One of the first scientific theories about homosexuality is the "Third sex" theory, developed in late 1900's. According to this theory, the homosexual male had a "female soul in a male body". With the aim of analyzing if and how contemporary theories about homosexuality are related to the idea of the "Third sex", we reviewed 211 papers about homosexuality, published in the Archives of Sexual Behavior from 1971 to 2006. The papers were classified in five categories: Psychological, Biomedical, Sociological/Cultural, HIV and Others. We analyzed two groups of papers included in the "Biomedical" category. The first group encompasses studies that try to establish a relationship between homosexuality in adulthood and "atypical gender behavior" in childhood (for instance, how girls preferred balls instead of dolls and vice-versa). In the second group, the studies searched for anatomic differences between homosexual and heterosexual individuals (such as length of the fingers and of the penis) that would indicate "abnormal levels" of pre-natal hormones, these being responsible for the cerebral lateralization that defines sexual orientation. We concluded that both groups of papers are based on a view of heterosexuality as a distinctive characteristic of gender that links homosexual males to heterosexual females and vice versa. This type of approach has great similarity with the 19th century theory of the "Third sex"; however, vague and metaphysical concepts, such as "soul" and "mind", have been replaced by more concrete and physical concepts such as "brain".
Homosexuality; science; biomedical research
