He completed the D.Phil in 1938 under the supervision of Vincent Harlow. His doctoral thesis was titled "The Economic Aspects of the Abolition of the Slave Trade and West Indian Slavery", and was published as "Capitalism and Slavery" in 1944. It was both a direct attack on the idea that moral and humanitarian motives were the key facts in the victory of British abolitionism, and a covert critique of the idea common in the 1930s, emanating in particular from the pen of Oxford Professor Reginald Coupland, that British imperialism was essentially propelled by humanitarian and benevolent impulses. Williams's argument owed much to the influence of C. L. R. James, whose "The Black Jacobins", also completed in 1938, also offered an economic and geostrategic explanation for the rise of British abolitionism.