Eugenics is sometimes broken into the categories of positive eugenics (encouraging reproduction among those designated "fit") and negative eugenics (discouraging or prohibiting reproduction among those designated "unfit"). Both positive and negative eugenic programs were advocated and pursued during the early 20th century. Negative programs were responsible for the
compulsory sterilization of hundreds of thousands of persons in many countries, and were a cornerstone of
Nazi eugenic policies of
racial hygiene which ultimately led to
genocide. New eugenics generally supports
genetic modification or genetic selection of individuals for traits that are supposed to improve human welfare. The underlying idea is to improve the genetic basis of future generations and reduce the incidence of genetic diseases and other traits considered undesirable. Some of the practices included in new eugenics are: prenatal testing, pre-implantation diagnosis and embryo selection, selective breeding, and human embryo engineering and gene therapy. ==Ethical status ==