Sexual Orientation, Sexual Arousal, and Finger Length Ratios in Women. Luke Holmes, Tuesday M. Watts-Overall, Erlend Slettevold, Dragos C. Gruia, Jamie Raines & Gerulf Rieger. Archives of Sexual Behavior, Jul 23 2021. https://link.springer.com/article/10.1007%2Fs10508-021-02095-5
Abstract: In general, women show physiological sexual arousal to both sexes. However, compared with heterosexual women, homosexual women are more aroused to their preferred sex, a pattern typically found in men. We hypothesized that homosexual women’s male-typical arousal is due to their sex-atypical masculinization during prenatal development. We measured the sexual responses of 199 women (including 67 homosexual women) via their genital arousal and pupil dilation to female and male sexual stimuli. Our main marker of masculinization was the ratio of the index to ring finger, which we expected to be lower (a masculine pattern) in homosexual women due to increased levels of prenatal androgens. We further measured observer- and self-ratings of psychological masculinity–femininity as possible proxies of prenatal androgenization. Homosexual women responded more strongly to female stimuli than male stimuli and therefore had more male-typical sexual responses than heterosexual women. However, they did not have more male-typical digit ratios, even though this difference became stronger if analyses were restricted to white participants. Still, variation in women's digit ratios did not account for the link between their sexual orientation and their male-typical sexual responses. Furthermore, homosexual women reported and displayed more masculinity than heterosexual women, but their masculinity was not associated with their male-typical sexual arousal. Thus, women’s sexual and behavioral traits, and potential anatomical traits, are possibly masculinized at different stages of gestation.
The present data confirmed that homosexual women had more male-typical sexual arousal patterns than heterosexual women, as indicated by both their genital arousal and their pupil dilation. However, there was no evidence that they had more male-typical digit ratios, or that digit ratios mediated the relationship between women’s sexual orientation and their male-typical sexual arousal patterns. Moreover, even though homosexual women were more masculine than heterosexual women in their self-reports or via observer ratings, this pattern, too, did not explain their male-typical arousal patterns.
The finding that homosexual women had stronger responses to their preferred sex than heterosexual women is consistent with previous research both for genital arousal and pupil dilation (Chivers et al., 2004, 2007; Rieger et al., 2015, 2016). However, the finding that 2D:4D was not significantly lower in homosexual women than heterosexual women is puzzling, as it was confirmed previously in a meta-analysis (Grimbos et al., 2010). This may have been due to methodological reasons: Although between-rater reliability was high, and computer-assisted measurement techniques, such as those employed in the current study, have been shown to have the highest reliability compared to other methods of measuring 2D:4D (Allaway et al., 2009), we cannot say with certainty that our measure was valid.
Indeed, there is an ongoing debate about the utility of 2D:4D: Although it is regarded as a valid measure with respect to sex differences and female sexual orientation differences, it is also the case that there is much variability in this measure across individuals, and findings only apply on aggregate and do not apply to single people (Swift-Gallant et al., 2020). Furthermore, the aforementioned meta-analysis suggested a publication bias in reported relationships of sexual orientation with 2D:4D (Grimbos et al., 2010), and the true effect could therefore be smaller than usually published. In the present data, the strongest linear relationship of sexual orientation with 2D:4D was r (or β) = − 0.12 in the right hand. With this effect, post hoc power analyses indicated a minimum sample of 542 women for it to be significant. If our a priori sample size estimate had returned such a large number, we would have considered it an unreasonable goal for a laboratory-based study like ours.
Another possible explanation for the present null finding with respect to 2D:4D is the ethnic makeup of the sample. We did not factor this into planning the present study because the meta-analysis pointed to an ethnicity effect only in men and not in women (Grimbos et al., 2010), although other research has found an influence of ethnicity on 2D:4D in women (Lippa, 2003). Indeed, excluding all non-Caucasian participants from the present sample made the association between 2D:4D and sexual orientation stronger (although still nonsignificant) in both hands (Fig. 4). Thus, future research measuring the relationship between 2D:4D and sexual orientation may wish to either employ a racially homogenous participant sample, or recruit enough participants that per-race comparisons are feasible. Note that even within the white sample, 2D:4D did not appear to explain (mediate) any relationship of women's sexual orientation with their sexual response patterns.
