Left-handedness has been called everything from a boon to a brain defect, with the overwhelming majority of research supporting the former. But if you are a lefty, or if you've ever tried to steal second base with a good lefty on the pitcher’s mound, the bigger question surrounding hand preference is why. In an effort to solve this riddle, resent studies have looked at the brains of both humans and animals. In most animals, mental processing seems to be equally divided between the brain's left and right hemispheres. Not so with humans, whose hemispheres tend to specialize in certain tasks. Scientists have observed that in right-handers, language is generally processed in the left hemisphere, while lefties tend to process language in the right. 　One Oxford University researcher argued that the beginnings of right-hand preference in humans could be traced back 200,000 years to a single mutation, an epic event that also introduced hemispheric specialization, higher reasoning power, and language. Not your everyday mutation. But this doesn’t explain why 10 % of humans are lefties. Chris McManus of University College London believes that left-handedness in humans first appeared between 20,000 and 100,000 years ago, the result of a second mutation that canceled out the bias to the right. Those who inherit the mutation have a fifty-fifty chance of becoming left-handed.   Primatologist Bill Hopkins studied primates for 10 years in order to determine whether apes, like humans, have hand preference. He found that a third of all chimpanzees are lefties. Interestingly, though, the more primitive primates tend to be left-handed, indicating that left-handedness may predate right-handedness. Hopkins also found that the younger a chimp is within a group of siblings, the more likely the chimp is to be a lefty. 　This is the first conclusive evidence linking birth order and hand preference. “More than 50 % of chimps that have at least five older siblings are left-handed,” says Hopkins. He concludes that left-handed apes and people either lack the gene that makes them right-handed or they possess the gene, but, sometime during development, the gene is prevented from expressing itself.