Using a mega-analytic approach, structural MRI data from 803 non-hormonally treated transgender and cisgender participants were analyzed. Transgender people differed significantly from cisgender people in subcortical and cortical brain volumes and surface area, but not cortical thickness. The authors report that the pattern varied by brain region and by the direction of gender identity, rather than showing a single simple shift toward one sex category.

This systematic review and meta-analysis examined neuroimaging literature on cisgender versus transgender brains before hormonal treatment and found that current results do not allow a specific brain phenotype to be concluded for each group. The review states that a clear pattern accompanied by consistent structural changes is still to be found.

The study reports that the INAH3 subnucleus volume “was 1.9 times larger in control males than in females and contained 2.3 times as many cells.” It then states: “We showed for the first time that INAH3 volume and number of neurons of male-to-female transsexual people is similar to that of control females. The female-to-male transsexual subject had an INAH3 volume and number of neurons within the male control range.” The authors conclude: “We propose that the sex reversal of the INAH3 in transsexual people is at least partly a marker of an early atypical sexual differentiation of the brain and that the changes in INAH3 and the BSTc may belong to a complex network that may structurally and functionally be related to gender identity.”

This peer‑reviewed article notes: “We have shown previously that the bed nucleus of the stria terminalis (BSTc) is female in size and neuron number in male-to-female transsexual people.” It also states: “We showed for the first time that INAH3 volume and number of neurons of male-to-female transsexual people is similar to that of control females. The female-to-male transsexual subject had an INAH3 volume and number of neurons within the male control range.” The authors write: “The data show that there is indeed a sex difference in the INAH3 subnucleus that is reversed in transsexual people … We propose that the uncinate nucleus may be part of a brain network that is involved in gender identity.”

The brains of transgender women ranged between cisgender men and cisgender women, although they were still closer to cisgender men, and the differences to both cisgender men and cisgender women were statistically significant. These findings add support to the idea that the underlying brain anatomy in transgender people is shifted away from their biological sex toward their gender identity.

Our findings suggest that the neuroanatomical signature of transgenderism is related to brain areas processing the perception of self and body ownership. The transgender groups showed sex-typical FA-values after controlling for sexual orientation, with the only exception being the right inferior fronto-occipital tract, a pathway linking parietal and frontal regions involved in own-body perception.

Contrary to the primary hypothesis, no sex-atypical features with signs of 'feminization' were detected in the transsexual group. Instead, the study found significant volume reductions of the thalamus and putamen in male-to-female transsexuals and significant increases in gray-matter volume in the right angular gyrus, posterior superior temporal gyrus, right insular cortex, and inferior frontal cortex.

This review describes post‑mortem work on sexually dimorphic hypothalamic regions: “Regarding grey matter, the main sexually dimorphic areas associated with the development of gender identity are represented by the central subdivision of the bed nucleus of the stria terminalis (BNST) and the third interstitial nucleus of the anterior hypothalamus (INAH3).” It summarizes earlier findings: “Post-mortem studies reported that the BNST is smaller and with low somatostatin neurons in ciswomen and transwomen compared with cismen. Regarding the INAH-3… one study reported this area to be smaller in transwomen than in cismen and to have less neurons.” However, it cautions that “the role of BNST and INAH-3 in the determination of sexual differentiation remains unclear because of the small size of the samples and because part of the subjects enrolled had received hormonal treatment previously.”

In this cross-sectional study of 2165 adolescents from the Netherlands general population, no significant differences in total brain volumetric measures were observed between youths who reported gender diversity and youths who did not. However, among youths who were assigned female, those who reported gender diversity had smaller thalamic volumes in both the left hemisphere and right hemisphere compared with adolescents who did not report gender diversity. In whole-brain, vertexwise analyses among adolescents assigned male at birth, thicker cortices in the left inferior temporal gyrus were observed among youths who reported gender diversity compared with those who did not. The authors conclude that gender diversity in the general population correlates with specific brain morphologic features in certain regions, but not with global brain size.

Manzouri et al. (2017) found that the FtMs left putamen was larger than both female and male cisgenders, and Savic and Arver (2011) found that among all subcortical structures, MtF’s putamen and thalamus were smaller than those in both female and male cisgender groups. A study published in 2020 involving 26 males and females concludes that the nucleus accumbens, left thalamus, right hippocampus, and right caudate nucleus were smaller in transgenders than in cisgenders. However, the results from this systematic review and meta-analyses do not allow the authors to conclude on specific brain phenotypes differential for each of the groups. They state that although some MRI studies indicate that certain fronto-parietal and cingulo-opercular regions are differentially relevant in transgenderism, a clear pattern accompanied by consistent structural changes is still to be found.

