Left Handed People

March 8, 2022
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Left Handed People

Left Handed People

Minimal 400 words. Within the first two sentences of your paper, you will identify for the reader the singular sub-topic that your entire paper will encompass. Do not provide a definition or explanation of the subtopic. You can think of your reaction paper as having a discussion with the instructor about the topic.

A reaction/reflection paper is not a summary of what you have read. As a reminder, you will want to identify the topic of your paper without providing a definition or summary of what the topic encompasses. Instead, your writing should reflect a concise, organized, and well-written expression of your independent thoughts following the reading of one sub-topic.

cite all of these references in APA style, reaction/reflection paper is based on the expression of your critical thinking skills (not the thoughts of another individual). Points will be deducted if more than one line of quoted or paraphrased text is included in your paper

In human biology, handedness is an individual’s preferential use of one hand, known as the dominant hand, due to it being stronger, faster or better in dexterity. The other hand, comparatively often the weaker, less dextrous or simply less subjectively preferred, is called the non-dominant hand.[2][3][4] Right-handedness is by far more common; about 90% of the human population are right hand dominant.[5][6] Handedness is often defined by one’s writing hand, as it is fairly common for people to prefer to do some tasks with each hand. There are examples of true ambidexterity (equal preference of either hand), but it is rare — most people prefer using one hand for most purposes.

Most of the current research suggests that left-handedness has an epigenetic marker — a combination of genetics, biology and the environment.

Because the vast majority of the population is right-handed, many devices are designed for use by right-handed people, making their use by left-handed people more difficult.[7] In many countries, left-handed people are or were required to write with their right hands. Left-handed people are also more prone to certain health problems. However, left-handed people have an advantage in sports that involves aiming at a target in an area of an opponent’s control, as their opponents are more accustomed to the right-handed majority. As a result, they are over-represented in baseball, tennis, fencing, cricket, boxing[8][unreliable source?] and MMA.[9]

Right-handedness is by far the most common type. Right-handed people are more skillful with their right hands. Studies suggest that approximately 90% of people are right-handed.[6][10] Left-handedness is far less common than right-handedness. Studies suggest that approximately 10% of people are left-handed.[6][11] Mixed-handedness or cross-dominance is the change of hand preference between different tasks. This is very uncommon in the population with about a 1% prevalence.[12] Ambidexterity refers to having equal ability in both hands. Those who learn it still tend to favor their originally dominant hand. This is very uncommon, with about a 1% prevalence.[13] Measurement
Handedness may be measured behaviourally (performance measures) or through questionnaires (preference measures). The Edinburgh Handedness Inventory has been used since 1971 but contains many dated questions and is hard to score. The longer Waterloo Handedness Questionnaire is not widely accessible. More recently, the Flinders Handedness Survey (FLANDERS) has been developed.[14]

There are several theories of how handedness develops. Occurrences during prenatal development may be important; researchers studied fetuses in utero and determined that handedness in the womb was a very accurate predictor of handedness after birth.[15] In a 2013 study, 39% of infants (6 to 14 months) and 97% of toddlers (18 to 24 months) demonstrated a hand preference.[16]

Language dominance
One common handedness theory is the brain hemisphere division of labor. In most people, the left side of the brain controls speaking. 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. As speech is a very complex motor control task, the specialised fine motor areas controlling speech are most efficiently used to also control fine motor movement in the dominant hand. As the right hand is controlled by the left hemisphere (and the left hand is controlled by the right hemisphere) most people are, therefore right-handed. The theory implies that left-handed people have a reversed organisation.[17]

However, this theory does not address the fact that the majority of left-handers have left-hemisphere language dominance—just like right-handers.[18][19] Only around 30% of left-handers are not left-hemisphere dominant for language. Some of those have reversed brain organisation, where the verbal processing takes place in the right-hemisphere and visuospatial processing is dominant to the left hemisphere.[20] Others have more ambiguous bilateral organisation, where both hemispheres do parts of typically lateralised functions. When tasks investigating lateralisation are averaged across a group of left-handers, the overall effect is that left-handers show the same pattern of data as right-handers, but with a reduced asymmetry.[21] This finding is likely due to the small proportion of left-handers who have atypical brain organisation.

Genetic factors
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 being left-handed.[22] A large study of twins from 25,732 families by Medland et al. (2006) indicates that the heritability of handedness is roughly 24%.[23]

Two theoretical single-gene models have been proposed to explain the patterns of inheritance of handedness, by Marian Annett[24] of the University of Leicester, and by Chris McManus[22] of UCL.

