Evolution Of Left-Handedness?
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At Edge.org, Jonathan Gottschall offers a theory for why natural selection can't seem to make up its mind about left-handedness:
Which brings me to Charlotte Faurie and Michel Raymond, a pair of French scientists who study the evolution of handedness. Left-handedness is partly heritable and is associated with significant health risks. So why, they wondered, hasn't natural selection trimmed it away? Were the costs of left-handedness cancelled out by hidden fitness benefits? 
The scientists noted that lefties have advantages in sports like baseball and fencing where the competition is interactive (but not in sports, like gymnastics or swimming, with no direct interaction). In the elite ranks of cricket, boxing, wrestling, tennis, baseball and more, lefties are massively over-represented. The reason is obvious. Since ninety percent of the world is right-handed, righties usually compete against each other. When they confront lefties, who do everything backwards, their brains reel, and the result can be as lopsided as my mauling by Nick. In contrast, lefties are most used to facing righties; when two lefties face off, any confusion cancels out. 
Faurie and Raymond made a mental leap. The lives of ancestral people were typically more violent than our own. Wouldn't the lefty advantage in sports—including combat sports like boxing, wrestling, and fencing—have extended to fighting, whether with fists, clubs, or spears? Could the fitness benefits of fighting southpaw have offset the health costs associated with left-handedness? In 1995 Faurie and Raymond published a paper supporting their prediction of a strong correlation between violence and handedness in preindustrial societies: the more violent the society, the more lefties. The most violent society they sampled, the Eipo of Highland New Guinea, was almost thirty percent southpaw.


Think about throwing a spear at an animal you are chasing like a quarterback throwing the football while rolling out. Righthanded quarterbacks can throw better while running to the right than the left. (Indeed, Robert Griffin III of Baylor likely won the Heisman last year because, more than any other single play, a remarkable pass he threw that beat Oklahoma on national TV. After rolling to his left, this righthander surprised the defense by throwing a long bullet into the right corner of the end zone (video). Having a lefthander in your hunting party ups your chances that somebody will connect.

Here are the top ten baseball hitters of all time according to one measure where the average batter gets a 100:
Seven lefties, a switchhitter, and two righties. On the other hand, four positions in baseball: 2nd, short, third, and catcher are largely off limits to lefties because most throws wouldn't be cross body.
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