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About 10% of the world population is left-handed. But why does handedness exist and what determines which hand is dominant? Scientists have suggested several theories, but the answer may well lie with evolution.

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Odds are that between 10 and 15 percent of you right now watching this are lefties. That is, unless you're a major league baseball pitcher, in which case there's a 30 percent chance that you're a southpaw, or if you happen to be president of the United States, 'cause seriously, five of the last seven presidents have been left handed. It's practically a prerequisite at this point. But, like 90 percent of humans, I am right-handed. So why do I have a preference for one hand over the other in the first place and what is it that determines our dominant hand? The answer may be evolution. Handedness, or hand dominance, is unique to humans among primates. Close relatives, like chimps and gorillas, do not consistently favor one hand over the other, and while many animals like cats and dogs will often favor one paw over another, the breakdown between righties and lefties is roughly equal. Though many believe one exists, scientists have yet to discover a gene that determines the dominant hand, and though handedness runs in families that doesn't necessarily mean that it's hereditary. Identical twins who share the same genes don't always have the same dominant hand. There are a lot of theories out there, including the idea that our right-handed world is an evolutionary byproduct of cooperation among humans. According to one study released in 2012, cooperative acts like sharing tools have favored the presence of a dominant hand representing a group so everyone can use the same tools and stuff. The more widely accepted theory is that hand dominance is connected to the asymmetry of our brains. Our brains are contralateral, which means that the left hemisphere of the cerebral cortex, where language and logical processing is usually localized, controls the movement of the right side of our body, and the right hemisphere, where spacial recognition usually takes place, controls the left side of our body. The brains of our primate relatives worked the same way but scientists believe that the human brain is more asymmetrical. Though it may not be a coincidence that the side of our brain that is in charge of speech also controls the hand used to produce written language. It could be that natural selection produced a majority of righties because our left brain is telling our right hands how to write. This theory is helped along by studies that have found that many lefties tend to process language more evenly across both hemispheres of their brains, and in some cases completely on the right side. So it could be that lefties don't exactly inherit their hand dominance. Instead they inherit a lack of neurological bias toward a dominant left brain. And that would be how you end up with 10 percent of the population being annoyed with the design of spiral notebooks. I just think we need to give more credit to our less dominant hand because it's still working hard. I can't imagine trying to eat a meal or type a script without my left hand. (Said to left hand) "You're important to me, okay?" Thanks for watching this episode of SciShow. If you have any questions, comments or suggestions for us you can find us at Facebook or Twitter and of course down in the comments below. And if you want to keep getting smarter with us here at SciShow, you can go to and subscribe.