The first law providing for the permanent, involuntary institutionalization of “feeble‐minded” individuals was passed in Illinois in 1915. This bill represented the first eugenic commitment law in the United States. Focusing on the consequences of this 1915 commitment law within the context of intelligence testing, eugenics, and the progressive movement, this paper will argue that the then newly devised Binet–Simon intelligence test facilitated the definition and classification of feeble‐mindedness that validated feeble‐mindedness theory, enabled the state to legitimize the eugenic diagnosis and institutionalization of feeble‐minded individuals, and especially empowered psychologists to carve out a niche for themselves in the courtroom as “experts” when testifying as to the feeble‐mindedness of individuals.