Hypothesized Subgroups of Sexual Orientation
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| Hypothesized biodevelopmental pathways for sexual orientation subgroups. VanderLaan et al. (2023a) |
In "Carving the Biodevelopment of Same‑Sex Sexual Orientation at Its Joints", VanderLaan et al. (2023a) describe how some "studies indicate that same-sex sexually oriented people can be divided into subgroups who likely experienced different biological influences." To be precise, it might be that sexual orientation has different subgroups associated with gender expression (masculine or feminine). In males, the subgroups might also be associated with anal sex role orientation ("top" or "bottom").
Three different theoretical possibilities are depicted in the figure shown above:
- Theory A: Nonheterosexuals are "shifted" in the direction of the opposite sex, independent of gender expression
- Theory B: Nonheterosexuals are "shifted" in the direction of the opposite sex, but the "dose" or degree of this "shift" is a function of gender expression
- Theory C: Nonheterosexuals are "shifted" in the direction of the opposite sex, "but these shifts are associated with different biodevelopmental mechanisms that correspond with differences in gender expression" (VanderLaan et al., 2023a)
VanderLaan et al. (2023a) dedicate most of their attention to the male sexual orientation subgroups, as can be found under the subsection "Evidence of Biodevelopmental Subgroups". However, they also make some mention of the female sexual orientation subgroups, stating the following:
"Findings pertaining to female sexual orientation have also provided inconsistent results. For example, some research has reported that, compared with more feminine (femme) lesbians, more masculine (butch) lesbians have more male typical 2D:4D (Brown et al., 2002; cf. Holmes et al., 2022), have higher circulating levels of testosterone (Singh et al., 1999), have higher scores on measures of child and adult gender nonconformity (Singh et al., 1999; Zheng & Zheng, 2016), are more likely to be tops (i.e., more likely to conduct and orchestrate the sexual episode rather than respond to another’s sexual initiatives; Singh et al., 1999), have higher (more masculine) waist-to-hip ratios (Pearcey et al., 1996), and perform better on tests of mental rotation (Zheng et al., 2018). In contrast, a meta-analysis of transgender men (i.e., females who identify as men) found no difference in 2D:4D from a comparison group of cisgender women (Siegmann et al., 2020). In addition, Thai research found that groups of female gynephiles—including lesbian and bisexual women, dees, and toms—tended to be similar to heterosexual women with regard to 2D:4D and height/long-bone growth (Skorska et al., 2021a). The same pattern was found for mental rotation test performance (Thurston et al., 2021). Yet, there were some differences found between groups of Thai female gynephiles. Toms and lesbian women showed evidence of greater long bone growth than dees (Skorska et al., 2021a), and lesbian and bisexual women outperformed dees on mental rotation (Thurston et al., 2021). These patterns might reflect gender expression differences given that dees tend to be more gender-conforming than these other groups of Thai female gynephiles (VanderLaan & Coome, 2018). Thus, although some studies relevant to hormonal mechanisms support the possibility of dosage effects or multiple, distinct same-sex sexual orientation biodevelopmental pathways delineated by gender expression, other studies do not. Given the existing literature is inconsistent regarding which pattern is more supported for both male and female sexual orientation, further research on this topic is required in both sexes." (VanderLaan et al., 2023a)
Although VanderLaan et al. (2023a) claim the existing literature is "inconsistent", a close examination of reveals a more consistent pattern for the subgroup of masculine gynephilic females. Evidence of masculinized biodevelopment is greater in butches and toms, compared to femmes and dees (Skorska et al., 2021). The only inconsistency described here is transgender men, as Siegmann et al. (2020) did not find evidence of masculinized 2D:4D. However, this could be due to the unreliability of 2D:4D measurements.
VanderLaan et al. (2023a) themselves note:
"Digit ratio has been a particularly contentious biomarker given the debate regarding mechanisms that influence this trait, idiosyncratic findings across the left- and right-hand 2D:4D, and the possibility that sex differences in the 2D:4D are simply a consequence of allometry (i.e., reflect sex differences in physical size). For further discussion of these issues and data analyses indicating left and right 2D:4D are most appropriately analyzed separately from one another as well as from measures of physical size (e.g., hand length, height), we refer readers to Skorska et al. (2021a)."
The decision to only include evidence from this Siegmann et al.'s (2020) digit ratio study is a strange one. A more thorough review of the evidence suggests that gynephilic Female-to-Male transsexuals are masculinized (Guillamon et al, 2016) (Roselli, 2018) (Hines, 2020) (Frigerio et al., 2021). When this is taken into account, a less "inconsistent" pattern of masculinization is found in butches, toms, and gynephilic FTMs, supporting the likelihood that female gynephilia has a masculine subgroup.
VanderLaan et al. (2023a) state the following of female gynephilia:
"In relation to this point, given the relatively less consistent evidence to suggest that female gynephilia is underpinned by similar biodevelopmental mechanisms across cultures, researchers should also bear in mind that the expression of female, compared with male, same-sex sexuality might hinge to a greater extent on sociocultural factors (Skorska et al., 2021a)."
The comment regarding "sociocultural factors" does not apply to the masculine subgroup, as Skorska et al. (2021) found evidence of masculinization in toms: "Groups of individuals who tend to be more masculine (i.e., toms, lesbians) showed more male-typical patterns on weight and leg length than some groups of individuals who tend to be less masculine (i.e., heterosexual women, dees)." In fact, said comment does not seem to apply to gynephilic women in general, as masculinized traits were also found in lesbians. Skorska et al.'s (2021) findings only suggest that "sociocultural factors" could explain differences between androphilic (heterosexual) women and dees, who can be compared to mostly-androphilic women.
Severi Luoto has been urging researchers to differentiate between the phenotypes of female sexual orientation, especially masculine (butch) and feminine (femme) subgroups:
"Therefore, studies on female nonheterosexuality should always seek to account for any butch/femme differences so that the results are not confounded by conflating very different types of nonheterosexual subjects...This is an extremely important point for being able to analyze the potential heterogeneity in biodevelopmental pathways leading to variation in the female sexual orientation spectrum." (Luoto, 2022)
A similar sentiment has been expressed by Francine Burke and her colleagues:
"There is evidence based on digit ratios (i.e., more male-typical digit ratios among “butch” identifying com pared to “femme” identifying lesbians, reviewed in Swift Gallant et al., 2020), that there may also be subgroups of same-sex oriented women who differ in the biodevelopmental pathways that led to their sexual orientation." (Burke et al., 2023)
In their Reply to Peer Commentaries, VanderLaan et al. (2023b) remark "there appears to be consensus that the possible existence of biodevelopmental subgroups of same-sex sexually oriented people is an impor tant avenue for further study." The Target Article, Peer Commentaries, and Reply to Commentaries are all valuable resources for those interested in the study of sexual orientation.
References
Roselli, C. E. (2018). Neurobiology of gender identity and sexual orientation. Journal of neuroendocrinology, 30(7), e12562. https://doi.org/10.1111/jne.12562
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