This review says that structural and functional brain characteristics are more similar between transgender people and control subjects with the same gender identity than between individuals sharing the same biological sex. It cites local differences in neurone number and the volume of subcortical nuclei, as well as gray- and white-matter differences, as evidence that gender identity can be associated with brain structure.

The authors argue that the human brain is not 'sexually dimorphic' in a simple binary sense. They also state that current evidence does not establish a direct link between brain sex differences and gender identity, emphasizing that many reported differences are small, overlapping, and not uniquely predictive of identity.

At the structural level, the study analyzed regional gray-matter volumes and white-matter microstructure in sexually dimorphic brain structures. In the regions of interest, gray-matter volumes of both gender-dysphoric groups deviated from the volumetric characteristics of their birth sex toward those of individuals sharing their gender identity.

The sample consisted of 803 structural brain MRI scans. The results reported that transgender persons differed significantly from cisgender persons in subcortical and cortical brain volumes and surface area, but not cortical thickness. The paper also notes that some regions showed patterns concordant with gender identity, while others were closer to sex assigned at birth.

Zhou et al. published an article in Nature in which they claim that there is a difference in the human brain in relation to the sexes; their findings show that the BSTc region of the brain is larger in size in males than in females. Neuroscientists discovered that in transsexual females, those considered male at birth but who had a strong conviction that they were female, the BSTc region was similar in size to the female BSTc, and transsexuals considered female at birth but who were certain they were male had a BSTc volume similar to the male BSTc. Another experiment carried out by Kruijver et al. further supports the Zhou et al. findings, showing that the number of neurons in the BSTc of male-to-female transsexuals was similar to that of females, while female-to-male transsexuals was in the male range. The paper notes that sex difference in the BSTc and its reversal in the transsexual brain have been interpreted as pointing to a neurobiological basis of gender identity disorder.

This article summarizes the same mega-analysis and states that applying a mega-analytic approach to more than 800 participants uncovered distinct structural brain patterns among transgender people. It emphasizes that the findings depend on the specific brain measure and brain region examined.

Summarizing the state of research, neuroscientist Jonathan Vanhoecke is quoted: “It’s not always true that your brain structure matches your gender identity, or vice versa. There is no clear consensus in the field.” The article explains that “some evidence suggests differences in gender identity could be linked to ways the brain develops in childhood and adolescence, and that the observed patterns correspond to gender identity. Other studies have indicated that neural patterns generally match the sex a person is assigned at birth. Yet other studies found evidence that doesn’t seem to support either of these, but rather that there are unique neural patterns in transgender people.” This highlights that findings about structural correlations are mixed and not uniform across regions or individuals.

Reviewing sexually dimorphic nuclei, this article notes that the third interstitial nucleus of the anterior hypothalamus (INAH3) “is larger in men than in women, both in volume and in number of neurons,” and that this nucleus has been implicated in sexual orientation and gender identity. It also references studies showing that in some transgender women, “INAH3 volume and neuron number have been reported to resemble those of cisgender women.” However, the review stresses methodological limitations such as “small sample sizes, lack of control for hormonal treatments and comorbid conditions,” and concludes that while these nuclei are candidate regions, “their precise role in the development of gender identity remains unresolved.”

This study aims to investigate how masculine and feminine identities influence cortical structures and personality traits, mainly focusing on the neural basis of self-identity. The authors report associations between measures of structural connectivity and scores on gender identity scales, suggesting that specific patterns of cortical connectivity may correlate with the degree to which individuals identify with masculine or feminine traits. The abstract describes gender identity as being linked to structural features in networks involved in self-perception, although detailed regional findings and effect sizes are limited in the conference summary.

In a large sample, the authors found that female sex, independent of continuum scores, was associated with larger proportion-adjusted volumes for the basal ganglia, hippocampus, and other regions, while gender-related psychological traits showed more subtle associations with cortical thickness and surface area. They report that sex assigned at birth explains more variance in global and subcortical brain measures than self-reported gender traits, but that certain regional measures do show small correlations with nonbinary gender characteristics. The paper emphasises that brain structure–gender associations are complex, distributed, and of small effect size rather than a simple one-to-one mapping between a single brain region’s size and gender identity.

