Is Gender Identity Actually In The Brain?

Is Gender Identity Actually In The Brain?

Abstract

A common claim for the etiology of gender dysphoria is that the sense of being a man or women is determined by brain structure and function, and that people with gender dysphoria have an opposite sexed brain, caused by abnormalities in prenatal hormone exposure. There is almost no evidence to support this idea, with commonly referenced brain imaging findings being in specific regions of the brain (rather than the brain as a whole), involving participants on puberty blockers or hormonal treatment, and failing to be replicated. Additional evidence in people with masculinized brains raised female is weak, and could be explained by other reasons than gender identity being biological. 

Brain Imaging

One common claim in attempting to explain transgenderism is that brain imaging studies find that trans women have "female brains" and trans men have "male brains", and that gender identity is biological. The brain is most definitely sexually dimorphic, and attempts at "debunking" sexual dimorphism of the brain are logically flawed, politically motivated, and tend to misrepresent evidence (Del Giudice 2021a, 2021b). But does gender identity exist biologically? To answer this, I reviewed brain imaging studies on transgender individuals vs controls [Table 1]. Not any of these studies found that the brain of transgender individuals is typical of the opposite sex. Of the 16 studies, only 6 found any pattern of brain volume or activity trending toward the opposite sex, however these findings were in only in 1-2 areas in each study, all failed to be replicated, and in 4 of these studies, participants were on puberty blockers, hormone treatment, or were homosexual (attracted to own biological sex). The remaining studies generally found differences in specific brain regions in both trans men & women compared to cis men & women.

[Table 1]

Study	Finding	Comments
Manzouri & Savic (2019)	Reduced functional connectivity in both trans men & women compared to cis men & women in PACC & IFOF, reduced cortical thickness	Excluded persons with autism or autistic traits
Feusner et al. (2018)	Reduced default mode network connectivity in trans men compared to cis heterosexual men & women	Excluded persons with autism or autistic traits
Nota et al. (2017)	Patterns of functional connectivity in RSMA & RPCG in transwomen similar to cis women, and RSMA in trans women smilar to cis men	Participants were on puberty blockers
Burke et al. (2017)	Functional connectivity in IFOF in trans persons similar to opposite sex	Excluded persons with autism or autistic traits
Burke et al. (2016)	Reduced right frontal activation in trans men during mental rotation compared to cis women	Participants were on puberty blockers, all trans participants had same sex attraction
Nota et al. (2017)	Functional connectivity in RCN in trans persons trend to opposite sex; nonsignificant	Participants were on puberty blockers
Hahn et al. (2015)	Reduced hemispheric connectivity ratios of subcortical and limbic areas in both transmen & women
Case et al. (2016)	Reduced activation of SSC and SG, increased temporal pole in trans men, for chest sensation	50% were taking testosterone, did not exclude persons with any disorder
Baldinger-Melich et al. (2020)	Machine learning can correctly determine biological sex using brain imaging data in 82.6% of cisgendered persons, but only 67.5% of transgender persons
Krans et al. (2014)	White matter microstructure shows a similar biological sex difference in trans persons that is shown in cis persons, however the difference is smaller
Hoekzema et al. (2015)	Gray matter distribution & volume in transgender individuals not different from that of natal sex; small differences found for FtMs from cis girls in reduced right cerebellum and increased medial frontal cortex; reduced bilateral cerebellum and hypothalamus in MtFs than cis males	69% of FtM and 70% of MtF were either on puberty blockers or sex hormone replacement
Luders et al. (2009)	Gray matter distribution in MtF not different from men, except for volume in the right putamen
Flint et al. (2020)	Structural differences in putamen & insula in trans women from cisgender men and women
Spizziri et al. (2018)	Reduced gray matter volume in the lower bilateral insula in treatment naive trans women compared to cis women
Clemens et al. (2021)	Increased putamen volume in transgender people compared to cis	Some participants on hormone replacement
Fukao et al. (2021)	Increased volume in RPCG, left occipital pole, and reduced volume in the LMTG in trans men compared to cis women	Trans women or cis men not included, not clear if participants had any hormone treatment (only abstract available)

IFOF = right inferior fronto-occipital tract

PACC = pregenual anterior cingulate cortex

RSMA = right supplementary motor area 

RPCG = right posterior cingulate gyrus

RCN = right caudate nucleus

SSC = secondary somatosensory cortex

SG = supramarginal gyrus

LMTG = left middle temporal gyrus

Gender Identity in People With Opposite Gendered Brains

In addition to trans people not showing "oppositely gendered brains", there is little evidence that those with sexually incongruent brain development are more likely to identify as the opposite sex purely due to having a brain typical of the opposite sex. Congenital adrenal hyperplasia (CAH) is a disorder where a person is excessively exposed to prenatal testosterone, and in girls this causes a "masculinized brain" with behaviors, interests, and personalities typical of men. A review of transgender identity in CAH found that while the majority of CAH girls prefer male playmates, toys, and games, only a small minority had significant indication of gender dysphoria, and the vast majority identified and lived as female (Cloudfindings 2021a). While the minority was slightly larger than in the general population, there is a likely possibility that this could be due to psychological alterations found in CAH which are predictive of gender dysphoria (Cloudfindings 2021a).

