Why Are Some People Left-Handed?
by Josh L Davis
Photo credit: Evgeny Dubinchuk/shutterstock
Scissors, can openers, spiral bound notebooks—the world, it often seems, is designed for right-handed people. There have been many studies looking into the basis for handedness, and yet we still don’t really understand what’s going on. Maybe it’s simply because it’s so puzzling that we find it so fascinating. One thing we do know for certain, however, is that only about one in 10 people are lefties.
Amazingly, that statistic is not a one-off, nor country dependent. It appears to hold true for the world over that roughly 10% of the population is always left-handed. This should therefore imply that there is a genetic basis for the behavior. But is there? In a talk given to the Royal Society, Silvia Paracchini, a geneticist from the University of St. Andrews, thinks that things are not as black and white as they may seem.
When researchers want to look into the genetic basis behind a trait, they often turn to twins. The use of twins, who are frequently raised in the same household, is vital in separating shared genetic and environmental factors that might affect the trait of interest. Researchers found in twin studies that only about 25% of handedness can be explained by a person’s genes. This, claims Paracchini, is surprisingly low. But as more and more studies that have looked into this have discovered, it is also amazingly consistent.
For many years, people have assumed that there must be a gene directly responsible for handedness, but with no real way to test it, researchers were left in the dark. However, with more than 100,000 human genomes now sequenced, the data set has skyrocketed. As of yet, a single culprit that directly determines handedness still remains elusive.
But steps have been made. When Paracchini was looking into the genetic basis of dyslexia, one of her PhD students found something else: an association between handedness and the same gene that sets up the bodies left/right axis. The particular gene is responsible for the condition situs inversus, where the asymmetry of the body is switched, resulting in, for example, the heart being on the right, rather than the left as is normal.
This raises the possibility that the same system that controls the structural asymmetry of our body plan might be recycled and used to control behavioral asymmetry. Whilst there is an association, we don’t yet know for certain if there is a link. Muddying the waters still further, another study looking at the rate of left handedness in patients with situs inversus found no difference when compared to normal people. However, if there is a link between the two conditions, Paracchini points out, it would only be a very tiny fraction in the underlying genetics of handedness.