Discussing hypothalamic nuclei, this review notes that several small regions, including the INAHs and the bed nucleus of the stria terminalis, show marked average sex differences in size. It also comments on studies of gender dysphoria: “Post‑mortem studies of individuals with gender dysphoria have reported that some of these sexually dimorphic nuclei resemble those typically observed in individuals who share their gender identity rather than their natal sex.” Nevertheless, the article cautions that such findings “derive from very small samples” and that broader imaging studies “suggest a mosaic of male‑typical and female‑typical features rather than a simple categorical shift.”

This review on sex/gender differences and gender identity states that early post‑mortem work found “a female‑typical size and neuron number of the central subdivision of the bed nucleus of the stria terminalis (BSTc) in male‑to‑female transsexual people.” It continues: “Similarly, the INAH3, another sexually dimorphic hypothalamic nucleus, has been reported to be smaller and to contain fewer neurons in male‑to‑female transsexuals compared to cisgender men, and to be in the range of cisgender women.” However, the authors emphasize that “evidence remains limited to very small samples and cannot on its own establish a causal relationship between these structural differences and gender identity.”

Reporting on structural and functional studies, the article notes that some work on the BSTc and INAH3 “found that in a handful of transgender women, these hypothalamic regions looked more like those of cisgender women than cisgender men.” It adds that more recent MRI studies “suggest a more complex picture, with transgender people often showing brain traits that are partly like their natal sex, partly like their gender identity and partly unique.” A quoted researcher explains that these results indicate “associations between certain brain regions and gender identity,” but that “no single brain structure has been found that reliably ‘determines’ gender identity in individuals.”

This paper reviews how brain studies of transgender identity are interpreted and notes that the literature commonly reports regional differences in cortical thickness, surface area, and connectivity, but that these findings are often heterogeneous and not reducible to a single brain marker of gender identity.

A large functional MRI study of nearly 5,000 children found that machine learning could predict sex assigned at birth more accurately than gender. The study reported that networks linked to sex were sensory and motor, while networks linked to gender were distributed across the brain and involved emotional processing, social cognition, and attention.

Early post-mortem work by Swaab and colleagues on the third interstitial nucleus of the anterior hypothalamus (INAH3) reported that INAH3 volume and neuron number differed between cisgender men and women, with trans women showing INAH3 measures closer to cisgender women than to cisgender men. These results were widely cited as suggesting a correlation between the size of this hypothalamic region and gender identity, but subsequent reviews have noted limitations including small samples, confounding by long-term hormone treatment, and lack of replication in larger, modern imaging cohorts.

This student paper summarizes the field and says studies generally reflect differences in cortical thickness and in structural and functional connectivity in transgender participants compared with cisgender individuals. It is a secondary discussion source rather than primary evidence.

A popular comment summarises scientific findings by stating that a study highlighted male-to-female transsexual individuals possessing a notably greater volume of gray matter in the right putamen compared to cisgender men, and references the presence of a female-sized BSTc (bed nucleus of the stria terminalis) in male-to-female transsexuals, unaffected by adult sex hormones and independent of sexual orientation. The commenter concludes that these findings provide evidence that brain anatomy is associated with gender identity, where measures in MTF transsexuals appear to be shifted away from gender-congruent men. Although this is a secondary, informal source, it reflects public interpretation of peer-reviewed studies on region-specific correlations between brain structure and gender identity.

This discussion cites older work reporting that the INAH-3 region in the anterior hypothalamus is about three times larger in males than in females and that male-to-female transgender individuals had INAH-3 characteristics closer to the typical female range. This is secondary discussion rather than a primary source.

The article summarizes that some MRI studies have reported sex-atypical features in transgender people in regions such as the putamen, thalamus, precentral gyrus, and central sulcus, but it also notes that these findings do not uniquely distinguish gender identity from sexual orientation and remain insufficient to establish a definitive neuroanatomical basis for gender identity.

In this talk, the speaker says meta-analyses found group differences in some brain volumes and that different structures may be larger or smaller in ways that are sometimes more like natal sex and sometimes more like gender identity. The speaker also emphasizes that the brain is not binary and sex differences exist along a continuum.