Handedness

In human biology, handedness is an individual's preferential use of one hand, known as the dominant hand, due to and causing it to be stronger, faster or more dextrous. The other hand, comparatively often the weaker, less dextrous or simply less subjectively preferred, is called the non-dominant hand. In a study from 1975 on 7,688 children in US grades 1–6, left handers comprised 9.6% of the sample, with 10.5% of male children and 8.7% of female children being left-handed. Globally, about 90% of people are right-handed. Handedness is often defined by one's writing hand. It is fairly common for people to prefer to do a particular task with a particular hand. Mixed-handed people change hand preference depending on the task. Natural ambidexterity does exist, but it is very rare—most people prefer using one hand for most purposes. Those who intentionally train their non-dominant hand still tend to favor their originally dominant hand, although if taught young to use their non-dominant hand it can become functionally dominant in what they trained. Most research suggests that left-handedness has an epigenetic marker—a combination of genetics, biology and the environment. In some cultures, the use of the left hand can be considered disrespectful. 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. In many countries, left-handed children are required to write with their right hands. However, left-handed people have an advantage in sports that involve 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, and mixed martial arts.

Types Right-handedness is the most common type. Right-handed people are more skillful with their right hands. Approximately 90% of people are right-handed. Left-handedness is less common. Left-handed people are more skillful with their left hands. Studies suggest that approximately 10% of people are left-handed. Ambidexterity refers to having equal ability in both hands. Natural ambidexterity is uncommon, with about a 1% prevalence. Mixed-handedness or cross-dominance is the change of hand preference between different tasks. This is about as widespread as left-handedness. This is highly associated with the person's childhood brain development.

Measurement Handedness may be measured behaviourally (performance measures) or through questionnaires (preference measures). The Edinburgh Handedness Inventory has been used since 1971 but contains some dated questions and is hard to score. Revisions have been published by Veale and by Williams. The longer Waterloo Handedness Questionnaire is not widely accessible. More recently, the Flinders Handedness Survey (FLANDERS) has been developed by Mike Nicholls and team from Flinders University and Victoria University of Wellington.

Evolution Handedness has been found in dozens of non-human vertebrates. While data for fish is contentious, handedness has been found in amphibians, reptiles, birds, and various mammals. Some non-human primates have a preferred hand for tasks, but most do not display a strong right-biased preference like modern humans, with individuals equally split between right-handed and left-handed preferences. A phylogenetic comparative meta-analysis of manual lateralization in 41 anthropoid species found that modern humans are an outlier among primates in both the strength and rightward direction of handedness, and suggested that this pattern may be associated with bipedalism and encephalization. The same study predicted that strong individual hand preference may have appeared early in hominin evolution, whereas a pronounced right-handed population bias increased later, particularly with the emergence of the genus Homo. When exactly a right handed preference developed in the human lineage is still unknown, although it has been found through dental scratch patterns and other means that Neanderthals had a right-handedness bias like modern humans. Attempts to determine handedness of early humans by analysing the morphology of lithic artefacts have been found to be unreliable..

Causes There are several theories of how handedness develops.

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 their child being left-handed. A large study of twins from 25,732 families by Medland et al. (2006) indicates that the heritability of handedness is roughly 24%. Two theoretical single-gene models have been proposed to explain the patterns of inheritance of handedness, by Marian Annett of the University of Leicester, and by Chris McManus 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. From these studies, McManus et al. now conclude that handedness is polygenic and estimate that at least 40 loci contribute to the trait. 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. 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). 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.

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".

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