The Discordant Development of Sexual Orientation in Identical Twins

The present thesis investigated how genetically identical twins with discordant sexual orientations differed in correlates of their sexual orientation in order to understand to what degree non-genetic factors affect the formation of sexual orientation. Because identical twins share 100% of their genes, factors other than genetics may contribute to any differences within these pairs. Chapter 1 gives an overview of the literature. Chapter 2 focuses on childhood gender nonconformity (femininity in males, masculinity in females), which predicts a non-straight (gay, lesbian, or bisexual) sexual orientation in adulthood. In order to avoid the limitations of self-report measures, gender nonconformity of these twins was assessed via observer ratings of their photographs from childhood and adulthood. In addition, although genetically identical twins can differ in their self-reported sexual orientations, it is unclear to what degree these self-assessments reflect observable differences in sexual arousal such as genital response or pupil dilation patterns while viewing sexual stimuli. Chapter 3 focuses on these responses in identical twin pairs with discordant sexual orientations. Finally, differing intrauterine environments during the early development could result in a discordant development of sexual orientation in identical twins. This includes varied prenatal hormonal exposures between twins. Chapter 4 highlights a putative biomarker of early hormonal exposure: the ratio of index to ring finger length (2D:4D), within these twin pairs In combination, findings suggested that these twins differed in many (but not in all) correlates of their sexual orientation, suggesting non-genetic influences. However, there were also subtle similarities within pairs that pointed to potential familial (e.g., genetic) influences.