The Lack of Lefties: Nature or Nurture?

Jackie Marano's picture

       All of my life it has been brought to my attention that I inherently possess a privilege that lends me distinct, yet terrifically underestimated advantages: right-handedness. On many occasions, my brother, my mother, and her entire immediate family, all of whom are left-handed, would, in jest, divert my attention to the fact that nearly every device, appliance, and physical construction in existence was designed for right-handed people. The casual yet unarguably valid observations of my left-handed relatives have made me aware of such phenomena in my own experiences; never in all of my years of school did I consider the amount of time and thought that I was spared on a daily basis when I grabbed any available pair scissors for my art project, when I comfortably sat at any one-armed desk that I desired, or when I navigated with a mouse on any and every computer. My repeated exposure to the collective experiences of my left-handed relatives has had, and will continue to have, a profound influence on my interest in and my appreciation of this curious phenomenon.


      While it is evident that a consistently and considerably small percentage of the entire human population is left-handed, the reasons for these circumstances are somewhat less established. Is it possible that the unchanging status of the lefty minority is a consequence of long-established social constructs? Could there be a discrepancy between the evolutionary fitness of righties and lefties? Or is this observed occurrence a representation of some pattern of randomness? Such a curious phenomenon demands consideration in the areas of sociology, neurobiology, and evolution.


      The potential influences of social constructions on the phenomenon of left-handedness become perceivable when they are considered in the context of history. The practices and traditions of our ancestors of ancient times demonstrate an unmistakable partiality to rightward positions and rightward orientations. For example, in the modern English language, the slighting word ‘sinister’ is derived from the Latin word for ‘left,’ while the complimentary word ‘dexterous’ is derived from the Latin word for ‘right’ (2). While such ancient language derivatives indicate that this aversion to all leftward orientations was enduring, further consideration of social circumstances reveals that it was also widespread; vocabulary words from the German, French, Spanish, Chinese, and Slavic languages equates rightward positions with skill, civility, and sophistication, while they associate leftward directions with sloppiness, lack of coordination, and disaccord (4).


       The connotations associated with rightward and leftward positions help to explain why, in the history of many societies, the seemingly backwards nature of lefties was often deemed inappropriate and unacceptable. For example, during the Industrial Revolution in the United States, a time when literacy and communication skills were emphasized, there was an estimated decline in the percentage of the population that was lefty (5). However, despite the apparently persistent and worldwide distaste for everything ‘left’, it is noteworthy that the left-handed tendency has not been eradicated from the human population. Could there be some element of the left-handed tendency that makes it resistant to absolute abolishment? Lefties such as Albert Einstein, Ben Franklin, Isaac Newton, Picasso, Napoleon, Jimi Hendrix, and Leonardo da Vinci to name a few, have certainly influenced the modern world in significant ways, so in this sense the role and contributions of lefties cannot be undervalued (2,3,4). While social pressures undoubtedly influenced on the population control of lefties to some extent, they do not account for the sustained resistance to elimination that is observed. Attention to studies in the field of neuroscience may explain the discrepancy; the characteristics associated with being a lefty transcend handedness.


       Brain lateralization refers to the idea that the brain is composed of two nearly-symmetrical halves, or cerebral hemispheres, each with its own specialized and localized functions (6). In the mid-1800s, French neurosurgeon Paul Broca first identified that, based on autopsies performed on individuals who suffered speech deficits, speech production is mainly concentrated in the left hemisphere (1,6). German neurologist Carl Wernicke built upon Broca’s observations and concluded that, also based on autopsies performed on impaired individuals, that language comprehension is a function of the left hemisphere (1). While more modern and sophisticated scientific methods, such as the Wada test and functional magnetic resonance imaging, support the observations of Broca and Wernicke, they contest the implication that functions of the brain are consistently restricted to one hemisphere (6). Increased scientific analysis has enabled scientists to associate distinct functions with specific hemispheres, but is a commonly accepted fact that each hemisphere cannot function in isolation and does not act independently; the corpus callosum and many axon tracts connect the two hemispheres and serve as a means of cross-hemisphere cooperation and coordination (6). While consistent division of labor between hemispheres has not been determined, general trends in scientific observations indicate that functions such as speech production and language comprehension are localized differently in the brains of righties and lefties (4). Such findings propose that it is not the structure of the brain that separates lefties and righties, but rather that it is the degree to which the functions are shared between the two cortical hemispheres.


