Gender is one of the first things we notice about the people around us. But where do our ideas about gender come from? Can gender differences be explained by genes and chromosomes, or are they the result of upbringing, culture and the environment? In this encore episode from October 2017, we delve into debates over nature vs. nurture, and meet the first person in the United States to officially reject the labels of both male and female, and be recognized as "non-binary."