Most of the population have a strong preference for using one hand to complete most unimanual activities - and it’s usually the right hand. But why is that the case, and how are different degrees of handedness formed?

Neural Asymmetry

The asymmetry of the human brain is relevant to many areas of psychology and neuroscience, spanning several domains of research [3]. The two hemispheres of the brain are thought to carry out different functional responsibilities and are divided by the mid-sagittal plane [4]. Research has used neuroimaging to identify neural asymmetry involvement in motor behaviour and handedness, such as magnetoencephalography and functional magnetic resonance imaging (fMRI) [5], [6]. One study reported a positive relationship between the degree of handedness and the degree of cerebral lateralisation when using fMRI [7]. Using positron emission tomography, the left hemisphere demonstrated significantly more activity in the motor cortex and premotor cortex when compared to the right hemisphere in right-handers for a hand rotation task [8]. The reverse finding was also true in left-handers, suggesting the dominant hemisphere was more involved than the non-dominant hemisphere.

Mixed-handedness vs Ambidexterity

Ambidexterity is often confused with mixed-handedness but is a rare phenomenon. Ambidextrous individuals show no preference for using either hand for all activities, e.g. writing on a whiteboard with their left hand, then switching to the right hand on the right side of the board. An ambidextrous individual will use the hand that is favourable to their environment, with no internal influence [9]. In comparison, ‘mixed-handedness’ is a term for people who use one hand or the other for specific tasks. Whilst a mixed-handed person and an ambidextrous person may display similar behaviours with their hand role selection, it is important to question which of the two categories they fall under.

Multiple studies have reported neuroanatomical differences between the categories of handedness, direction, and degree. Weak-handers have less asymmetry for grey and white matter, cortical surface area, thickness, and gyrification compared to consistent-handers [10]. These findings support functional differences in lateral brain organisation, leading to behavioural output of a weaker or stronger degree of handedness. Weaker handers also have greater interhemispheric interactions during cognitive tasks, suggesting they are less lateralised between hemispheres [11]. These results were found across both weak left-handers and weak right-handers, with no difference between the direction of handedness. Furthermore, differences in the basal ganglia structures have been reported in left-handers who were forced to convert to using their right hand [12]. Specifically, the putamen and pericentral cortex showed abrupt atrophy in individuals forced to convert from left-handedness to right-handedness. Interestingly, left-handers who were not converted to right-handedness did not differ in the volume of their putamen compared to right-handers, suggesting that conversion was the main factor of structural plasticity.

Theories of Handedness

Global Dominance

The global dominance hypothesis was the most widely accepted theory of handedness. This hypothesis states that the left and right hemispheres compete to gain dominance and majority control in motion [13]. Therefore, handedness arises from the dominant hemisphere, resulting in a dominant contralateral hand [14]; for example, a dominant left hemisphere would result in a dominant right hand. This theory also names the non-dominant hemisphere as the inferior hemisphere, with less sway in motor control [15]. During bimanual tasks, the dominant hand is usually selected to perform harder movements and is also selected during unimanual tasks [16]. The global dominance hypothesis suggests this is due to the overall better performance of the dominant hand for all activities.

Complementary Dominance

The complementary dominance hypothesis is a more recent theory suggesting that each hemisphere specialises in different skills and works together to produce cohesive movement [16]. Specifically, the non-dominant hemisphere specialises in stabilising and the dominant hemisphere specialises in high-precision motion [17]. Multiple studies have supported this theory in both bimanual and unimanual tasks [18], [19], [20]. Other support for this hypothesis comes from the non-dominant hand showing superior positional accuracy of objects when compared to the dominant hand [17].

Factors

Genetic

Research has also explored the genetic basis of handedness. The most established genetic approach is the right shift theory, in which a ‘right shift’ (RS) gene influences handedness [21]. This theory states that the stronger the RS gene is expressed in an individual, the stronger the degree of right-handedness will be. Whilst this theory is still prevalent in the understanding of handedness, recent research has expanded to polygenetic models of handedness, suggesting up to 340 loci are involved in establishing handedness [22]. An example of this is the polymorphism PCSK6 involved in the development of brain lateralisation, as well as the expression of PCSK6 linked to the degree of handedness, but not the direction of handedness [23]. Whilst the genetic basis of handedness is inconsistent in the literature, there is agreement that some heritability is involved.

Environmental

Aside from genetic influences on handedness, other biological factors such as sex and parental handedness are involved. Multiple studies have reported that males are more likely to be left-handed compared to females, at 12% and 10% respectively [24]. This finding doesn’t contradict or directly support genetic theories of X-linked maternal influence on left-handedness, but does illustrate the complex nature of handedness formation. Other research has investigated that various combinations of parental handedness produce different offspring handedness.

Cultural

The distribution of left-handedness varies globally across cultures and generations [3]. Some cultures report higher negative associations of the left hand and higher positive associations of the right [3]. For example, a 4% prevalence rate of left-handedness was reported in the Middle East, while a 12% prevalence was reported in the Americas [3]. Older generations have shown lower frequencies of left-handedness in their populations compared to younger generations [25], [26]. This is likely due to the social stigma of being left-handed in the past centuries, with many left-handers being forced to use their right-hand [12].

The cultural pressure of right-handedness conformance is suggested to be higher in cultures that value conformity, with one study comparing Brazilian and Canadian participants’ views of left-handedness [27]. Negative views of left-handedness are still prevalent with one study reporting 87.6% of participants in Malawi thought that left-handers should be forced to switch their hands [28]. The same study reported that 75% of the participants agreed that the left-hand should not be used for everyday activities. Surveys of Chinese students in the 1980s consistently reported less than 1% were left-handed [29]. A review of the Chinese social and cultural views on left-handedness found that there are no accommodations for left-handed equipment in China, as reported separately in China Daily: “In the US, you have left-handed desks, left-handed guitars, and all sorts of other left-handed devices, but in China we have none of the sort.” A cultural study in Ivory Coast and Sudan reported 1% left-hand use in youth (18-22), but 14% left-hand use in teenagers (12-15) [30]. These findings argue that a social influence between these age groups is responsible for such a drastic change in hand preferences.

Conclusion

Handedness is a complex and intricate process that is affected by a variety of factors in life. Research still does not definitively answer some of the basic questions of hand preference, but has provided insight to how we may develop our handedness.