Arthur R. Jensen (1923–2012) defended the idea that racial differences in intelligence were biologically based. He based his ideas on what he claimed were sound population genetics and evolutionary biology. Viewing his work through the lenses of those disciplines reveals that his arguments for biological racial differences did not meet the minimum evidentiary requirements needed to show that socially defined races were genetic populations. His evidence was from 19th-century race science and the race science of the Nazi regime. His reliance on such evidence supported Jensen’s fears that the country was in danger of collapse because of dysgenic breeding by those of low intelligence. Jensen’s well-known associations with scientific racists were not incidental to his scientific work, but central because he cited their work throughout his career.