Why Is Left-Handedness So Rare?
Ah, the debate rages on. According to this article in Scientific American, "What causes some people to be left-handed, and why are fewer people left-handed than right-handed?" the issue is solidly the way genetics influence who is right/left handed.
According to the article, handedness is based on two genes. One of the genes they call "D" for "dextral," meaning right-handed, and the other they call "C" for "chance," meaning it's a matter of chance. The D gene is more frequent in humans, but each human has two of these genes.
The possibilities are DD, DC, and CC. People with DD or DC will both be right-handed, though the DD person will be more strongly so. People with CC could be either right- or left-handed. The result? Many more right-handed people than left-handed.
That's the genetics.
But why would it be that more people have Ds than Cs? And why would we have a genetic code that was biased in favor of right-handedness?
The LiveScience article, "Life's Extremes: Left- vs. Right-Handed" talks about a lot of issues, including the language dominance of the left hemisphere of the brain. Generally, the hemispheres of the human brain use the left side for speech and language. This is the case for about 95 percent of right-handed people, but only for about 65 percent of left-handed people. The other 5 percent of righties and 35 percent of lefties, have their speech and language in their right brain hemisphere, or on both sides.
When we speak of "handedness," we generally mean the small motor coordination of our hand. It turns out that not everyone has dexterity and strength in the gross motor (big muscle) abilities on the same side as their small motor abilities. I, for one, am considered right-handed, because I can only legibly write with my right hand. But my left ARM is my dominant arm in many things, but not everything. If I reach for a cup on a shelf, I'll instinctively reach with my left. And if I am painting, it turns out I can paint equally well with either hand! I'm lousy at throwing with either hand, but slightly better with the right.
It is possible that extreme handedness might be connected to the fact that most people are taught to write in school. In other words, maybe before humans started writing (only a few thousand years ago), we weren't so obviously one-handed. It is also possible that we were always right-hand dominant for writing, but just didn't know it until we started writing.
Another issue that comes up in this debate is the fact that our bodies look symmetrical on the outside (one of anything in the middle, two of everything not in the middle of our bodies) but on the inside, our organs do not look symmetrical. Our hearts are on the left (except for about 1 in 12,000 people, who have it opposite), liver is on the right, and even our lungs are shaped differently on the right and on the left.
There are some differences between right- and left- handed people, but according to the Huffington Post, lefties are not more prone to being artistic or creative. There are a few small benefits (eg. certain sports) and a few very minor correlations to problems (eg. slightly higher risk of certain sleep problems from limb movements), but less differences than most people expect.
Bottom line: We really still do not know why humans seem statistically prone to be 90 percent right-handed, and 10 percent left-handed. Even if the genetics described above are right, why did it happen that way? It could be part of the brain hemisphere system, or related to the lack of symmetry of our internal organs, but no one has found a way to prove causation, only correlation — at least so far.
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