Both higher and lower than average exposure to male sex hormones increases same sex attraction in male mice and in men

Carving the Biodevelopment of Same-Sex Sexual Orientation at Its Joints. Doug P. VanderLaan, Malvina N. Skorska, Diana E. Peragine & Lindsay A. Coome. Archives of Sexual Behavior, Aug 12 2022. https://link.springer.com/article/10.1007/s10508-022-02360-1

Abstract: Sexual orientation is a core aspect of human experience and understanding its development is fundamental to psychology as a scientific discipline. Biological perspectives have played an important role in uncovering the processes that contribute to sexual orientation development. Research in this field has relied on a variety of populations, including community, clinical, and cross-cultural samples, and has commonly focused on female gynephilia (i.e., female sexual attraction to adult females) and male androphilia (i.e., male sexual attraction to adult males). Genetic, hormonal, and immunological processes all appear to influence sexual orientation. Consistent with biological perspectives, there are sexual orientation differences in brain development and evidence indicates that similar biological influences apply across cultures. An outstanding question in the field is whether the hypothesized biological influences are all part of the same process or represent different developmental pathways leading to same-sex sexual orientation. Some studies indicate that same-sex sexually oriented people can be divided into subgroups who likely experienced different biological influences. Consideration of gender expression in addition to sexual orientation might help delineate such subgroups. Thus, future research on the possible existence of such subgroups could prove to be valuable for uncovering the biological development of sexual orientation. Recommendations for such future research are discussed.

Notes

We recognize that “gynephilia” and “androphilia” are terms that denote sexual attraction toward adults, and as such these terms do not apply in the case of those who are sexually oriented toward minors (i.e., children and/or adolescents). In studies of those sexually oriented toward minors, the terms “homosexual” and “heterosexual” have been used to denote attraction to the same- and opposite-sex, respectively (e.g., Blanchard et al., 2020). We, however, did not choose to use these terms because here we review to a greater extent the considerable cross-cultural literature on transgender, nonbinary, or third gender populations for which gynephilia and androphilia have been the terms typically employed.

By using same-sex sexual behavior as a proxy for sexual orientation, Ganna et al. (2019) were able to maximize inclusion of genetic data from 477,522 individuals. Importantly, the authors also reported supplemental analyses on genetic associations when using sexual attraction, fantasy, and identity measures available for subsets of participants, and the results generally aligned with those found when using the proxy behavioral measure in the full sample.

Digit ratio has been a particularly contentious biomarker given debate regarding the mechanisms that influence this trait, idiosyncratic findings across left- and right-hand 2D:4D, and the possibility that sex differences in 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).

Gender expression has also often been regarded as a psychological marker of pre-/perinatal hormone exposure (Hines, 2020), but it has also been linked to the genetic and immunological mechanisms discussed in this section (e.g., Alanko et al., 2010; Blanchard, 2018; Coome et al., 2018). We refer readers to the section entitled “One Biodevelopmental Pathway or Many?” for in-depth discussion regarding the importance of gender expression to uncovering the bases of sexual orientation biodevelopment.

A large Dutch national probability sample reported a fraternal birth order effect among women belonging to female-female civil unions, suggesting birth order may be related to female sexual orientation as well (Ablaza et al., 2022); however, caution is warranted in interpreting this finding given prior inconsistencies, and primarily null findings, in birth order studies of female sexual orientation (Blanchard, 2022; Bogaert & Skorska, 2011; Semenyna et al., 2022).

Neuroscience research on transgender populations has often not included information regarding participants’ sexual orientations or reported that participants’ sexual orientations were heterogeneous (for a recent review, see Frigerio et al., 2021). Here, in addition to comprehensive review articles, we only cite examples of individual neuroscience studies that either reported on transgender samples of individuals described as being sexually oriented toward the same sex assigned at birth or that explicitly examined transgender participants’ brain features in relation to varying sexual orientation.

In the study by Rahman et al. (2020), prevalence rates varied depending on how sexual orientation was defined. For example, if sexual orientation was defined by heterosexual, bisexual, and homosexual identity, then prevalence rates were estimated at 90.7, 7.2, and 2.1% for women and of 90.0, 5.1, and 4.9% for men. In contrast, if defined by sexual attractions that were predominantly not toward the same sex, moderately toward the same sex, or predominantly toward the same sex, then prevalence rates were estimated at 66.2, 27.3, and 6.5% for women and 82.6, 10.2, and 7.2% for men.

To further verify the meaningfulness of the subgroups derived from their latent profile analysis, Swift-Gallant et al. (2019a) compared the subgroups on several psychological variables, including gender expression. Details regarding subgroup differences in gender expression are described in the subsection of the present article entitled “Research on Gender Expression and Sexual Orientation Biodevelopment.”.