Neurohormonal functioning and sexual orientation: a theory of homosexuality-heterosexuality.

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-hormonal, pharmacological, maternal stress, immunological, and social experiential. From this evidence, we propose a theory of how the entire spectrum of human sexual orientation (vs. simply homosexuality) is determined.