What do neuroimaging studies reveal about brain differences between trans women and cisgender males, and are these differences attributed to prenatal hormonal influences or environmental factors?

Brain Imaging Differences Between Trans Women and Cisgender Males

The evidence suggests that trans women present with a unique brain phenotype that is intermediate between cisgender men and cisgender women, rather than simply matching either group, and current research points toward a combination of prenatal hormonal influences and neurodevelopmental factors rather than purely environmental causes.

What Neuroimaging Studies Show

Structural Brain Differences

Trans women demonstrate distinct neuroanatomical patterns that differ from both cisgender males and cisgender females 1, 2. The largest mega-analysis to date, examining 803 individuals, found that transgender persons show significant differences from cisgender persons in subcortical brain volumes and cortical surface area, though not in cortical thickness 1.

Critically, a 2022 study using multivariate classification found that trans women's brains ranged between cisgender men and cisgender women—closer to cisgender men but significantly different from both groups 2. This challenges the simplistic notion that trans women have "female brains" and instead suggests a distinct neurobiological profile.

Functional Connectivity Patterns

Trans women show reduced global connectivity measures compared to cisgender men, including lower small-worldness, modularity, and clustering coefficients 3. They also demonstrate decreased connectivity in superior parietal regions within the salience and executive control networks compared to cisgender men 3.

Notably, trans women display singular features not seen in either cisgender group, including:

• Reduced thalamus and putamen volumes
• Elevated gray matter in the right insular and inferior frontal cortex
• Changes in the right angular gyrus region 4

Origins: Prenatal vs Environmental Factors

Evidence for Prenatal Hormonal Influences

The Endocrine Society guidelines acknowledge that neither biological nor psychological studies provide a completely satisfactory explanation for gender identity development 5. However, the evidence leans toward prenatal factors:

Studies of individuals with disorders of sex development provide indirect support for hormonal influences on gender identity 6. For example:

• Individuals with complete androgen insensitivity syndrome (unable to respond to androgens) consistently develop feminine gender identity despite XY chromosomes
• Some individuals with 5α-reductase deficiency develop male gender identity at puberty despite female upbringing

However, the relationship is complex: most 46,XY individuals with prenatal androgen exposure who are raised female do NOT develop male gender identity, and trans women develop female gender identity despite typical male prenatal androgen exposure 5. This suggests prenatal hormones are not deterministic.

The Neurodevelopmental Hypothesis

Current evidence supports a neurodevelopmental cortical hypothesis suggesting different brain phenotypes emerge during development 3, 7. Research indicates that:

• Brain differences in transgender individuals involve complex mixtures of masculine, feminine, and unique patterns rather than simple sex-reversal 7
• The years between childhood and mid-adolescence represent a critical period when puberty-related factors influence neural characteristics in both sex- and gender identity-specific ways 8
• Changes in connectivity within body perception networks may give rise to gender dysphoria 9

Limited Role of Environmental Factors Alone

The evidence does not support purely environmental causation. Studies examining potential psychosocial factors are retrospective and rely on self-report, making them unreliable 5. Behavioral genetic studies suggest heritable components to atypical gender identity development 5.

Clinical Implications

The brain differences observed appear to be:

1. Present before hormone treatment (studies examined hormone-naive individuals) 1, 2, 4
2. Not simply explained by social factors (structural and functional differences are measurable)
3. Consistent with a neurodevelopmental origin involving prenatal and early developmental periods 8

Important Caveats

• These are group-level differences with substantial individual variation
• Brain phenotypes in transgender individuals are unique rather than simply shifted toward the opposite sex 1
• The specific mechanisms linking brain structure to gender identity remain incompletely understood 5
• Cross-sex hormone treatment does produce additional marked effects on brain structure 7

The current scientific consensus, as reflected in the 2023 Annual Review of Medicine guidelines, is that gender identity expressions are not mental health conditions per se 10. The neurobiological evidence supports an intrinsic, developmentally-determined basis for gender identity that likely involves prenatal hormonal influences interacting with genetic factors, rather than being purely environmentally determined.