Interesting Findings from Manzouri & Savic

As I have continued to research the neurohormonal hypothesis, some key observations have been made by the research team of Amirhossein Manzouri and Ivanka Savic.

In 2000, Meredith Chivers and Michael Bailey observed differences between homosexual and nonhomosexual FTMs. While the finding could reflect a true difference between the groups, the study relied on self-reported measures of sexual orientation, childhood gender nonconformity, etc. Self-reported measures are often unreliable, since transsexuals coach each other on misrepresenting themselves in order to conform with gender stereotypes. What Chivers & Bailey may have observed is not a true difference in etiology, but the tendency for FTMs who exaggerate their stereotypical masculinity to also exaggerate a sexual interest in women.

Chivers & Bailey defined “homosexual transsexual” as Kinsey>3. There might be stronger evidence that Homosexual FTMs should be defined as Kinsey>4, not Kinsey>3. In a paper published in 2017, Manzouri et al. studied a sample of 28 female-to-male transsexuals who were “mostly gynephilic”, scoring “between 4 and 6 (4.3 ± 1.9)” on the Kinsey scale. “More precisely, 13 FtM-TR [female-to-male transsexuals] scored Kinsey 6, whereas the remaining 15 scored 3.5–5 and none described self as truly bisexual.” In this sample, they did not find evidence of male-like brain phenotypes. Their observations “do not support the hypothesis that the sexual differentiation of the brain in FtM-TR would be away from the gender they were assigned at birth.” (Manzouri et al., 2017)

In a paper published in 2019, Manzouri & Savic studied a sample of male-to-female and female-to-male transsexuals. “Among the 40 TrM subjects, 24 scored as gynephilic (scores 4–6), 10 as bisexual (score 3), and 6 as androphilic (scores 0–2).”  This time, “homosexual transsexuals” were defined as having a Kinsey score 5–6. In this sample, Manzouri & Savic only found evidence of androgynous brain phenotypes in the Homosexual Transsexuals: “While Cth [cortical thickness] and FA [fractional anisotropy] values showed signs of a less pronounced sexual dimorphism among both TrW [male-to-female transsexuals] and TrM [female-to-male transsexuals], this pattern did not remain when taking into account sexual orientation and comparing each of the transgender groups with “all” cisgender controls.” (Manzouri & Savic, 2019)

Additionally, the finding of sex-atypical brain phenotypes was “more pronounced” in homosexual males (including MTFs) than homosexual females (including FTMs). Unlike MTFs, the boundary between “homosexual” and “nonhomosexual” FTM is less well-defined, in line with females being “the more bisexual sex”.

Whenever I mention these brain studies, I feel the urge to bring up this quote from Ray Blanchard:

"...I would say we could basically shut our ears to MRI findings for the next 20 years, until people start producing studies with thousands of cases instead of dozens of cases. So much of this early work that was done in neuroimaging...a lot of it was just done with sample sizes that are too small. I would think anybody, whichever side of this debate they're on, would be ill-advised to base their arguments on the biological data currently available, because it could change in a flash and their whole argument will be left in tatters..."

This is especially true for FTMs.

It is plausible (but not proven) that Homosexual FTMs display slightly "male-shifted" brain phenotypes, but this is only because they are homosexual. Due to sex differences in homosexuality, it is also possible that the neurohormonal hypothesis holds greater validity for MTFs than it does for FTMs.

References

Chivers, M.L., Bailey, J.M. Sexual Orientation of Female-to-Male Transsexuals: A Comparison of Homosexual and Nonhomosexual Types. Arch Sex Behav 29, 259–278 (2000). https://doi.org/10.1023/A:1001915530479

A. Manzouri, K. Kosidou, I. Savic, Anatomical and Functional Findings in Female-to-Male Transsexuals: Testing a New Hypothesis, Cerebral Cortex, Volume 27, Issue 2, February 2017, Pages 998–1010, https://doi.org/10.1093/cercor/bhv278

A Manzouri, I Savic, Possible Neurobiological Underpinnings of Homosexuality and Gender Dysphoria, Cerebral Cortex, Volume 29, Issue 5, May 2019, Pages 2084–2101, https://doi.org/10.1093/cercor/bhy090

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