Commentary on Diamond (2021) "The New Genetic Evidence on Same-Gender Sexuality"

Lisa Diamond is an expert on female sexual malleability. In 2021, she wrote an interesting article in response to the largest-ever genome-wide association study (GWAS) on same-sex sexual behavior. She describes how the study's findings have implications for our conceptualization of sexual orientation and sexual fluidity.

Diamond (2021) describes how human sexuality can be conceptualized. The first is as sexual orientation, a "constitutional" "trait" that is influenced by "endogenous" factors ("born this way"). The second is as sexual fluidity, a "facultative" "state" that is influenced by "exogenous" factors (socioculturally-influenced). Rather than an "either/or" situation, the GWAS suggests that both conceptualizations are partially correct. 

Ganna et al.'s (2019) GWAS found that "sex-hormone regulation may be involved in the development of same-sex sexual behavior", as they reported evidence of genetic links to "biological pathways that involve sex hormone regulation and olfaction". Instead of a single, smooth continuum, the GWAS suggests that there are two distinct traits to nonheterosexuality:

  • Whether one has "ever/never" had same-sex sexual relations
  • The lifetime proportion of same-sex sexual partners to total sexual partners 
As illustrated by the graphic below, a smooth spectrum might exist from moderate bisexuality to exclusive homosexuality. However, the full range of nonheterosexuality does not lie on this spectrum, as mostly-heterosexuals are excluded. 
Spectrum from "one-third same-sex behavior" to "exclusive same-sex behavior"
Diamond (2021)

Diamond (2021) speculates that "openness/risk-taking" is the trait associated with mostly-heterosexuality, which isn't necessarily involved in the moderate bisexuality to exclusive homosexuality spectrum. She also makes the following observation: "Importantly, the fact that different genes contribute differently across different points along the Kinsey scale does not necessarily mean that there are fundamentally different processes at work along these different points. There may be different sets of genes involved, but relatively similar neurological and/or psychological mechanisms, and the nature of these mechanisms remains a critical unanswered question in this area." (Diamond, 2021)

In terms of sex differences, Diamond (2021) notes that "although "proportion of same-gender [same-sex] sexual partners" is heritable in both men and women, there appear to be different genes responsible for this trait in each gender [sex]." She describes different aspects of sexual variability and how they are more applicable to the female sex: "instead of describing women as “more bisexual,” it is actually more informative to describe women as “less exclusively same-gender attracted."

Towards the beginning of the article, Diamond (2021) describes important limitations of the GWAS, which restrict its generalizeability. However, there are some parts where I disagree with her. 

In her introduction, Diamond (2021) includes a lengthy paragraph where she argues that sex research has used restrictive definitions "that center and privilege the experiences of cisgender individuals". She argues that, "Accordingly, the term “same-gender” has been used to denote cisfemale-cisfemale or cismale-cismale pairings, while the term “other-gender” has been used to denote cisfemale-cismale pairings. Yet the terms “same- gender” and “other-gender” fail to adequately describe pairings involving transgender individuals (whose gender identities and/or presentations do not correspond to their birth- assigned sex/gender) and nonbinary individuals (who may experience and express their gender as both male and female, neither male nor female, or fluid)." For this reason, she opts to replace the terms "same-sex" and "opposite-sex" with "same-gender" and "other-gender".


"Rapidly changing and poorly justified acceptable terminology impedes scientific communication in several ways. First, the acceptable terminology is vetted not by sex researchers who understand the underlying scientific issues, but by activists who do not seem to care much about scientific precision. Second, it makes connecting past, present, and future scientific researches much more difficult. Third, it wastes scientists’ time."

As Semenyna et al. (2022) explain: "Efforts to respect individuals’ identities are understandable, but scientists nonetheless need frameworks for objectively understanding characteristics of individuals who feel sexual attractions, as well as the objects of their sexual desire."

