Homosexuality and Heterosexuality in Identical Twins

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Abstract
Twin studies were used to demonstrate the extent of the genetic contribution to human behavioral traits and disorders. As they have in other psychiatric conditions, such studies in the case of homosexuality have indicated a high degree of concordance in monozygotic male twins. However, a genetic predispositon always expresses itself by way of a long and complex interactional process with the physical and social environment, including the family. In those rare cases, therefore, where there is an overt dissimilarity in sexual object choice despite genetic identity, the developmental factors which led to this divergence should be particularly outstanding and amenable to study. After reviewing theories of homosexuality on the one hand and special psychodynamic problems of twins on the other, this paper describes 2 pairs of twins, in each of which the members differ in preferred sexual role. In addition to cytological, biochemical, and psychological tests, they were investigated by the free-associational method and by family and social studies looking for the patterns of family relationship which influenced their psychosexual development. Data were sought regarding their psychopathology, their self-concept and body image as derived from the family transactions, their attitudes toward being a twin and to separation from the twin, their fantasy and dream life, important experiences, and significant relations with others. Similar life factors leading to a hostile identification with the parent of the opposite sex were found in the homosexual member of each pair.