Nature Vs. Nurture

Adding to the age-old debate of nature vs. nurture is the controversy over human hand preference. Is it a matter of genetics which yields the handedness of an individual, or is it determined by the person's surroundings and their environment? Those that believe nature is the main factor that decides handedness argue that it is decided by the the genetic makeup of the individual which has been passed down through the family. Those that argue on the side of nurture feel that each person has a clean slate and that it is the surroundings and experiences that the person encounters that decide whether they are right or left handed. Many believe that it is a combination of both, though the results of this study attempt to prove this an improbable possibility.

Many different studies have been conducted relating to handedness in human beings in the past decades. This article first emphasizes that it is important to understand the definition of handedness. Many different research groups have made the mistake of assuming that handedness only refers to the hand with which the person prefers to write. It is easy for an individual to determine which hand they prefer, but to satisfy the requirements of research methods handedness cannot be determined solely by which hand the person writes with. This research uses the definition of handedness as described by a study done by D.C. Rife in 1940. According to Rife, a person is right handed if they use the right hand to throw a ball, use a spoon, saw, sew, shoot marbles, bowl, cut with a knife, cut with scissors, hammer, and write. This way all aspects of preference toward one side or the other are exploited. It is important, however, to account for the fact that because of discrepancies between different methods of determining handedness between studies, there is a 9-11% variation in the prevalence of non-right-handers between different studies.

This article takes a clear “nature” stance on the debate of the influences on handedness in human beings. In other words these researchers feel that genetics play a more important role in determining which hand is preferred than does the persons environment. Despite the fact that this article is out to prove that genetics is the cause of handedness, they do provide the evidence and arguments that the environment has something to do with it. For example, a study done by Rife in 1940 shows that “up to one half of the children born to left-handed * left-handed parents were right handed”, and even more interestingly enough, 18% of monozygotic twins were discordant, meaning that one twin was RH and the other was LH.

This article recognizes the fact that the ideal way to prove that handedness is determined genetically would be to conduct a study using genetic mapping. The problem with this, however, is that you must know exactly how many genes are involved before such a study can be conducted. This information is not yet available. Instead of using genetic mapping, the researchers in this study set out to find something to compare handedness to within an individual. That is to say, that the individual would be their own control. They looked for another body feature that had the same left-right body asymmetry and that was not “culturally influenced.” What they decided to use was the direction of the scalp hair-whorl rotation. Not only does this trait demonstrate radial asymmetry but also left-right asymmetry.

The researchers examined 500 individuals, predominantly in Maryland, for their direction of hair-whorls, finding that 91.6% of them had clockwise rotation. This statistic was consistent with another survey done decades earlier among newborns that found 93.8% had clockwise rotation. The prediction of this research was that the “scalp hair-whorl rotation should be uncoupled in NRH (non right-handed) individuals such that one-half of them should exhibit a counterclockwise pattern.” The study found that this prediction was indeed correct, with 22 of 49 having a counterclockwise rotation, or 44.9%. This is much different than the 8.4% of the overall population with a counterclockwise rotation. To further show that genetics play a more important role in deciding handedness, they predicted that of individuals that have counterclockwise rotation, one-half of them should be non right-handed, contrasting with 9-11% of the general population that is non right-handed. The study they conducted showed that of 23 individuals with counterclockwise whorl rotation, 12 were RH and 11 were LH, proving their prediction correct.

These researchers also take another step to further test the hypothesis that genetics play the most important role in determining handedness. They recognized that a strong argument against genetic etiology was the fact that 18% of monozygotic twins were discordant, when genetic theory would suggest that the twins should have the same handedness because they share