Sunday, July 15, 2007

The southpaws’ dilemma

Left-handedness in a world of “right”

Even as recently as in my childhood, the left-hander or “southpaw” had an implicit negative social connotation; a “sub” status of sorts. Teachers, parents, and even religious advisers openly discouraged left-handedness, with varying degrees of success. The reasons for these suppressive pressures against left-handedness were never made clear.


Indeed, the quality of left-handedness has been associated with all types of negative human social behaviors. These include decreased mental capacity to associations with hygienic standards and demonic connections. There have even been suggestions of left-handedness being correlated with shortened life-spans and other behavioral anomalies (i.e.; dyslexia and psychosis).


Even today, with our expanded knowledge of brain functionality and much wider acceptance of the left-handed individual, there remains a palpable impression that the ghostly negative stigma still permeates, at some level, our general societal consciousness.


That the quality of left or right handedness is essentially a function of brain hemisphere interaction, genetic influence, varying and sometimes competing cognitive abilities (i.e.; language versus spatial skills) all interacting together under the sway of human cultural pressures opens the doors to a fascinating and nuanced understanding of left-right handed phenomena. Understanding these complex interactions may help those of us “afflicted” with left-handedness comprehend why this quality may have been so shunned- and perhaps decrease the ill will some might still hold against their teachers or parents.


Studies in comparative brain structure are beginning to reveal that the human brain, instead of evolving through some wholesale structural change via systems “add-ons” or massive re-wirings, is in fact the result of general systematic re-organizations, elaborations or reductions of existing structures, and proportional changes in existing connections.


Among the bubbling hominid evolutionary pressures, developing language and other human qualities played a large role in defining many general socio-cultural elements. Somewhere around 90% of humans depend on the left side of the brain for speech and the same number use the right hand for writing- also a generally left hemisphere function. Interestingly, the right hemisphere has predominant functions related to recognizing faces, understanding spatial relationships, and responding to emotional cues.


What is interesting here is that there is a population of individuals where the hemispheric influences are less clear- left handers and mixed-handers for instance. Theories abound, often connecting these observations to abnormalities, diseases, or handicaps. There are some observations supporting this concept to some extent. Early brain injuries do increase the probability of left-handedness; hence the observed increase with left handedness and mentally handicapped individuals. However, other explanations are needed in light of the fact that there are any numbers of normal and exceptionally gifted left-handers. Add to this the fact that left-handedness occurs naturally in about half of the populations of our closest primate relatives, and the need for broader theories becomes evident.


Marian Annett notes “Handedness in all species with hand and paw preferences, including humans, may result from chance differences between the two sides of the body that arise during early growth. In most humans an additional factor gives a slight advantage to the left hemisphere over the right one. This is sufficient to displace the chance distribution in a dextral direction.”


Known as the “right-shift principle”, Annett describes a Menedelian approach that may provide a better understanding of "handedness". This theory suggests a gene based concept as the foundation to this “dextral direction” where a combination of a dominant gene that increases selective bias for right-handedness and a recessive allele giving to no bias on either side exist work together to reflect the observed human "handedness" patterns.


Annett adds “The link between brain specialization and handedness could thus depend on a factor (the rs+ allele) that give the left hemisphere an advantage for speech and incidentally increases the chances of right-handedness. Those who do not carry the gene have equal chances of developing speech in either hemisphere , and perhaps both; they should be equally often left- and right-handed for skill, but cultural pressures to use the right hand would make more right- than left- handed.”


This obviously begs the question as to why then, if this gene favors speech, hasn’t it spread throughout the population relegating other options to the dust bin of extinction? The reason may have something to do with some variant of the evolutionary stable strategy theory. Though a bit of an extreme example, consider the increased resistance to malaria in heterozygous individuals and the invariably associated balance with those unfortunate homozygous who suffer from sickle cell anemia.


Annett explains “Perhaps the gene carries disadvantages as well as advantages, because it works by handicapping the right hemisphere. The estimates of genotype proportions suggest that there might be advantages for those with one copy, in comparison with those with no copies and those with two copies.” In other words, there seems to be disadvantages to both end of the “handedness” phenomena, and a blend of brain hemisphere function may be most efficacious, even if it appears right-handers predominate.


These findings add color and nuance to the complexity and function of the human brain, and at the same time, reveal how immersed we are within a sea of vast evolutionary influences everywhere we look.


Ref: Annett, Marian. The brain and left-handedness.Human evolution 9th ed.Cambridge university press, UK. 2005. pg122

1 comment:

Linda said...

One way to be a southpaw and kool is to use a Left handed FrogPad. www.frogpad.com