The field of Eugenics is concerned with the genetic quality of humans. People who are in favor of eugenics promote the reproduction of those with positive genetic traits (such as intelligence, attractiveness, and height) and fewer offspring for those with undesirable traits. Eugenics can be brought into practice through birth control or sterilization of those with undesirable traits to prevent reproduction. In extreme circumstances eugenics can lead to extermination of people with traits or qualities that are deemed undesirable, such as the murder of millions of people by the Nazis before and during World War II.