Nature or Nurture?
The cause or causes of left handedness have been obscure, with science having proposed everything from handedness being caused by early conditioning as a child to having the left side of the brain slightly damaged during birth.[1] Although both these and other arguments may seem to be reasonable at first glance, the truth is probably much more complex given the varying statistics.
For instance, it seems apparent after a quick look at several different sets of data that left handedness tends to develop regardless of cultural influences. Sonograms of unborn children, for example, show that 85% of them prefer to suck their right thumb, and follow up studies on these children as they grew older showed that these children kept that hand preference.[2] This, and other similar data, would point to a genetic rather than a cultural cause of handedness.
Yet if that is the case, it quickly becomes obvious that the typical dominant-recessive gene pattern is too simplistic to provide an adequate explanation. Children of two left handed parents (presumably the recessive gene given its rarity) might have a much higher chance of being left handed (26.1%) than do the children of two right handed parents (9.5%), but their handedness is hardly guaranteed.[1] And identical twins, despite sharing the same genome, do not always develop the same handedness.[1,2]
Two separate, but strikingly similar, theories have been developed to deal with this difficulty: one called the Right-Left Shift theory (developed by Professor Marian Annett)[2] and another one developed by Dr. Chris McManus [1]. Essentially, both models propose that a single gene (here called R+) was responsible for the development of right handedness, and that children with the R+ gene from both parents would be guaranteed to be born right handed.[1,2]
On the other hand, rather than also having a simple gene that coded for left handedness, it was proposed that the alternate gene, R-, simply allowed for either hand to be selected without bias. Children born with R- genes from both parents would simply have a 50:50 chance for which hand was selected.[1,2]
The theories begin to differ slightly when deciding how to deal with those who have the R+,R- combination. Both agreed that the genes would be co-dominant, but disagreed in the exact way such co-dominance would manifest. Annett proposed that those with the R+, R- genes would have a slightly more "mixed" handedness. They would, perhaps, write right-handed, but throw balls, and hammer nails left handed, or follow some other combination.[2] McManus, on the other hand, suggested that children with both genes would have a 25% chance of developing fully fledged left handedness, and a 75% chance of right handedness.[1]
Both of these models work very well, although Annett's model seems to be slightly more likely given that handedness is not so much an issue of one or the other as it is a spectrum of different degrees of left handedness (see below). Each of them, however, sufficiently explains why a right handed parent can still have children who favored their left hands; even if the parent used the right hand, so long as they had an R- gene to pass on to their child, the child would still be able to determine his or her own handedness independently. Also, identical twins, if born with the R-, R- combination (or possibly the R+, R- combination depending on which theory is true), would be able to prefer different hands.[1,2]
For instance, it seems apparent after a quick look at several different sets of data that left handedness tends to develop regardless of cultural influences. Sonograms of unborn children, for example, show that 85% of them prefer to suck their right thumb, and follow up studies on these children as they grew older showed that these children kept that hand preference.[2] This, and other similar data, would point to a genetic rather than a cultural cause of handedness.
Yet if that is the case, it quickly becomes obvious that the typical dominant-recessive gene pattern is too simplistic to provide an adequate explanation. Children of two left handed parents (presumably the recessive gene given its rarity) might have a much higher chance of being left handed (26.1%) than do the children of two right handed parents (9.5%), but their handedness is hardly guaranteed.[1] And identical twins, despite sharing the same genome, do not always develop the same handedness.[1,2]
Two separate, but strikingly similar, theories have been developed to deal with this difficulty: one called the Right-Left Shift theory (developed by Professor Marian Annett)[2] and another one developed by Dr. Chris McManus [1]. Essentially, both models propose that a single gene (here called R+) was responsible for the development of right handedness, and that children with the R+ gene from both parents would be guaranteed to be born right handed.[1,2]
On the other hand, rather than also having a simple gene that coded for left handedness, it was proposed that the alternate gene, R-, simply allowed for either hand to be selected without bias. Children born with R- genes from both parents would simply have a 50:50 chance for which hand was selected.[1,2]
The theories begin to differ slightly when deciding how to deal with those who have the R+,R- combination. Both agreed that the genes would be co-dominant, but disagreed in the exact way such co-dominance would manifest. Annett proposed that those with the R+, R- genes would have a slightly more "mixed" handedness. They would, perhaps, write right-handed, but throw balls, and hammer nails left handed, or follow some other combination.[2] McManus, on the other hand, suggested that children with both genes would have a 25% chance of developing fully fledged left handedness, and a 75% chance of right handedness.[1]
Both of these models work very well, although Annett's model seems to be slightly more likely given that handedness is not so much an issue of one or the other as it is a spectrum of different degrees of left handedness (see below). Each of them, however, sufficiently explains why a right handed parent can still have children who favored their left hands; even if the parent used the right hand, so long as they had an R- gene to pass on to their child, the child would still be able to determine his or her own handedness independently. Also, identical twins, if born with the R-, R- combination (or possibly the R+, R- combination depending on which theory is true), would be able to prefer different hands.[1,2]
Degrees of Left-Handedness
It used to be thought that left-handedness was something that was fairly easy to measure. After all, a person who wrote with his left hand would naturally be left-handed, and those who didn't would not. But after closer scrutiny this explanation actually proves to be as simplistic as the early theories for the causes of left handedness.
