Hofstadter earned his PhD in 1942. In 1944, he published his dissertation "Social Darwinism in American Thought, 1860–1915". It was a commercially successful (200,000 copies) critique of late-19th-century American capitalism and its ruthless "dog-eat-dog" economic competition and Social Darwinian self-justification. Conservative critics, such as Irwin G. Wylie and Robert C. Bannister, disagreed with his interpretation. The sharpest criticism of the book focused on Hofstadter's weakness as a research scholar: he did little or no research into manuscripts, newspapers, archival, or unpublished sources, relying instead primarily on secondary sources augmented by his lively style and wide-ranging interdisciplinary readings, thus producing well-written arguments based on scattered evidence he found by reading other historians.