Duration: 04:53 | Today, about one-tenth of the world's population are southpaws. Why are such a small proportion of people left-handed -- and why does the trait exist in the first place? Daniel M. Abrams investigates how the uneven ratio of lefties and righties gives insight into a balance between competitive and cooperative pressures on human evolution. [Directed by TED-Ed, narrated by Addison Anderson, music by Michael Dow].