive correlation between left-handedness and several specific physical and mental disorders and health problems, including: 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.
[94][clarification needed] Some studies examining the relationship between handedness and sexual orientation have reported that a disproportionate minority of homosexual people exhibit left-handedness,[95] though findings are mixed.
[101][102][103][104] 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.
[105] 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.
[107] 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.
John W. Santrock has written, "For centuries, left-handers have suffered unfair discrimination in a world designed for right-handers.
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.
[citation needed] There are many negative connotations associated with the phrase left-handed: clumsy, awkward, unlucky, insincere, sinister, malicious, and so on.
[citation needed] 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.
Conversely, right-to-left alphabets, such as the Arabic and Hebrew, are generally considered easier to write with the left hand.