It is impossible to draw any conclusions from the present data about whether the relationship between 2D:4D and sexual orientation mediates the relationship between sexual orientation and sexual responses, simply because 2D:4D in itself did not relate to sexual orientation. With regard to masculinity–femininity, if anything, statistically controlling for any of the three masculinity–femininity variables made the correspondence of women’s sexual orientation with their male-typical sexual arousal stronger. This pattern—a strengthening of the effect of sexual orientation on sexual response when measures of behavioral masculinity are statistically controlled for—has been previously noted (Rieger et al., 2016). In combination with present findings, it appears unlikely that it was previously a chance finding.
If one assumed for a moment that the present findings are accurate, what could be their reasons? For females, it is possible that there exist several “sensitive periods” of masculinization during prenatal development, and that these periods differ for different traits (McCarthy et al., 2018; Xu et al., 2019). At least in non-human primates, exposure to testosterone at different stages of gestation may masculinize sexual behaviors independently from non-sexual behaviors (Goy et al., 1988). Specifically, Goy et al. reported that female rhesus macaques exposed to testosterone during their prenatal development had different behavioral outcomes depending on the timing, with those exposed early in gestation displaying male-typical sexual behaviors (e.g., mounting other females) and those exposed late in gestation displaying male-typical non-sexual behaviors (e.g., rough play). It is possible that behavioral traits and sexual arousal patterns are masculinized at different stages of development in humans also, and thus, are not necessarily interlinked within individuals—for example, those who have male-typical arousal may not have male-typical gender-related behaviors and vice versa.
A final point concerns bisexual women, who were intermediate between heterosexual and homosexual women in their sexual arousal and masculinity–femininity, but were significantly more feminine in their 2D:4D. One hypothesis is that due to intermediate dosages of genetic or prenatal hormonal influences, bisexual individuals, who could be considered to have sexual orientations between heterosexual and homosexual, also fall intermediate with respect to correlates of sexual orientation (Rieger et al., 2020). Thus, regarding bisexual women's 2D:4D, we assumed that they could also be intermediate between heterosexual and homosexual women on this measure. Contrary to this assumption, bisexual women had more feminine 2D:4D than both heterosexual and homosexual women (Table 1). It has been proposed that personality differences between homosexual and heterosexual women may be caused by exposure to androgens during prenatal development, whereas the distinct personality traits of bisexual individuals (e.g., higher sociosexuality compared to heterosexual and homosexual) may be a correlate of their higher levels of postnatal androgens (Lippa, 2020). If the present findings are valid, they would suggest that bisexual women also differ from heterosexual and homosexual women with respect to prenatal androgenization, but this would imply that they have been less masculinized than other groups, and we cannot offer an explanation for why this would be the case.
In sum, the findings of the present research suggest that there is no link between the male-typical sexual responses of homosexual women and putative markers of prenatal androgenization. Other purported markers of androgen exposure may reveal a different pattern than the one reported here. Such markers include the distance between the anus and the genitalia (Barrett et al., 2018) and otoacoustic emissions, which are tiny sounds emitted by the inner ear (McFadden & Pasanen, 1998). Another avenue for future research would involve individuals with conditions affecting the availability of androgens, or their sensitivity to them. To our knowledge, no studies to date investigated the arousal patterns of women with CAH. If androgen exposure does indeed impact sexual responses—and given the apparent impact of excessive androgens on the sexual orientation of women with CAH (Meyer-Bahlburg et al., 2008; Zucker et al., 1996)—women with CAH may show male-typical specificity in their sexual arousal.
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