In the 1960s, scientists discovered a genetic condition in which males have an extra Y chromosome, a phenomenon aptly named “XYY syndrome.” Because the Y, or “male,” chromosome is responsible for things such as testosterone production—and because an early study suggested the Danish prison population had a higher incidence of the condition—some hypothesized the mutation led to increased aggressiveness and criminality. To date, studies have largely debunked the myth of the “supermale,” though the syndrome remains a subject of interest among researchers. Learn more about the genetic underpinnings and the history of XYY syndrome with this Vox explainer.