This article examines the work of anthropologist Franz Boas who, in the early twentieth century, argued against the existence of the stability of the cephalic index, a measure of head shape, and its relation to the mental and moral capacities of human races. The article claims that Boas successfully shifted the burden of proof to his opponents and set the stage for the scientific rejection of belief in innate racial differences in intelligence. The article urges rhetorical scholars to attend to the notions of burden of proof and presumption in scientific controversies over neurological differences.