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Reflections Magazine : Reflections Issue 61, 2015
Why do lefties exist? It is commonly held that 10 to 12% of the global human population is left-handed, although there are geographical variations and cultural biases. Handedness polymorphism (the phenomenon of difference in hand preference) is found in all societies, with left-handers always present, yet at a substantially lower frequency than right-handers. Left-handedness seems to occur at a percentage somewhere between the low single digits and roughly a quarter of the population. There is no single definitive answer as to why handedness polymorphism and right-hand dominance exist in humans, but it has been demonstrated right back to at least the Upper Palaeolithic period. The low-level yet consistent presence of left-handers suggests that there is evolutionary pressure resulting in the continuance of this trait. Left-handers have an advantage in hand-to-hand combat, for example (and its modern-day expression, competitive sport), because both left and right-handers are accustomed to mostly battling right-handers, so lefties have the element of surprise on their side. Left-handers are therefore often over-represented in the elite levels of interactive sports. However, the downside for left-handers is that the world is subtly set up in a way that makes many tasks more difficult or problematic than they are for the right-handed majority. There is no single factor that determines hand preference in individuals, but genetic, hormonal, developmental and cultural factors may contribute to an individual's handedness. Interestingly, approximately 18% of identical twins have different hand preferences, suggesting factors other than genetics are relevant. Males are more likely to be left-handed than females. The heritability of left-handedness is complex, but two left-handed parents have a 30-40% chance of their child being left-handed, with both genetic and environmental factors coming in to play. In practise, this means that parents often have to learn how to help and guide a child whose handedness they do not share. Mixed-handed or strong-handed? While most people identify themselves as being right or left-handed, usually as a function of which hand they write with (at least in our society, where writing is culturally important), handedness is not a binary phenomenon. There are degrees of, and variations within, handedness and overall bodily laterality. In addition to the distinction between left and right-handedness, there is also a distinction between mixed and strong-handedness. Strong-handedness is where an individual exhibits a strong preference for one hand across almost all tasks. Mixed-handedness (a.k.a."cross-dominance" or "inconsistent handedness") is where one hand is preferred for some manual tasks, and the other hand is preferred for other tasks. Whether or not these individuals are classified as left or right-handed largely comes down to what hand they use for specific culturally important tasks, such as writing and eating. Mixed-handed people may consider themselves to be ambidextrous, although in the majority of cases this term is a misnomer. True ambidexterity -- the capacity to use either hand with equal dexterity in all situations -- is rare, applying to one or two per thousand people. Right-handers tend to be strong-handed, while left-handers tend towards mixed-handedness. On the Edinburgh Handedness Inventory, performing at least one of the ten activities with the non-dominant hand would indicate mixed-handedness. Some researchers, including psychologist Stephen Christman, even believe that the degree of handedness (strong/mixed) is a more appropriate and powerful classification than the direction of handedness (left/right), and has more implications regarding cerebral structure and behaviour. Handedness and the brain There are neurological/cognitive features associated with the different kinds of handedness (left/right, strong/mixed). The brain itself is not an entirely symmetrical structure, and while the body's motor functions are largely controlled by the opposite brain hemisphere, left and right-handed people do not simply have mirror-image brains. The different hemispheres have their own particular functional specialisations, a phenomenon known as brain lateralisation. Left-handers and mixed-handers tend towards more varied and diffuse cerebral functions than strong-handers and right-handers, and tend towards more interhemispheric communication, a lesser degree of brain lateralisation, and a larger corpus callosum (the structure that joins the hemispheres of the brain). This potentially has wide-ranging (if usually subtle) implications, from what musical instrument an individual is best suited to, to how they remember and recall events and how they process language. Greater interhemispheric interaction has benefits and drawbacks. To use a musical example, mixed-handed people may do better with tasks requiring the two hands to temporally cooperate (such as stringed and woodwind instruments), while strong-handed people will do better than mixed-handers at tasks requiring the hands (and brain hemispheres) to perform separately, such as tapping out two separate rhythms simultaneously. This may predispose them to be more musically suited to keyboard and percussion instruments. A potential drawback of more interhemispheric communication is more "background noise" and stimuli overload. Strong-handers may have an advantage on tasks requiring independent processing by the hemispheres. 13 REFLECTIONS • GOWRIE AUSTRALIA • SUMMER 2015 - ISSUE 61
Reflections Issue 60 Spring 2015