The invisibility and erasure of bisexuality arguably have their roots in the work of the early sexologists. Nonetheless, these early sexologists’ theories have created the foundations of bisexual invisibility. Although during second-wave sexology there began to be the potential for bisexuality to be understood in different ways, it nonetheless remained largely invisible within subsequent sex research. Psychologists have continued to find it problematic to locate bisexuality (and pansexuality, asexuality, and plurisexualities) within this model because it does not allow for understandings beyond binaries. Before the work of the early sexologists the concepts which were understood were masculinity and femininity within the individual. Therefore, within early sexology, a bisexual person was understood to be someone with the characteristics of both masculinity and femininity.