View full lesson: http://ed.ted.com/lessons/why-are-some-people-left-handed-daniel-m-abrams 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.