Sunday, September 23, 2007

Hand Dominance

It seems some studies have been done to determine the genetic predisposition of a person to being right-handed or not right-handed (i.e., left-handed or ambidexterous). In this study, for example, the researchers concluded the following:
Therefore, we suggest that humans are born with hand preference that is prenatally determined. This work should also impact the social aspects of handedness. With this knowledge, it should be possible for the parents to predict that their newborn with a counterclockwise hair whorl will develop into a left-handed or ambidextrous person in 50% of the cases.
That is, if the hair at the crown of your child's head swirls in a counter-clockwise direction, s/he has a 50% chance of being left-handed or ambidexterous. While the number seems to correlate with random chance, it's actually more significant because left-handed people make up anywhere from 10-20% of the population, depending on the source of your statistics (e.g., Anything Left-Handed, ABC News, and the University of Washington).

This reminded me of a Discover Channel program titled
In The Womb: Multiples. It addressed mirror image identical twins, identical twins whose traits are identical, but reveresed. For example, the hair whorl flows in one direction (e.g., clockwise) on twin A, and the opposite on twin B; twin A's birth mark is the right shoulder, and twin B's is on the left; or twin A is right-handed while twin B is left-handed. Per the program, the theory is that some left-handed people are the surviving mirror image twins of identical twins. "Some fetuses spend the first trimester of pregnancy with a companion that later disappears—a phenomenon called the 'vanishing twin syndrome.'"

Per North Suburban Parents of Multiples:
In approximately 23 percent of identical twins, the fertilized egg splits later than 7 days following conception. The original right half of the egg becomes one twin and the original left half becomes the other.

This may be a partial explanation for the fact that a little over one third of identical twins are left-handed, double the rate in the general population. In extreme cases, all of the internal organs are reversed in one of the twins, with the heart on the right, the liver on the left, and the appendix on the left.
Virgin Media's "fascinating twin facts" states that "A higher rate of twins (18-22%) are left handed compared to non-twins (10%)."

This image from Genetics shows an example of hair whorls. "The counterclockwise parietal scalp hair-whorl on the left is of a NRH person, and the one on the right of the RH author swirls clockwise."

Per those photos, MetaBaby's hair whorl looks like it's counter-clockwise to me. As of yet, he's still an equal opportunity hand user, so we'll see where this leads us.


Bryan said...

Hello I ran across your blog while searching VTS because I have been trying to research my mental illness more and I have thought for years that I might be a part of VTS. I am being treated for bipolar and schizophrenia, I have a double crown and have had a third set of extra teeth removed on the top of my mouth. When I was younger I was left handed but when I started school they only taught me to write right handed. I can do some thing with both hands but was never truly ambidextrous. I would like to try to find out more about this condition... maybe working together we can figure something out.

Have a great day