Since right-handers outnumber southpaws by approximately 9 to 1, it’s not hard to imagine why there’s a bias against lefties. Yet there are plenty of left-handed role models – two of the last three presidents (and both of this year’s contenders) favor the left manually, if not necessarily politically. Celluloid hero Luke Skywalker (Mark Hamill) dispatched the Death Star with his left, while Neo (Keanu Reeves) favored his sinister side in harnessing the power of the Matrix. And after granting a mortal man divine powers, guess which hand God (Morgan Freeman) used to snatch them back?

Which hand do you use to hold your coffee cup? Is it the same hand you write with?.

Though most people define left- or right-handedness by the hand that grips the pen, it turns out there are varying degrees of handedness. Want to find out if you should actually consider yourself either purely left- or right-handed, or somewhere in between? Take the new 23andWe survey on handedness and find out.

The non-right-handed 10 percent of the population has proved useful to scientists who want to understand how the brain works. In her “right-shift gene” theory, for example, British psychologist Marian Annett argues that handedness is merely a side effect of having a single (unidentified) gene evolve to assign speech control to the brain’s left side (which governs movement in the body’s right side). Her proposal is based on a study of hand preference when performing various tasks, and then resulting subgroup classifications. Analyzing the distribution, she noticed that there was a tendency toward right-handedness even among some lefties, which led to the name of her theory. If the gene is damaged early in development, she says, it would affect the left hemisphere of the brain and could be responsible for conditions such as autism and schizophrenia.

For Australian New Zealand psychologist Michael Corballis however, handedness might have been part of what helped separate man from monkeys. He thinks language and right-handedness evolved together, and ties this relationship in to the asymmetry of the brain. Most people, Corballis notes, are right-handed and have their language center in the left hemisphere of the brain. He posits that they have two copies of a “right-handed” gene. Corballis says that lefties, however, have one copy of the right-handed gene and what he calls a “chance” gene, so their language center may be in the left hemisphere, the right hemisphere, or even in both. The disruption of brain asymmetry, he says, explains the link between left-handers and language problems such as dyslexia and stuttering.

Both psychologists’ ideas stress the importance of understanding how the brain is organized, and looking at brain development on a genetic level may give researchers an even better understanding of the subject. Though genetic evidence to support either Annett’s or Corballis’ theories has yet to be found, last year researchers at Oxford’s Wellcome Trust Center for Human Genetics announced that they’d found a gene that is associated with left-handedness if children inherited their father’s copy. Known as LRRTM1, the gene has also been associated with dyslexia and schizophrenia.

Though LRRTM1 is the only gene that has been associated with left-handedness to date, plenty of other interesting traits have been linked to it that also offer hints to its biology. Consider these other findings that, as yet, haven’t been linked to a gene or genes:

• there are more left-handers in England than there are in Wales and Scotland combined;
• there’s a link between studying architecture and left-handedness …
• … and between creativity in painting and music and left-handedness;
• the percentage of left-handers among homosexuals is higher than in the general population;
• an increased incidence of breast cancer among post-menopausal left-handed women in Australia and America; and,
• slightly more left-handed major league baseball players were born in June than in any other month.

Some of the findings above may seem trivial, but some are also controversial. One study that’s been hotly contested for the last 20 years, for example, is whether or not right-handers live longer than left-handers. Several researchers have since published reports that either support or contradict the data.

Images from: bigfoto.com and PLoS ONE: Kalisch et al, 2006.

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