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Biology 202
2004 First Web Paper
On Serendip

Theories on Left-handedness and Laterality

Hannah Messkoub

Every time I walk into a classroom I am faced with the same challenge. It's not that I haven't done my homework or that the professor is boring, but that I can't find the right chair to sit in. Some may argue that the problem is inherently caused by a neurobiological difference. Others might say that it is the work of the devil. My crime be told, I am left-handed.

For 20 years I have suffered from the writing discomfort associated with right-handed desks and notebooks. I have also had difficulty playing the guitar and have been forced to try and play sports in a right-handed fashion. Scissors are always a battle unless they are specifically designed for left-hand use and power tools are not recommended as they have an usually high accident level with left-handedness. I have managed to overcome most of these minor obstacles without much difficulty, but perhaps this is due to my socio-cultural background which is relatively understanding of left-handed differences.

Throughout history, people with left-handedness have been persecuted for a variety of reasons. Deemed social deviants or mentally defected, left-handedness has been considered an undesirable attribute that can be "corrected" through persistent repetition of right-handed behaviors. Adroit, according to the Miriam Webster dictionary, is derived from the Latin word droit (translated as 'right') and in contemporary use means correct or proper. 1 Comparatively, the French word gauche, literally 'left' is used to express someone who is "lacking social grace or is not tactful". 2 Being left-handed has historically been grounds for discrimination although it in recent years these biases have grown more subtle. As I considered the issue of being left-handed more deeply I was convinced that this clearly had not only physical and socio-cultural implications but was also related to neurology, specifically the lateralization of the certain functions in the cerebral hemispheres.

According to Dr. M.K. Holder, the Director of the Handedness Research Institute at Indiana University, left-handedness can be understood in terms of brain lateralization and the functional specialization areas for speech. Although there is not one definitive explanation as to why an individual has a specific handedness, there seems to be evidence that there are several factors such as genetic background, socio-cultural influences, and neurobiological implications.

For example, the Kerr clan of Scotland is infamous for their predisposition towards left-handedness. 3 This feature, known by the Kerr as being "Corry Fisted" or "Kerr-Handed" can be understood as a culturally reinforced trait that was supposed to aid these Scottish warriors in methods of warfare. Due to their common gene pool, however, this trait can also be understood a genetic characteristic of the Kerr clan.

According to Oldenfield (1971), the statistics for left-handedness are also higher in males than in females. Geschwind and Galaburda (1987), developed the "G-G theory" which builds upon the notion of sex-differences, arguing that higher levels of testosterone can affect cerebral lateralization by causing the normal dominance pattern to change. 4 For the majority of right-handed individuals, the language is associated with dominance in the left-hemisphere and visio-spatial skills in the right-hemisphere. Gorski et al expands upon this idea, noting the important role that the levels of the testosterone can have on lateralization.
The hormone can affect the growth of many tissues, and has an inhibitory effect on the growth of immune structures, such as the thymus gland and the burse of the Fabricus. Testosterone is also capable of changing the structure of specific nuclei in the hypothalamus and limbic system. (Gorski, 1986)

From what I understood, the G-G theory argues that testosterone levels can increase for many reasons and one of the effects can be a delay in the growth of the left-hemisphere. This delay can in turn produce what neurologists have coined "Anomalous Difference" which is characterized by "left-handedness, right hemispheric language dominance, left-hemispheric visuo-spatial dominance.." 4 In short, when there are higher levels of testosterone the normal patterns of dominance associated with language in the majority of the population are switched. Their explanation can be categorized as a chemical model based on the changing variable of testosterone in an effort to understand the creation of specific functional lateralization with regards to handedness. Although the G-G theory is widely supported, I would argue that it is not the definitive explanation for left-handedness but rather one of many important factors in determining this disposition.

Both the French neurologist Paul Broca, 5 and German neurologist Carl Wernicke made important discoveries in the 18th century that identified areas in the prefrontal cortex of the left hemisphere as being associated with areas that are primarily used for speech production. Compared to other primates, this area of the brain in humans is greatly enlarged. 6 Although handedness used to serve as a basis for establishing which lateralization individuals had for language, it later became clear with use of the sodium amytal (Wada) tests of the 1960's that lateralization for language in some left-handed individuals can also occur in the left-hemisphere. 7 By injecting patients with this contrast dye, the areas of the brain which are associated with language and memory become visible with use of an x-ray. 8 The explanation for this still remains unknown.

This point raised various questions for me on an individual level. As a left-handed person it may be significant that my disposition for fine arts and foreign language is high, a function that according to my research seems to be associated with the right hemisphere. In line with the G-G theory, this can be understood as an overcompensation in the right hemisphere as a "compensatory growth mechanism" because the left hemisphere growth has been delayed.

Whether or not the G-G theory correlations between testosterone to specific functional lateralization proves causation is debatable. From the research that I have done I would argue that left-handedness can not be understood simply in terms of neurology, genetics, or socio-cultural factors alone but as a combination of all these. The G-G theory also fails to explain why the left hemisphere is more sensitive to levels of testosterone or if there are more testosterone receptors in this area of the brain.



1)Miriam Webster Dictionary

2)Miriam Webster Dictionary

3)Kerry Clan Lineage

4)Theories About Handedness Causation

5)Biography of Paul Broca

6)Lateralization and Language II

R7)Medical College of Georgia, MCG Wada Protocol: Clinical Core

8)The Biological Basis for Langauge

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