Gender identity refers to a person’s internal sense of being male, female, both, or neither, separate from the physical characteristics assigned at birth. Neuroimaging techniques offer a window into the brain’s biological structures and functions, allowing researchers to explore physical correlates of this internal sense of self. Brain scans, such as Magnetic Resonance Imaging (MRI) and functional MRI (fMRI), are employed to map these characteristics, helping scientists investigate the neurobiological landscape of individuals whose gender identity does not align with their sex assigned at birth.
Purpose of Transgender Neuroimaging Research
Researchers use neuroimaging to seek objective, quantifiable biological evidence that aligns with the subjective experience of gender identity and gender dysphoria. This work aims to move beyond purely psychological or social models by examining the physical organization of the nervous system. Studies compare the brains of transgender individuals to those of cisgender men and women, looking for patterns that might correlate with the identified gender rather than the assigned sex.
A range of scanning methods provides different levels of detail about the brain. Structural MRI reveals anatomical features, such as the volume of gray matter or the size of specific regions. Functional MRI (fMRI) measures brain activity by detecting changes in blood flow, indicating how different areas communicate during rest or specific tasks. Diffusion Tensor Imaging (DTI) maps the white matter tracts, the brain’s internal communication cables, to assess connectivity and organization.
Researchers typically study transgender individuals before they begin gender-affirming hormone therapy (GAHT). By focusing on hormone-naïve participants, researchers attempt to isolate neurobiological patterns that may have developed early in life. This comparative design allows for testing hypotheses regarding the neurodevelopmental origins of gender identity. Findings often suggest that the brains of transgender individuals display features that are unique, or intermediate between, the typical patterns of cisgender men and women.
Key Structural Findings in Brain Anatomy
Research into the anatomy of the hormone-naïve transgender brain suggests complex patterns that often shift toward the identified gender. Studies examining gray matter volume (GMV) have found differences in various regions. For example, some adolescent transgender individuals exhibit GMV patterns in areas like the superior medial frontal and pre/postcentral cortex that deviate from their sex assigned at birth toward their identified gender.
Post-mortem studies first highlighted structural differences in the hypothalamic region known as the bed nucleus of the stria terminalis (BSTc). This area was found to be similar in size to that of cisgender women in transgender women, despite their assigned male sex. This finding suggested a neuroanatomical correlate for gender identity, though subsequent in-vivo neuroimaging studies have demonstrated a more nuanced picture across the whole brain.
White matter organization, studied using DTI, also provides evidence of structural distinctions. White matter tracts have been shown to display intermediate microstructural patterns in transgender adolescents compared to cisgender controls. Structural connectivity analyses have revealed decreased hemispheric connectivity ratios in subcortical and limbic areas for both transgender men and women before hormone treatment. These findings are often described as a mosaic of masculine and feminine traits depending on the specific brain region being analyzed.
Functional Connectivity and Hormonal Effects
Functional neuroimaging focuses on how different brain regions interact, providing insight into self-perception and body awareness. Functional connectivity studies, often conducted during a resting state, frequently find that interaction patterns between brain networks are more consistent with the lived gender identity than the sex assigned at birth. These differences are particularly noted in networks related to the sense of self and own-body perception, such as the default mode network and the salience network.
Specific regions involved in processing emotions and body representation, including the insula and supramarginal gyrus, have shown altered functional connections in transgender individuals. For instance, activation patterns in the hypothalamus in transgender adolescents during certain tasks have been observed to resemble those of their experienced gender. These functional patterns suggest a difference in the processing of self-referential and interoceptive information.
Gender-affirming hormone therapy (GAHT) can dynamically alter the brain’s structure and function. Studies on transgender women receiving estradiol show the hormone can strengthen connections in the thalamus, a major relay station for sensory and motor information. For transgender men, testosterone therapy has been linked to an increase in cortical thickness in certain posterior brain regions.
GAHT administration has also been shown to increase functional connectivity within the self-referential and body perception networks in both transgender men and women. These changes, which occur over the course of treatment, suggest a sensitivity of various brain areas to the effects of sex hormones. The observed structural and functional shifts may reflect the brain’s plasticity, potentially resulting in patterns that align more closely with the individual’s identified gender over time.
Context and Limitations of Research Findings
Interpreting neuroimaging results requires careful attention to inherent limitations. A significant challenge is the typically small sample size, which can limit statistical power and lead to mixed or non-replicated findings across different laboratories. Furthermore, controlling for external variables, such as psychological stress, social exclusion, or the precise timing of puberty, remains difficult in human research.
Neurobiological differences are correlational; they show a relationship but do not prove a cause for gender identity. A different brain pattern does not mean that the pattern caused the identity, as experience and environment also shape the brain. The discovery of these neurobiological patterns should not be used as a diagnostic tool or to determine the legitimacy of an individual’s identity. These findings contribute to a growing understanding that gender identity has a biological basis, alongside psychological and social factors.