Eugenicists considered people with developmental disability genetically inferior. People with intellectual disability were seen as a threat. The invention of intelligence quotient (IQ) testing helped eugenicists to segregate people with intellectual disability. Thomas Armstrong (2010) explains how IQ testing came about:
In 1905 psychologist Alfred Binet was asked by the Paris public school system to devise a test that would help predict which students would be in need of special education services. He developed the original test, upon which IQ scores would be based, but his belief was that students could improve their performance on the test through further development and learning. It was a German psychologist, William Stern, who actually gave the test a ‘score’ that became the intelligence quotient of an individual. The most significant changes in IQ testing, however, took place when American psychologist Henry Goddard brought Binet’s test and Stern’s score to the United States. In contrast to Alfred Binet, Goddard believed that the IQ test represented a single innate entity that could not be changed through training. (pp. 141-142)
Eugenicists thought that intelligence was genetic, unchangeable, related to social and financial success, and necessary for moral citizenship (Roige, 2014). IQ tests were used to label people as “feeble-minded,” which put them at risk of being institutionalized and sterilized (Roige, 2014). According to Kim Nielson (2012), “Many in power… used Gregor Mendel’s scientific work on plant genetics and the newly developed Binet-Simon intelligence test to argue that criminality, feeble-mindedness, sexual perversions, and immorality, as well as leadership, responsibility, and proper expressions of gender, were hereditary traits” (Nielson, 2012, p. 101). By saying that morality and intelligence were passed down through families, scientists argued for laws that restricted people who fell outside of the norm. IQ scores were treated as evidence for the kind of lives people were allowed to lead.