Opposition to eugenics began even as the movement was being organized into a scientific discipline. By 1910, the equilibrium model developed by Godfrey N. Hardy and Wilhelm Weinberg disproved the claim that degenerate families were increasing the societal load of dysgenic genes. The Hardy-Weinberg equation also showed that sterilization of affected individuals would never appreciably reduce the percentage of mental defectives in society. At the same time, George Shull, at the Carnegie Station for Experimental Evolution, showed that hybrid corn plants are more vigorous than pure-bred ones. This refuted the notion that racial purity offers any biological advantage or that race mixing destroys "good" racial types.
Work by a number of scientists countered the simplistic assertion that complex behavioral traits are determined by single genes. Herman Muller's survey of mutations in Drosophila and other organisms, from 1914-1923, showed variation in the "gene to character" relation: A single gene might affect several characters (traits) at one time; conversely, mutations in several different genes can affect the same trait in similar ways. The environmental contribution to behavior was pointed up by twin studies — conducted in the 1930s by Horatio Newman, Frank Freeman, and Karl Holzinger — showing that identical twins raised apart after birth had different IQs. Lionel Penrose found that most cases at a state-run institution in Colchester, England resulted from a combination of genetic, environmental, and pathological causes.
A review panel, convened by the Carnegie Institution in 1935, concluded that the vast majority of work sponsored by the Eugenics Record Office was without scientific merit and recommended a halt to its propagandising for eugenic social programs, such as sterilization and immigration restriction. In retrospect, it is easy to recount these lines of evidence that refuted key eugenic tenets. It is much harder to understand why eugenic social programs continued unabated in the United States — until they were directly discredited by association with the Nazi eugenic program, whose "final solution" led to the Holocaust.
Many sophisticated geneticists — including some who provided refuting evidence — supported some form of eugenic program at one point or another. Although he denounced the negative eugenics of the American movement, Herman Muller remained committed to a personal brand of positive eugenics based on individual worth. Despite the fact that the Hardy-Weinberg showed that sterilization would have little effect on incidence of feeblemindedness, most geneticists still believed that affected individuals should not be allowed to reproduce. The Catholic Church opposed eugenics from the outset, and helped to ward off eugenic social legislation in much of Europe. However, the Catholic viewpoint held little sway in Protestant America. With Buck vs. Bell providing the full approval of the U.S. Supreme Court, state legislatures continued to enact new eugenic sterilization laws up until WWII.