However, growing evidence from linkage and genome-wide association studies suggests that genetic variance in handedness cannot be explained by a single genetic locus.[25][26][27][28][29][30][31][32] From these studies, McManus et al. now conclude that handedness is polygenic and estimate that at least 40 loci contribute to the trait.[33]

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.[34] Brandler and Paracchini suggest the same mechanisms that determine left/right asymmetry in the body (e.g. nodal signaling and ciliogenesis) also play a role in the development of brain asymmetry (handedness being a reflection of brain asymmetry for motor function).[35]

In 2019, Wiberg et al. performed a genome-wide association study and found that handedness was significantly associated with four loci, three of them in genes encoding proteins involved in brain development.[36]

Epigenetic factors
Twin studies indicate that genetic factors explain 25% of the variance in handedness, and environmental factors the remaining 75%.[37] While the molecular basis of handedness epigenetics is largely unclear, Ocklenburg et al. (2017) found that asymmetric methylation of CpG sites plays a key role for gene expression asymmetries related to handedness.[38][39]

Prenatal hormone exposure
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”.[40][41][42][43]

Prenatal vestibular asymmetry
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,[44] and asymmetry of the vestibular cortex is strongly correlated with the direction of handedness.[45]

Another theory is that ultrasound may sometimes affect the brains of unborn children, causing higher rates of left-handedness in children whose mothers receive ultrasound during pregnancy. 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.[46]

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Developmental timeline
Infants have been observed to fluctuate heavily when choosing a hand to lead in grasping and object manipulation tasks, especially in one- versus two-handed grasping. Between 36 and 48 months, there is a significant decline in variability between handedness in one-handed grasping; it can be seen earlier in two-handed manipulation. Children of 18–36 months showed more hand preference when performing bi-manipulation tasks than with simple grasping.[47]

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.[47] Scharoun and Bryden noted that right-handed preference increases with age up to the teenage years.[5]

Correlation with other factors
Further information: Handedness and mathematical ability and List of musicians who play left-handed
In his book Right-Hand, Left-Hand, Chris McManus of University College London argues that the proportion of left-handers is increasing, and that an above-average quota of high achievers have been left-handed. He says that left-handers’ brains are structured in a way that increases their range of abilities, and that the genes that determine left-handedness also govern development of the brain’s language centers.[48]

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. 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.[49]

Conversely, Joshua Goodman found that evidence for left-handers was overrepresented amongst those with higher cognitive skills, such as Mensa members and higher-performing takers of SAT and MCAT tests, due to methodological and sampling issues in studies. He also found that left-handers were overrepresented among those with lower cognitive skills and mental impairments, with those with intellectual disability (ID) being roughly twice as likely to be left-handed, as well as generally lower cognitive and non-cognitive abilities amongst left-handed children.[50] In a systematic review and meta-analysis, Ntolka and Papadatou-Pastou found that right-handers had higher IQ scores, but that difference was negligible (about 1.5 points).[51]

The prevalence of difficulties in left-right discrimination was investigated in a cohort of 2,720 adult members of Mensa and Intertel by Storfer.[52] 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.[52] The study also revealed an effect of age, with younger participants reporting more problems.[52]

Early childhood intelligence
Nelson, Campbell, and Michel studied infants and whether developing handedness during infancy correlated with language abilities in toddlers. In the article they assessed 38 infants and followed them through to 12 months and then again once they became toddlers from 18 to 24 months. What they discovered was that when a child developed a consistent use of their right or left hand during infancy (such as using the right hand to put the pacifier back in, or grasping random objects with the left hand), they were more likely to have superior language skills as a toddler. Children who became lateral later than infancy (i.e., when they were toddlers) showed normal development of language and had typical language scores.[53] The researchers used Bayley scales of infant and toddler development to assess all the subjects.

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.[54][55] There are also handedness differences in perception of musical patterns. Left-handers as a group differ from right-handers, and are more heterogeneous than right-handers, in perception of certain stereo illusions, such as the octave illusion, the scale illusion, and the glissando illusion. [56]

Left-handed people are more likely to have several specific physical and mental disorders and health problems. For example:

Lower-birth-weight and complications at birth are positively correlated with left-handedness.[57]

A variety of neuropsychiatric and developmental disorders like autism spectrum disorders, depression, bipolar disorder, anxiety disorders, schizophrenia, and alcoholism has been associated with left- and mixed-handedness.[39][58]

A 2012 study showed that nearly 40% of children with cerebral palsy were left-handed,[59] while another study demonstrated that left-handedness was associated with a 62% increased risk of Parkinson’s disease in women, but not in men.[60] Another study suggests that the risk of developing multiple sclerosis increases for left-handed women, but the effect is unknown for men at this point.[61]

Left-handed women have a higher risk of breast cancer than right-handed women and the effect is greater in post-menopausal women.[62]

At least one study maintains that left-handers are more likely to suffer from heart disease, and are more likely to have reduced longevity from cardiovascular causes.[63]

Left-handers are more likely to suffer bone fractures.[64]

One systematic review concluded: “Left-handers showed no systematic tendency to suffer from disorders of the immune system”.[65]

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.[66]

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