In this chapter, I discuss sexology’s construction of male inversion, highlighting its multiple, conflicting accounts. My aim is not to exaggerate sexology’s authority or agential role (Chitty154) but rather its considerable contradictions, the primary one of which is to place congenital, acquired, and situational theories of homosexuality alongside each other. Beyond arguing whether inversion was a hereditary condition, the result of childhood experiences, or a vice occasioned by situation, many sexologists proposed that it might be all three—i.e., that there could be different kinds of inversion, characterized by different degrees of pathology. Furthermore, over the course of their careers, sexologists might revise their understandings of inversion: the concept of homosexuality was highly “unstable,” the question of whether same-sex desires were pathological remaining ambiguous. After summarizing Euro-American sexological ideas about homosexuality, I then discuss Italian sexology in more detail, surveying a few representative texts. One is a sexologist’s pathologization of gender inversion; a second is a memoir that portrays inversion as congenital decadence; a third ties German belligerence to inversion by combining congenital and situational theories; a fourth, from the Fascist period, justifies the interment of homosexuals. I then offer a few brief comments on Italy’s progressive sexological movement.