Gender identity is the internal sense of being male, female, both, or neither, which can differ from the sex assigned at birth. For transgender individuals, their gender identity does not align with their assigned sex. A growing field of neuroscience is exploring the biological basis of this identity by investigating how differences in brain structure and function relate to a person’s gender.
Brain Structure and Gender Identity
Neurobiological research has identified differences in the physical structures of the brain, often by comparing cisgender men, cisgender women, and transgender individuals. Studies suggest the brains of transgender people show characteristics more similar to their experienced gender than their sex assigned at birth, indicating a neurological basis for gender identity.
Two specific areas, the bed nucleus of the stria terminalis (BSTc) and the third interstitial nucleus of the anterior hypothalamus (INAH-3), have been a focus of this research. Post-mortem studies show the size and neuron count in the BSTc of transgender women are comparable to those of cisgender women. Similarly, the INAH-3, a region involved in sexual behavior, also aligns with a person’s gender identity rather than their sex assigned at birth.
Research also examines the brain’s gray and white matter. Gray matter consists of neuronal cell bodies and processes information, while white matter is made up of nerve fibers that transmit signals between brain regions. The patterns of white matter microstructure in transgender individuals can be more similar to those of their identified gender. For example, studies on trans men have shown some white matter tracts are more comparable to those in cisgender men than in cisgender women.
These findings represent statistical averages and cannot determine an individual’s gender identity. Brains are not uniformly “male” or “female” but are complex mosaics of different characteristics. Research also shows variations in cortical thickness, with some studies indicating that transgender women have thicker cortices in certain areas compared to cisgender men.
Brain Activity and Self-Perception
Beyond physical structure, the brain’s functional activity provides further insight into the neurological basis of gender identity. Functional magnetic resonance imaging (fMRI) allows researchers to observe brain activation in real-time. Studies using fMRI reveal that when transgender individuals engage in tasks related to self-perception, their brain activity patterns often resemble those of their identified gender.
One study used fMRI to monitor brain activity while participants viewed images of their own bodies morphed to appear more masculine or feminine. Transgender participants showed brain activation patterns similar to cisgender individuals of their identified gender when viewing images that aligned with their gender identity. This alignment was observed in neural systems responsible for self and body processing.
The way the brain processes its own body image appears to be different in transgender individuals. This may be linked to the connectivity of brain networks involved in own-body perception. For instance, studies have found differences in the default mode network (DMN) and visual networks, which are engaged in self-awareness and body perception, in transgender people compared to cisgender individuals.
These functional differences are not limited to body image tasks. Brain activation patterns in transgender adolescents performing cognitive tasks, such as visual/spatial memory exercises, more closely resemble those of their identified gender. These findings support the idea that differences in brain function may occur early in development.
The Influence of Hormones on the Brain
Hormones are a significant factor influencing the brain both before birth and later in life. The theory of prenatal hormone exposure suggests that the hormonal environment in the womb plays a part in organizing brain development. Elevated levels of androgens, such as testosterone, during critical prenatal periods are thought to influence the development of brain structures that later affect gender-related behaviors and identity.
Studies of individuals with conditions causing atypical prenatal hormone exposure support this theory. For example, girls exposed to high levels of androgens in utero often show more male-typical play behaviors. This suggests that prenatal hormones can have a lasting impact on brain differentiation in ways that may influence the development of gender identity.
Hormones continue to affect the brain throughout life, and gender-affirming hormone therapy (GAHT) provides a clear example. When transgender individuals undergo GAHT, the administration of hormones like testosterone or estrogen can lead to changes in brain structure and function. These changes often result in the brain’s characteristics becoming more aligned with the person’s gender identity.
Research using neuroimaging has shown that GAHT can alter both gray and white matter, as well as brain connectivity. Testosterone therapy in transgender men has been associated with changes in white matter microstructure. Estrogen therapy in transgender women can also lead to shifts in brain structure and function, demonstrating the brain’s capacity to change in response to hormonal signals.
Neurological Basis of Gender Dysphoria
Gender dysphoria refers to the significant distress a person may feel when their gender identity does not match the sex they were assigned at birth. This distress is understood to have a neurological basis, distinct from gender identity itself. The feeling of a mismatch between one’s internal sense of self and physical body can trigger conflict signals within the brain’s networks.
Brain networks involved in body ownership and emotional regulation are relevant to understanding gender dysphoria. The insula and the anterior cingulate cortex are two brain regions that are part of the salience network, which processes bodily self-awareness and emotional responses. Altered activity in these areas is thought to be associated with the distress of gender dysphoria.
The experience of gender dysphoria can be seen as a neurological conflict between the brain’s internal map of the body and the physical reality. Medical transition, including gender-affirming hormone therapy and surgeries, can alleviate this distress by helping to align the physical body with the internal sense of self. This alignment can resolve the neurological conflict, leading to a reduction in the distress associated with gender dysphoria.