The Science Of How And Why Left-Handers Are So Uncommon
By Roxanne Palmer | December 4, 2012 9:41 AM EST
Signs of the vast right-hand conspiracy are everywhere. Blatantly discrepancies in scissor quality. Words for “left” also being used to signify evil (the Latin 'sinister') or clumsiness ('gauche,' in French). Languages that are read left-to-right, making writing with a pencil a smudgy mess for any southpaw that isn't transcribing Hebrew or Arabic.
No matter how many specialized corkscrews there are available, left-handers across most of the world will always be, to some degree, the odd ones out. Frustrated lefties of the world may wonder: how did right-handedness become so common?
Estimates for the exact percentage of people that are righthanded vary a bit between 70 to 90 percent. Hand preference doesn't seem to be a matter of simple genetic inheritance – if left-handedness was a simple recessive trait, then you'd expect to see two lefties making nothing but southpaws, and any right-handed person producing right-handed children. And identical twins should always prefer using one hand or the other. But that doesn't always happen.
Still, there's much work suggesting that there is some genetic factor at play.
In a 2009 paper in the journal Neuropsychologia, Researchers from the University of Helsinki in Finland and Indiana University examined data on 30,161 people aged 18 to 69 that were members of a massive Finnish twin study sample.
(Twins are especially useful to study when scientists want to figure out how much genes may determine a particular trait, since identical twins will be nearly 100 percent genetic matches, and fraternal twins will be nearly 50 percent genetic matches.)
The team found that left-handedness was more common in twins and triplets, with rates of 8.1 percent and 7.1 percent, respectively, than in people that are not part of multiple births, or singletons. In singletons, the rate of left-handedness was 3.5 percent.
One wrinkle in determining just how a person becomes right or left-handed is that handedness is also easier to permanently manipulate and change than other genetically derived traits, like eye and hair color.
“Teachers have been known to force children to switch from using their left hand to using their right hand for writing,” Indiana University primate researcher M.K. Holder wrote in Scientific American in 1997. “Also, some more restrictive societies show less left-handedness in their populations than other more permissive societies.”
One possible genetic model for left-handedness is called “random-recessiveness.” National Cancer Institute researcher Amar Klar's work suggests that a single gene with two variants, but with an interesting twist, could explain the imbalance. The dominant form of the gene confers right-handedness (and also, interestingly enough, a clockwise hair spiral). But the recessive form of the gene does not cause left-handedness or ambidextrousness – it means that there is no preference for either, and handedness becomes a 50-50 chance.
This model would explain how half of the children of a marriage between left-handers could be right-handed.
Even if the genetic mechanism can be explained, there's also the evolutionary aspect to consider, and there are a number of theories about the value of handedness and how it might have been selected for in human evolution.
In one theory that combines genetics and behavior, right-handedness becomes dominant thanks to the value of cooperation. Right-handers can share tools with ease, making shared handedness advantageous on a broader level.
“The more social the animal -- where cooperation is highly valued -- the more the general population will trend toward one side,” Northwestern University researcher Daniel Abrams said in a statement in April. “The most important factor for an efficient society is a high degree of cooperation. In humans, this has resulted in a right-handed majority.”
In a recent paper published in The Journal of the Royal Society Interface, Abrams and his colleague Mark J. Panaggio use a mathematical model to explain how the balance between cooperation and competition in human evolution has preserved left-handedness, but as a small minority.
If cooperation was the sole important factor in determining handedness, humans would be totally right-handed. But lefties can sometimes have an advantage in competition – an unexpected left cross from an ancient hominid may have helped it win fights and ensured that he or she survived and reproduced.
Left-handers might only make up a fraction of the population and find themselves more predisposed to mental illness, but they do tend to be overrepresented in a number of prestigious professions, including sports (except, according to one study, tennis, where left-handedness seems to confer little advantage at elite levels). U.S. chief executives also tend to swing to one side – current and former Presidents Barack Obama, Bill Clinton, George H.W. Bush, and Ronald Reagan were all southpaws.
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