What is it that makes a person a boy or a girl? From our cradles to our graves, a pair of letters, either XX or XY, will define much of our lives. “It's a girl!" or “It's a boy!" will be the first label applied to us, the first thing said about who we are as an individual. For every person in every society, gender has a fundamental affect on what we choose, how we live, and how we think about the world and how the world sees us. Sex is one of the most powerfully defining concepts that we have.