'All the progressive forms of life are built up on the attraction of sex': Development and the social function of the sexual instinct in late 19th- and early 20th-century Western European sexology.

This article explores the relationship between sexual science and evolutionary models of human development and progress. It examines the ways in which late 19th- and early 20th-century Western European sexual scientists constructed the sexual instinct as an evolutionary force that not only served a reproductive purpose, but was also pivotal to the social, moral, and cultural development of human societies. Sexual scientists challenged the idea that non-reproductive sexualities were necessarily perverse, pathological, or degenerative by linking sexual desire to the evolution of sociality, often focusing on forms of relationality and care that exceeded biological kinship. As a result, non-reproductive sexual expressions, including homosexual and non-reproductive heterosexual behaviours, were interpreted as manifestations of a sexual instinct operating in the service of human development. These claims were reliant on cross-cultural and historical comparisons of sexual values, behaviours, and customs that rehearsed and reinforced imperial narratives of development premised on racialized, gendered, and classed hierarchies. Sexual scientists mapped diverse sexual behaviours in terms of their perceived evolutionary benefits, contributing to colonial narratives that distinguished between different cultures according to imagined trajectories of development. These contestations around the sexual instinct and its developmental functions played a vital role in allowing sexual science to authorize itself as a field of knowledge that promised to provide expertise required to manage sexual life and secure the global development of human civilization.