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Following a historical sketch of attempts to explain homosexuality, we review evidence indicating that the process of determining human sexual orientation is fundamentally the same in all mammals. In this process, four phenotypic dimensions of sexuality develop from two more or less distinct sex genotypes. Studies are reviewed that indicate how phenotypic deviations from these two genotypes (called sexual inversions) can occur. The causes of sexual inversions are categorized as genetic-hor-monal, pharmacological, maternal stress, immunological, and social experiential. From this evi-dence, we propose a theory of how the entire spectrum of human sexual orientation (vs. simply homosexuality) is determined. A consistent preference for sexual relations with ones own sex (homosexuality), the opposite sex (heterosexuality), or vary-ing degrees of ambivalence about the partners sex (bisexuality) may be called sexual orientation. Homosexuality should not be confused with occasional homosexual experiences. Homosex-ual experiences are fairly common, especially early in adoles-
Ellis et al. (Thu,) studied this question.