Meyer-Bahlberg et al. (2005) found that 22% of XY males with genital abnormalities not due to masculinization in the womb or hormonally, which underwent sex reassignment surgery to live as female, later choose to live as female, and this is taken as suggesting it could be due to their male brain. Reiner & Gearhart (2004) found that 8 of 14 subjects reassigned female at birth due to genital abnormalities at some point chose to live as male later on. Four of the patients did so by declaring they were male spontaneously, whereas the remaining four decided to live as male after becoming aware of being a biological male. All except one of the patients raised female had interests, behaviors, and psychological characteristics typical of males. This is taken as evidence that gender identity is biological, however there are many potential reasons why some patients chose to live as male. Simply knowing about being biologically male was enough for four patients to transition in Reiner & Gearhart (2004), and it's likely a significant portion of the cases in Meyer-Bahlberg et al. (2005) did so for the same reason, however it did not report on their reasoning.

1. Manzouri & Savic (2019) Possible Neurobiological Underpinnings of Homosexuality and Gender Dysphoria

2. Feusner et al. (2018) Intrinsic network connectivity and own body perception in gender dysphoria

3. Nota et al. (2017) Brain functional connectivity patterns in children and adolescents with gender dysphoria: Sex-atypical or not?

4. Burke et al. (2017) Structural connections in the brain in relation to gender identity and sexual orientation

5. Burke et al. (2016) Male-typical visuospatial functioning in gynephilic girls with gender dysphoria — organizational and activational effects of testosterone

6. Nota et al. (2017) Brain sexual differentiation and effects of cross-sex hormone therapy in transpeople:A resting-state functional magnetic resonance study

7. Hahn et al. (2015) Structural Connectivity Networks of Transgender People

8. Case et al. (2016) Altered White Matter and Sensory Response to Bodily Sensation in Female-to-Male Transgender Individuals

9. Baldinger-Melich et al. (2020) Sex Matters: A Multivariate Pattern Analysis of Sex- and Gender-Related Neuroanatomical Differences in Cis- and Transgender Individuals Using Structural Magnetic Resonance Imaging

10. Hoekzema et al. (2015) Regional volumes and spatial volumetric distribution of gray matter in the gender dysphoric brain

11. Krans et al. (2014) White Matter Microstructure in Transsexuals and Controls Investigated by Diffusion Tensor Imaging

12. Luders et al. (2009) Regional gray matter variation in male-to-female transsexualism

13. Del Giudice (2021a) Binary Thinking About the Sex Binary: A Comment on Joel (2021a)

14. Del Giudice (2021b) Ideological Bias in the Psychology of Sex and Gender

15. Cloudfindings (2021a) Gender dysphoria is not equivalent to gender nonconformity

16. Saraswat et al. (2015) Evidence Supporting the Biological Nature of Gender Identity

17. Meyer-Bahlberg et al. (2005) Gender identity outcome in female-raised 46,XY persons with penile agenesis, cloacal exstrophy of the bladder, or penile ablation

18. Spizzirri et al. (2018) Grey and white matter volumes either in treatment-naïve or hormone-treated transgender women: a voxel-based morphometry study

19. Flint et al. (2020) Biological Sex Classification With Structural Mri Data Shows Increased Misclassification In Transgender Woman

20. Clemens et al. (2021) Replication of Previous Findings? Comparing Gray Matter Volumes in Transgender Individuals with Gender Incongruence and Cisgender Individuals

21. Fukao et al. (2021) Gray matter volume differences between transgender men and cisgender women: A voxel-based morphometry study

22. Reiner & Gearhart (2004) Discordant Sexual Identity in Some Genetic Males with Cloacal Exstrophy Assigned to Female Sex at Birth

23. Coolidge et al. (2002) The Heritability of Gender Identity Disorder in a Child and Adolescent Twin Sample

24. Cloudfindings (2021b) Meta Analysis of Personality Disorders & Autism In Gender Dysphoria