Havelock Ellis's main work, the "Studies", constituted the most weighty contribution to sexological research in the late nineteenth and early twentieth centuries. One of the volumes was almost entirely devoted to the topic of sexual inversion, or homosexuality. And he felt, for instance, that there were both congenital and psychological factors influencing the predisposition to homosexuality. It has been suggested that Havelock Ellis paved the way for the significant sexological developments that were to begin with Freud.