Every single individual varies along a range of five personality traits. We don't know why, or how, only that we do. Agreeableness, conscientiousness, extraversion, neuroticism, openness: these five traits, and the variations within them, capture the range of human personality, and they do it quite well. They capture differences between men and women, between liberals and conservatives, between emotionally unstable artists and hard-working manager types, and everyone in between. Not only do...