Although Diamond (2021)'s intent was noble, the terms "same-gender" and "other-gender" misrepresent the GWAS, whose key phenotype of interest was "same-sex sexual behavior". Sex refers to whether one is male or female, while gender refers to masculine or feminine, as in the subtypes of homosexuality (VanderLaan et al., 2023). No matter how you define the term gender, "same-gender" is not synonymous with "same-sex". 

Consider the following example. There are two college students named "Veronica" and "Betty". "Veronica" is only interested in females because she is exclusively same-sex attracted. She finds "Betty" physically attractive. It turns out, "Betty" is actually "Bob": a female-to-male transsexual who is early in transition and doesn't "pass" for male. If we replace the term same-sex attracted with "same-gender" attracted, this implies that "Bob's" gender is female instead of his sex. To replace same-sex with "same-gender" is a misrepresentation, and inadvertently "misgenders" transsexuals.

In her "hypothesis generation", Diamond (2021) places strong emphasis on sociocultural factors. For example, under the section titled "Revisiting Gender Differences", Diamond (2021) observes sex differences in bisexual expression. In response to the question "What is the "something" responsible for this difference?", she only offers "social pressures" as "possible explanations". Diamond (2021) also claims:

"Heterosexual women occupy cultural environments replete with positive images of “occasional” female-female eroticism, including film and television depictions of heterosexually-identified women experimenting with and enjoying sporadic same-gender sexual contact." 

I found this section a little curious, as a paper she co-authored five prior addressed this exact question: 

"One possibility is cultural—namely, Western cultures produce omnipresent depictions of female beauty, which are often sexualized, and exposure to these images from an early age may sensitize both men and women to experience sexual arousal to the female body. Although this explanation might account for heterosexual women’s genital arousal to female stimuli, it cannot account for the fact that homosexual men, who have experienced just as much exposure to sexualized images of women, do not experience sexual arousal to female stimuli. Similarly, exposure to sexualized images of women fails to account for the fact that homosexual women show more genital arousal to male sexual stimuli than homosexual men show to female sexual stimuli." (Bailey et al., 2016)

Another quote from the same paper: 

"Another possibility is that the difference is based in fundamental evolved differences between female and male sexuality...although currently there is no direct evidence speaking to this question. Intriguingly, Goy and Goldfoot (1975) showed over 30 years ago that in many different mammalian species, bisexuality is an intrinsically dimorphic trait that develops (through prenatal hormonal pathways) in either the male or the female of a species, but never both. This suggests the provocative possibility that in humans, women are “the more bisexual sex,” whereas males are more likely to be exclusively heterosexual or homosexual." (Bailey et al., 2016)

Surely Diamond is aware of possible biological explanations for these sex differences, so why did she neglect to mention these? It is the consensus that sociocultural environment has some influence on the expression of nonheterosexuality, but it is a mistake to downplay its biological underpinnings. Doing so can "mislead one into ignoring important variation within groups." The dominant voice of the transgender and nonbinary community is captured by queer theory, and therefore wants to downplay any association between gender (masculine/feminine) and sexual orientation. So do some members of the gay (and lesbian) community, who want to deny an "association between homosexuality and gender nonconformity". This silences the voice of the feminine-male and masculine-female subgroups of homosexuality, including those who are transsexual (VanderLaan et al., 2023).

Another "nit-pick" critique I have is where Diamond (2021) describes another sex difference discovered in sex research, that being the "nonspecific" patterns of genital arousal observed in heterosexual women. She writes:

"...the definition of heterosexual women as "unique" depends on the classification of sexual stimuli as preferred or non-preferred, according to the participants' self-described patterns of attraction. Within this framework, heterosexual women are unique because they show stronger genital arousal to their non-preferred gender (i.e., women) than do all other groups. But what if we re-classified the sexual stimuli as simply "men" versus "women?" Using this re-classification, exclusively gay men are suddenly the outlier group...Whereas heterosexual women, lesbian women, bisexual women, heterosexual men, and bisexual men all show some degree of genital arousal to sexual stimuli depicting women, gay men do not."

While Diamond (2021) is not entirely wrong, she is downplaying an important sex difference: the lack of concordance that is only seen in the female sex.