For example, what are those who do everyday activities left-handed, from throwing a ball to striking a match, but leave the writing to the right hand, as Babe Ruth did?[2] Are these people also left handed? Or what about those who do the exact opposite, writing left handed but using the right hand for other important functions? How could both of these sets of people be left handed? Are they both left handed?
Looking at the various ways in which people can manifest left handedness, scientists eventually came to the conclusion that, while there are certainly people who do the vast majority of their day-to-day activities favoring only their right or left hands (often called 'strongly right handed' or 'strongly left handed'), there is also a considerable percentage of people who are naturally 'mixed.'[1, 2] In all, while about 10% of most populations would classify themselves as left handed, a full 35% fulfill the requirements for at least being weakly left handed by drawing, striking matches, or even sweeping a floor in ways that favor the left hand.[2]
To complicate matters further, there is also the fact that there is not only handedness, but entire ranks of varying sidedness in humans: a preference for a certain foot, ear, eye, even which side of the mouth one chews with.[1] In all of these, the right side is still favored over the left, but this preference is considerably less marked. For instance, only 80% of people favor their right hand, and only 70% favor the right eye.[1] Obviously, with only 10% of a population identifying itself as left handed, large numbers of right handed people must be "cross lateralized." What is truly baffling, however, is the fact that for all the high numbers of right handed people who switch in at least some degree, an even higher percentage of left handed people are inconsistently aligned.[2] Although studies of sidedness are much less common than those of handedness, there is evidence that cross eye and hand dominance, at least, gives an advantage in sports--and presumably other physical activities involving coordination-- thus enhancing fitness and possibly giving a slight evolutionary edge to such lopsidedness.[2]
For example, what are those who do everyday activities left-handed, from throwing a ball to striking a match, but leave the writing to the right hand, as Babe Ruth did?[2] Are these people also left handed? Or what about those who do the exact opposite, writing left handed but using the right hand for other important functions? How could both of these sets of people be left handed? Are they both left handed?
Looking at the various ways in which people can manifest left handedness, scientists eventually came to the conclusion that, while there are certainly people who do the vast majority of their day-to-day activities favoring only their right or left hands (often called 'strongly right handed' or 'strongly left handed'), there is also a considerable percentage of people who are naturally 'mixed.'[1, 2] In all, while about 10% of most populations would classify themselves as left handed, a full 35% fulfill the requirements for at least being weakly left handed by drawing, striking matches, or even sweeping a floor in ways that favor the left hand.[2]
To complicate matters further, there is also the fact that there is not only handedness, but entire ranks of varying sidedness in humans: a preference for a certain foot, ear, eye, even which side of the mouth one chews with.[1] In all of these, the right side is still favored over the left, but this preference is considerably less marked. For instance, only 80% of people favor their right hand, and only 70% favor the right eye.[1] Obviously, with only 10% of a population identifying itself as left handed, large numbers of right handed people must be "cross lateralized." What is truly baffling, however, is the fact that for all the high numbers of right handed people who switch in at least some degree, an even higher percentage of left handed people are inconsistently aligned.[2] Although studies of sidedness are much less common than those of handedness, there is evidence that cross eye and hand dominance, at least, gives an advantage in sports--and presumably other physical activities involving coordination-- thus enhancing fitness and possibly giving a slight evolutionary edge to such lopsidedness.[2]
Citations
1. McManus, I. Right Hand, Left Hand. Cambridge, MA: Harvard University Press, 2002. Print.
2. Roth, Melissa. The Left Stuff: How the Left-Handed Have Survived and Thrived in a Right-Handed World. New York, NY: M. Evans and Co., 2005. Print.
2. Roth, Melissa. The Left Stuff: How the Left-Handed Have Survived and Thrived in a Right-Handed World. New York, NY: M. Evans and Co., 2005. Print.
