Darwin's Use of Intellectual Disability in The Descent of Man

Abstract

When Charles Darwin turned his attention to writing about human descent in 1871 he attempted to narrow the fossil gap between human beings and higher primates by presenting persons with intellectual disabilities — "idiots" in the language of the day — as evidence in support of the theory of evolution. This paper explores the four ways that Darwin used persons with intellectual disabilities in The Descent of Man: 1) as intermediate rung on the evolutionary ladder connecting humans and primates; 2) as exemplars of the inevitable waste and loss produced by natural selection acting upon variability; 3) as the floor of a scale representing the "lowest", most unfit variety of any species when individuals were rank ordered by intelligence; and 4) as atavistic reversions to extinct forms whose study would reveal the characteristics of earlier stages of human evolution. Darwin's strategic use of intellectual disability is brought to bear on the controversy regarding the mental state of Darwin's last child.