A Sex Difference in the Specificity of Arousal
Chivers et al. (2004)

I mention this because it seems especially relevant to the discussion of sexual variability in females. Even if the sexual stimuli are reclassified, the lack of concordance is still most striking in heterosexual women. 

Still, Diamond (2021) is right to describe some of the possible sociocultural influences on sexual orientation. Previously, I reviewed Menelaos Apostolou's (2022) "The Evolution of Female Same-Sex Attraction", which makes some compelling points but leaves some lingering questions. Apostolou (2022) notes that the ancestral context of humanity imposed heterosexual relations on the female sex, weakening selection pressure against female same-sex attraction. However, most of the factors he describes also apply to the male sex, with the exception of 'male-male competition'. If men were also forced into arranged marriages and socially pressured into heterosexual unions, then why isn't selection pressure also weak for male same-sex attraction? Diamond (2021) is right to quote Bailey et al. (2000), in describing how "Something about male sexual orientation development makes extreme departures from heterosexual development especially likely compared with female sexual orientation development." Diamond (2021) also notes that most of the participants of the GWAS were over age 40. In the realm of "hypothesis generation", it is possible that some of the sex differences Ganna et al. (2019) found are due to sociocultural factors that this generation experienced.

Overall, Diamond (2021) provides key insight into the largest-ever GWAS on same-sex sexual behavior, as it pertains to sexual orientation and sexual fluidity. 

References

Apostolou, M. (2022). The Evolution of Female Same-Sex Attraction. In T. K. Shackelford (Ed.), The Cambridge Handbook of Evolutionary Perspectives on Sexual Psychology (pp. 28–51). chapter, Cambridge: Cambridge University Press.

Bailey, J.M. How to Ruin Sex Research. Arch Sex Behav 48, 1007–1011 (2019). https://doi.org/10.1007/s10508-019-1420-y

Bailey, J. M., Dunne, M. P., & Martin, N. G. (2000). Genetic and environmental influences on sexual orientation and its correlates in an Australian twin sample. Journal of personality and social psychology, 78(3), 524. http://doi.org/10.1037/0022-3514.78.3.524

Bailey, J. M., Vasey, P. L., Diamond, L. M., Breedlove, S. M., Vilain, E., & Epprecht, M. (2016). Sexual Orientation, Controversy, and Science. Psychological Science in the Public Interest, 17(2), 45-101. https://doi.org/10.1177/1529100616637616 (Original work published 2016)

Chivers, M. L., Rieger, G., Latty, E., & Bailey, J. M. (2004). A Sex Difference in the Specificity of Sexual Arousal. Psychological Science, 15(11), 736-744. https://doi.org/10.1111/j.0956-7976.2004.00750.x (Original work published 2004)

Diamond, L. M. (2021). The New Genetic Evidence on Same-Gender Sexuality: Implications for Sexual Fluidity and Multiple Forms of Sexual Diversity. The Journal of Sex Research, 58(7), 818–837. https://doi.org/10.1080/00224499.2021.1879721

Ganna, A., Verweij, K. J., Nivard, M. G., Maier, R., Wedow, R., Busch, A. S., ... & Zietsch, B. P. (2019). Large-scale GWAS reveals insights into the genetic architecture of same-sex sexual behavior. Science, 365(6456), eaat7693. https://doi.org/10.1126/science.aat7693

Semenyna, S. W., Vasey, P. L., & Honey, P. L. (2022). The Female Sexual Orientation Spectrum in Evolutionary Perspective. In T. K. Shackelford (Ed.), The Cambridge Handbook of Evolutionary Perspectives on Sexual Psychology (pp. 3–27). chapter, Cambridge: Cambridge University Press.

VanderLaan, D.P., Skorska, M.N., Peragine, D.E. et al. Carving the Biodevelopment of Same-Sex Sexual Orientation at Its Joints. Arch Sex Behav 52, 2939–2962 (2023). https://doi.org/10.1007/s10508-022-02360-1

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