Eugenics
Today we know that interpreting statistics of this type is a complex business, and that many of the studies published early in the 20th century have serious flaws. Nevertheless, what stopped the eugenics movement was not better science. It was the realization, after World War II, of the effects of Nazi policies on race in Germany and other countries occupied during the war. Such war crimes were not, of course, advocated by any eugenicist. All the same, there was a common theme. This theme was the growing interest in the rights of individuals as against the rights of the state.