British and American debates on the relationship between slavery and economic growth have had little interaction with each other. This article attempts intellectual arbitrage by joining these two literatures. The linkage turns on the neglected part two of the ‘Williams thesis’: that slavery and the slave trade, once vital for the expansion of British industry and commerce, were no longer needed by the nineteenth century. In contrast to recent assertions of the centrality of slavery for US economic development, the article argues that part two of the Williams thesis applies with equal force to nineteenth‐century America. Unlike sugar, cotton required no large investments of fixed capital and could be cultivated efficiently at any scale, in locations that would have been settled by free farmers in the absence of slavery. Cheap cotton was undoubtedly important for the growth of textiles, but cheap cotton did not require slavery. The best evidence for this claim is that after two decades of war, abolition, and Reconstruction, cotton prices returned to their prewar levels. In both countries, the rise of anti‐slavery sentiment was not driven by the prospect of direct economic benefits, but major economic interest groups acquiesced in abolition because they no longer saw slavery as indispensable.