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Hand dominance refers to the preference of a specific hand an individual chooses to use in everyday tasks. The majority of the human population (~90%) are right-hand dominant, with the respective ~10% being either left-hand dominant or mixed-handed.

Most children start showing a clear preference for one hand or the other at around the age of 2 or 3, while some do it as early as 18 months. However, Some children remain ambidextrous (using both hands equally) until they're 5 or 6.

There are several theories which attempt to explain the development of hand preference of individuals during both prenatal and post-birth early childhood.

Prenatal Developmental Theories
Specific occurrences during prenatal development could be of significant influence in the determination of a person's hand preference. Researchers have studied fetuses within the uterus, and it was suggested that hand preference within the uterus was an extremely accurate indicator of hand preference after birth. Interestingly, one study showed that 39% of infants (6-14 months) and 97% of toddlers (18 to 24 months) demonstrated an individual hand preference.

Prenatal Hormone Exposure
The Centres for Disease Control endorsed a study in 2003 which determined that males with prenatal exposure to an estrogen-based fertility drug (diethylstilbestrol) were more likely to be left hand dominant than those who were not exposed to the drug. It is suggested than estrogen affects the developing brain, specifically the parts related to sexual behaviour and right/left hand dominance.

Prenatal Ultrasound Exposure
Ultrasound scans use sound waves to build a picture of the baby in the womb in order to ensure healthy development and progress of the unborn child. It is suggested that ultrasound could have an effect on the brains of unborn children within the uterus. Children exposed to ultrasound during pregnancy may be more likely to be left hand dominant than those who were not exposed to ultrasound. Studies on this topic have shown a possible weak association between ultrasound exposure and left hand dominance.

Prenatal Vestibular Asymmetry
After reviewing a number of studies, Previc suggested that the fetal position during the final trimester of pregnancy, and a baby's subsequent birth position, could affect their hand preference. approximately 2/3 of fetuses come out with the left side of the back of the head present first. Previc suggests that different prenatal positioning in fact causes different vestibular stimulation, which is thought to be involved in the development of hand dominance.

It is shown that in every disorder in which a patient shows left handedness, there is either a vestibular irregularity or hindrance. A difference in the lateralization of the vestibular cortex is also shown to be strongly correlated with hand preference. .

Genetic/Lateralisation Developmental Theories
Genetics and functional lateralisation of the brain also act as an important influence in the determination of hand preference.

Genetic Influences
The hereditary pattern involved in hand dominance is far from simple. A large twin study from 25,732 families gives evidence that the heritability rate of handedness is approximately 24%. As of this moment, two theoretical single gene models have been proposed by McManus and Annett in a attempt to explain the complex patterns of the heritability of hand preference.

However it has been argued that single-gene theories are too simplistic, and perhaps the heritability of hand preference cannot be attributed to only one single gene. Several research teams carried out a series of linkage and genome-wide association studies which give evidence that the inheritance patterns of hand preference cannot be explained using one individual genetic locus.

Because of these findings, McManus has been strained to reconsider his single-gene theory, and now concludes that the heritability of handedness is in fact polygenic, with influences from at least 40 different loci.

Brain Function Lateralisation
Handedness can be linked to language and speech (verbal processing) functions of the brain. Verbal processing functions are usually located on the left hemisphere of the brain, whereas visual-spatial functions are located on the right. Because of this most people are right hand dominant. It is therefore predicted that left handed people have the opposite lateralisation of these functions, with verbal processing being controlled on the right side of the brain, as opposed to the usual left.

In those who are left hand dominant, the brain either works with the same pattern of verbal / visual-spatial processing functions but on opposite hemispheres, or the verbal processing functions are actually shared across two hemispheres, as opposed to just one. This means that on average, left handed people are less lateralised than their right-handed counterparts.

Handedness as an Early Indicator of Intelligence
Left-handedness may be a sign of intelligence and in turn, success. This has been supported with an array of research findings, and by a number of famous, talented and successful figures throughout history. Notable examples include entrepreneurs such as Bill Gates, Country leaders such as Barack Obama and David Cameron, legendary historical figures such as Leonardo Da Vinci and Aristotle, and highly successful musicians such as Jimi Hendrix, Kurt Cobain and Paul McCartney.

One study shows that earlier presence of hand dominance (<12 months), indicates a higher likeliness that those children will have greater language skills when they were toddlers (18-24 months) than those who showed hand preference later than infancy. The children who showed a hand preference later than infancy were found to possess normal language ability, and scored typically on language assessment.

Another study gives further evidence toward a higher intelligence in left-hand dominance. Ghayas & Adil observed the intelligence of 150 students of Sargodha district in Pakistan. Samples included an equal number of right and left handed students. Handedness was distinguished using the Sherman-Kulhavy Laterality Assessment Inventory (LAI) and intelligence was tested using the Raven Standard Progressive Matrices Test (RSPMT). Results suggested that left-hand dominant students were significantly more intelligent than those who were right-hand dominant, and time taken to complete the RSPMT was significantly higher for right handers than for left handers.

However, This higher intelligence supposedly possessed by left-handers is often over-represented by specific individuals who are very intelligent and/or have been highly successful. It is evident that there is also a disproportionate amount of left-handed people who suffer from mental disorders such as schizophrenia, dyslexia, child autism and Attention Deficit Hyperactivity Disorder (ADHD).