Sexual inversion is a term used by sexologists, primarily in the late 19th and early 20th century, to refer to homosexuality. Sexual inversion was believed to be an inborn reversal of gender traits: male inverts were, to a greater or lesser degree, inclined to traditionally female pursuits and dress and vice versa. The sexologist Richard von Krafft-Ebing described female sexual inversion as "the masculine soul, heaving in the female bosom". In its emphasis on gender role reversal, the theory of sexual inversion resembles transgender, which did not yet exist as a separate concept at the time.
Initially confined to medical texts, the concept of sexual inversion was given wide currency by Radclyffe Hall's 1928 lesbian novel The Well of Loneliness, which was written in part to popularize the sexologists' views. Published with a foreword by the sexologist Havelock Ellis, it consistently used the term "invert" to refer to its protagonist, who bore a strong resemblance to one of Krafft-Ebing's case studies.
According to this theory, gay men and lesbians were sexual "inverts", people who appeared physically male or female on the outside, but felt internally that they were of the "opposite" anatomical sex (according to the binary view of gender). Therefore, same-sex desires and attraction were explained as "latent heterosexuality", and bisexual desire was known as psychosexual hermaphroditism – in other words, gay men and lesbians were really just heterosexuals who were "born in the wrong body", and "bisexuals" were what modern-day sexologists would call intersexuals (formerly hermaphrodites) by this theory (the bisexual person's "male" part supposedly has attractions towards females, and the "female" part have attractions towards males).
- Havelock Ellis's definition was "sexual instinct turned by inborn constitutional abnormality toward persons of the same sex". Ellis, 1.
- Doan, 26.
- Taylor, 288–289.
- Prosser, passim.
- Prosser, 133; Taylor, 288–290.
- Eisner, Shiri (2 Jul 2013). Bi: Notes for a Bisexual Revolution. Seal Press. pp. 8–9. ISBN 9781580054751. Retrieved 14 March 2015.
- Doan, Laura (2001). Fashioning Sapphism: The Origins of a Modern English Lesbian Culture. New York: Columbia University Press. ISBN 0-231-11007-3.
- Ellis, Havelock (1927). Studies in the Psychology of Sex Volume II: Sexual Inversion. 3rd Ed. Project Gutenberg.
- Prosser, Jay (2001). "'Some Primitive Thing Conceived in a Turbulent Age of Transition': The Transsexual Emerging from The Well." Doan, Laura; Prosser, Jay (2001). Palatable Poison: Critical Perspectives on The Well of Loneliness. New York: Columbia University Press. pp. 129–144. ISBN 0-231-11875-9.
- Taylor, Melanie A. (1998). "'The Masculine Soul Heaving in the Female Bosom': Theories of inversion and The Well of Loneliness". Journal of Gender Studies. 7 (3): 287–296. doi:10.1080/09589236.1998.9960722.