Biology 202, Spring 2005
First Web Papers
On Serendip

Handedness is marked by the preference of one hand over another for fine motor tasks, especially writing. Usually, only one hand is considered dominant, however, there are individuals who exhibit the ability to use both hands equally (ambidexterity). It is generally the case that most people are right-handed, with left-handed people constituting approximately 2% to 30% of the population, depending on the criteria used, with the median estimate of about 10%. (2). Is this propensity to use the right hand genetic, or does it result from socio-cultural pressures?

Firstly, the preference for one hand over another is most likely the effect of brain lateralization. The human brain is divided into two hemispheres (right and left) connected by the corpus callosum. The hemispheres are contra lateral, meaning the left-brain controls the right side of the body and vice versa. (4). Different skills and abilities are associated with the two hemispheres; basically, the brain is compartmentalized, relegating specific abilities to one location. The "information" necessary to perform various tasks is stored within the brain. This is dependant on surface area: the greater the surface area, the greater "storage space." Since writing and other fine motor skills require "storage space," it is logical that these abilities are usually restricted to one hemisphere rather then having the same information stored in two places. For this reason, humans prefer one hand over the other; when this information is stored in the right brain, the individual displays a tendency to use the left hand (the reverse is also true). However, it should also be noted that just because people usually prefer one hand as opposed to the other, fine motor skills are not exclusively associated with that hand (i.e. right-handed people are capable of playing instruments, such as the violin, which rely on motor skills of the left hand). (4). Therefore, an individual will prefer to use one hand (especially for writing), but is still capable of using the other for other fine motor tasks.

Given that the brain is compartmentalized, many scientists believe there is a genetic basis for handedness. "Dominance" of one hemisphere over the other could be passed down through DNA (specifically, some alleles may be "coded" for handedness). "In humans, the inheritance of handedness fits well one-locus models where one allele causes right-handedness and another left- or right-handedness at random." (3). Thus, according to this model, handedness is determined by what is "coded" on the allele. Since one allele is specifically programmed for right-handedness, but the other can be programmed for either, right-handed people outnumber left-handed people because of a greater probability of having an allele coded for right-handedness. The percentage of right-handed people in the general population (~70 – 95%, depending on the criteria used) further affirms the assertions of this genetic model. Furthermore, since left-handed people are statistically more prone to diseases, handedness does indeed appear to be influenced by genetics. Statistically, left-handed people are more likely to be dyslexic, schizophrenic, have Crohn's disease, ulcerative colitis, and mental disabilities. (1). Since left-handed individuals are afflicted with these genetic disorders with greater frequency, it suggests that there may be a genetic basis for handedness. However, some assert that this increased frequency of diseases among those with left-hand dominance suggest that this is due to some sort of "trauma" during gestation or birth. "It could be that this early trauma is also the trigger behind health problems linked to left-handedness. Coren points to two famous left-handers, Presidents Bill Clinton and George H.W. Bush, as evidence. Both had histories of birth stress and have health issues from Clinton's severe allergies to Bush's Graves' disease." (1). Thus, the increased frequency of disease may be the result of genetics, or perhaps the result of "trauma" during gestation. However, if left-handedness truly were genetic, then why could some individuals switch from writing with the left hand to the right hand?

Some individuals have switched hand "dominance" due to socio-cultural pressures. Left-handed individuals were at one point considered evil, or even communist, for being left-handed. (2). In fact, the English language demonstrates this bias against lefties. The word "sinister," meaning evil or wicked, comes from the Latin word "sinistra," meaning left. This derivative alone shows the societal connection between "left" and "evil." Due to this societal belief, many left-handed individuals were forced to switch and use the right-hand instead. (2). Furthermore, there is a higher percentage of left-handed people in "permissive" societies than in "restrictive" ones. (5). This further reinforces the idea that handedness is susceptible to socio-cultural pressures, and in fact, may be changed. Therefore, it is possible that the human brain in malleable, meaning that it can be "rearranged" so that coordination of certain skills, like writing, can be switch from right brain to left-brain. Furthermore, humans seem to be the only species to demonstrate "population-level handedness," meaning that humans overwhelmingly seem to prefer right to left. (5). While some animals do indeed exhibit a preference for one hand/paw to another, they do not seem to have "population-level handedness." (5). This further suggests societal pressures may determine handedness (for, if it were purely genetically based, animals would probably exhibit the same "population-level handedness").

In conclusion, while handedness does appear to have a genetic basis, it is also influenced by socio-cultural pressures. The propensity to use the right hand demonstrates a left-brain "dominance," whether or not it is genetically imposed. Much is still unknown as to why an individual is right or left-handed, and further exploration is necessary before a definitive explanation can be offered. There is also the question of ambidexterity – how does the brain of an ambidextrous person differ from that of a left-handed or right-handed individual? Is ambidexterity genetic, or culturally influenced, or neither?

References

1)The Left-Handed Advantage, ABC News Article


2)Gauche, Left-Handers in Society


3)The Evolution of Brian Laterization: a game theoretical analysis of population structure, Genetic basis of handedness


4)Lateralization of functions in the vertebrate brain: A review, Lateralization in the human brain as compared to other vetebrates


5)Do scientists understand why there are so many more right-handers than left-handers? Do other primates show a similar tendency to favor one hand over the other?, Handedness in humans and primates


| Course Home | Serendip Home |