February 28, 2009

Lefties Have Evolutionary Benefits

A new study by scientists in France offers a possible explanation for left-handedness, finding that the trait survived since prehistoric times in part due to its rarity, which offered benefits.

The question of why some people are left-handed while the vast majority of humans are right-handed has puzzled scientists, who have long believed that genes that don't aid in the struggle to survive typically get squeezed out of the genetic code, leaving instead the ones that are fitter.

However, in prehistoric times, a left-hander might have had some advantages, for example the benefit of surprise in fighting against a right-hander, the French scientists say.

Furthermore, left-handers tend to be skilful with both hands, or even ambidextrous, compared with most right-handers who show a strong preference for their right hand -- a distinct disadvantage in situations that require intermanual coordination.

The study, led by Violaine Llaurens of the Institution of Evolutionary Sciences in Montpellier, France, estimates that left-handers comprise between five and 25 percent of the total population, with significant geographical variations.

The scientists believe that while genes for left-handedness are so far elusive, there is compelling evidence that the trait is heritable. Indeed, a person with two left-handed parents will have more than twice the chance of being left-handed than those with right-handed parents.

Developmental factors, such as exposure to hormones in the uterus, could also play a role in development of the trait, scientists say.

The study also revealed some good news for those who are left-handed, finding that lefties are relatively numerous among creative men and among children with IQs greater than 131.  Also, a number of those who excel at mathematics and music are left-handed.

"All these advantages may play a significant role in the social status of left-handers," the scientists wrote in a report about their study.

Indeed, some research indicates that left-handed men are better paid than their right-handed counterparts.

However, left-handedness also carries an evolutionary cost, the scientists say, pointing to statistical data that suggests right-handers typically live longer than left-handers, by anywhere from a few months to several years.

Although the underlying cause is not yet clear, this difference in longevity could be partly explained by the greater number of fatal accidents involving left-handed men using instruments, industrial tools and machines designed for right-handers.

Furthermore, left-handers tend to generally have smaller body sizes, something linked to less reproductive success, the scientists said.

And lefties tend to be more numerous in the homosexual population than in the general population as a whole.  Since homosexuals tend to have fewer children or none at all, "lefty" genes are less likely to be passed on to future generations, which is an evolutionary disadvantage.

The study appears in Philosophical Transactions of the Royal Society B.


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