Sexual Arousal Patterns of Identical Twins with Discordant Sexual Orientations

Tuesday M. Watts, Luke Holmes, Jamie Raines, Sheina Orbell, Gerulf Rieger
2018 Scientific Reports  
Genetically identical twins can differ in their self-reported sexual orientations. However, whether the twins' subjective reports reflect valid differences in their sexual orientations is unknown. Measures of sexual orientation, which are free of the limitations of self-report, include genital arousal and pupil dilation while viewing sexual stimuli depicting men or women. We examined these responses in 6 male twin pairs and 9 female twin pairs who reported discordant sexual orientations. Across
more » ... rientations. Across measures, heterosexual male twins responded more strongly to women than to men. Their homosexual co-twins showed an opposite pattern. Heterosexual female twins responded equally to both sexes, whereas their homosexual co-twins responded somewhat more to women than men. These differences within pairs were similar to differences between unrelated heterosexual and homosexual males and females. Our study provides physiological evidence confirming twins' discordant sexual orientations, thereby supporting the importance of the non-shared environment for the development of sexual orientation and sexual arousal. A recent review of twin studies indicates that although both genetic variations and the non-shared environment contribute to population-level differences in sexual orientation, the contribution of the non-shared environment is much stronger 1 . The non-shared environment is unique to each individual twin in an identical pair. Due to these unique influences, genetically identical twins can have discordant sexual orientations with one being heterosexual and the other homosexual. Past research on identical twins with discordant sexual orientations has largely depended on their self-reports, which can be inaccurate 1 . Homosexual orientations, especially, may not be correctly disclosed. Some people are unaware that they are homosexual 2 . Others misrepresent their homosexual orientations because of stigma 3, 4 . In some pairs of twins, both might identify as heterosexual, but one twin may not accurately disclose his/her homosexuality. In such cases, their true discordance would not be reflected in their identities. Or, both twins could be homosexual, but one twin may not disclose his or her same-sex preferences. In such cases, twins of a pair may be inaccurately identified as discordant. For these reasons, actual discordance could be different from what is implied by twins' self-reports. As Bailey et al. stated in their review 1 : "Research supporting the validity of twin discordance-that twins who report different sexual orientations truly have them-is currently lacking, and would be most desirable for MZ [monozygotic] pairs. " The goal of the present research was to investigate the validity of the reported discordant sexual orientations of these twins. Because self-report is not always accurate, some research has focused on assessing sexual orientation with other measures. Such measures include genital arousal to male and female sexual stimuli, assessed with penile gauges and vaginal plethysmographs. Another of these measures is the level of pupil dilation to sexual stimuli. In general, sex and sexual orientation differences in sexual response are comparable for these measures. Heterosexual men respond substantially more to females than males, and homosexual men respond substantially more to males than females [5] [6] [7] [8] ; thus, on average, genital arousal and pupil dilation patterns verify men's reported sexual orientations. Women of all sexual orientations respond, on average, with some level of arousal to erotic stimuli representing either sex. However, this effect is especially common in heterosexual women, whereas homosexual women respond somewhat more to the same sex than other sex. This pattern can be seen in women's genital responses and pupillary responses. However, this pattern can vary for genital response, depending on stimulus type 9 . Moreover, for unknown reasons it is stronger if assessed with pupil dilation than genital response 7, 8 . Pupil dilation is, on
doi:10.1038/s41598-018-33188-2 fatcat:dwnfy5oqvbcf5mbt5zhy4evmge