The empirical advancements of scientific lexicography, on whose principles the Oxford English Dictionary was founded, paralleled the developments made in a range of other sciences in the nineteenth century. Rejecting the overt linguistic prescriptivism of many earlier lexicographers, the OED’s editors aimed to approach language as a natural system akin to any other; like their fellow scientists, they were concerned only with objective fact. Yet the representation of any human behaviour will necessarily be selective and subjective, and the conflicting evidence of real linguistic usage would complicate the lexicographers’ ideals of impassive collection and analysis. The better to cast this problem into relief, this paper juxtaposes scientific lexicography with another, more controversial nineteenth-century science: sexology. Sexologists’ pathologisation of ‘deviant’ sexual desires gave rise to an extensive new taxonomy, which the OED began documenting in the early decades of the twentieth century. Drawing on unpublished draft material from the dictionary’s archives, this paper examines the scientific ideologies of lexicography and sexology as they interacted in the OED, exploring what they reveal about the tension between scholarly principles and social practice.