      One might argue then, that, in terms of language and speech functions, righties are less lateralized because their functions are more localized to the left hemisphere, while lefties are more lateralized because the functions are less localized to the left hemisphere (1). But can our classification of some brain lateralization patterns and their correlations to handedness explain the various behavioral complications that appear more often in lefties? Neurobiological investigations display consistent evidence of the link between the left-handed tendency and various other disorders such as mental illness, schizophrenia, and dyslexia to name a few (3). Righties and lefties certainly exhibit patterns of behavior that are similar enough that the two have managed to coexist. While this may be part of the reason why lefties have not been eliminated, there are some cases that may challenge this noted similarity. Is it the hand that writes a clear indication of brain lateralization in the first place? A counterexample might be in the case of a righty who must become a lefty as a result of permanent injury to the right arm; does the switch of the dominant hand alter the individual’s brain lateralization? Does the individual, as a result, increase his chances of contracting the mental and behavioral disorders associated with left-handedness? If this is not the case, can one’s innate handedness represent one of many characteristics of a distinct collection of behavioral patterns? It is clear that much about both brain lateralization and handedness remains unknown, and also undefined.


      The phenomenon of left-handedness is certainly curious, and in recent years there have been increased attempts to discover its cause. In 2007, some researchers claimed that they found a gene that could potentially be responsible for increased odds of being left-handed, while another theory states that higher levels of testosterone in a pregnant woman may possibly lead to a left-handed child. Additionally, there have been attempts to measure the percentage of lefties among populations in the context of geography, socio-economic background, genetics, comparison between generations, and also between males and females. However, no consistent trend of the incidence of left-handedness has been found or widely accepted in any of these contexts, except that it is continuously less frequent than the incidence of right-handedness in the human population.


      The inconclusiveness that remains after consideration of the impact of social constructions and observations in neuroscience prompts a broader approach that may actually encompass the observations in these two fields. If social pressures have had a proven effect on the percentage of lefties in a given population when literacy was introduced to society, and if patterns in neuroscience suggest that brain lateralization determines the distribution of the functions of speech and linguistics, perhaps there is need for comparisons between modern humans and their illiterate predecessors. The near impossibility of going about such a task implies that it may, instead, be of great use to study the left-handed phenomenon (or lack thereof) in our evolutionary partners; analysis of the relationship between brain lateralization and its consequences in apes and chimpanzees may provide us with more insight not only about these correlations in ourselves, but also about our evolutionary relationship with these organisms. In any case, even if the cause of the disparity between the proportions of lefties and righties in the global human population is not determined within my lifetime, I will continue to ponder, but not devalue, this curious minority.

Sources:

1. http://www.indiana.edu/~primate/brain.html “What does Handedness have to do with Brain Lateralization” (accessed 23 Jan 2008)


2. http://www.time.com/time/printout/0,8816,972749,00.html “The Perils of Being a Lefty.” Time Magazine (accessed 24 Jan 2008)


3. http://abcnews.go.com/Technology/story?id=498707&page=1 “The Left-Handed Advantage.” ABC News (accessed 2 Feb 2008)


4. http://en.wikipedia.org/wiki/Left-handed “Left-Handedness.” Wikipedia (accessed 15 Fed 2008)


5.http://www.philly.com/inquirer/magazine/20071029_The_perplexing_case_of_the_fluctuating_lefthanders.html “During the Victorian Era, Lefties Nearly Disappeared.” The Philadelphia Inquirer (accessed 20 Feb 2008)


6. http://en.wikipedia.org/wiki/Lateralization_of_brain_function “Lateralization of Brain Function.” Wikipedia (accessed 20 Feb 2008)


Comments

Paul Grobstein's picture

handedness and the brain

Thanks to both you and your family for calling attention to this matter. It IS curious that there are both persistent social constructions about handedness and a persistant bimodal distribution in the human population. Could it be related to reading/writing/language? I'd be surprised, since most of human culture developed before widespread literacy. And, correspondingly, there is a clear disconnect in this case between a brain characteristic and social construction. If handedness were a social construction left handedness would certainly have long since disappeared in the face of social pressure. So ... why does left handedness persist? And where does the social pressure against it come from?

Handedness exists not only in humans but in lots of organisms, including the fiddler crab. So too do various internal assymetries. Is that relevant? Somebody needs to come up with a good story.

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