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Somewhere between 0.5% and 1.3% of individuals identify as transgender, adopting a gender that does not match their sex at birth. Is this biologically determined or socially constructed? Or perhaps both nature and culture play a role?

Brain scans could provide an answer. If genetics hold sway, we might expect to find clear-cut differences between the brains of transgender people and those of their cisgender peers. If social factors dominate, there might not be such a clear distinction.

Over the last few years, scientists have begun to tackle this topic. In May, researchers from the University of Edinburgh amalgamated the existing studies to see what can be gleaned. Their aim was to "document the scientific evidence from neuroimaging techniques on brain features that might be distinctive in cisgenders compared to transgenders."

As the researchers reported in the journal Archives of Sexual Behavior, they identified and reviewed thirty applicable studies, conducted using various brain scanning techniques.

"From extracting and summarizing all the information available, differences were found between cisgender and transgender people in white matter microstructure, volumetric analyses, cortical thickness, and corpus callosum shape," they wrote, adding "in [male to female individuals] it was possible to find traits which are “feminine and demasculinized” and in female to male it was possible to find traits which are “masculine and defeminized.”

Still, they noted, "the majority of neuroanatomical, neurophysiological, and neurometabolic features in transgenders resemble those of their natal sex rather than those of their experienced gender."

"Due to conflicting results, it was... not possible to identify specific brain features which consistently differ between cisgender and transgender," they wrote.

The researchers cautioned that the studies conducted to date were conflicting, preliminary, and suffered from small sample sizes. They also found that some did not report negative results, omitting findings which suggested no brain differences between transgender and cisgender individuals.

Overall, it seems the science exploring the brain's link to gender identity is muddled and inconsistent, leaving the researchers to carefully proffer that both nature and culture affect an individual's gender identity.

"Both gender identity and sexual orientation seem to develop under two main types of influence: biological (genes, hormones, and gene expression) and environmental (influences of parents, peers, partners, and social models) factors as a result of the interaction between nature and culture."

Source: Frigerio, A., Ballerini, L. & Valdés Hernández, M. Structural, Functional, and Metabolic Brain Differences as a Function of Gender Identity or Sexual Orientation: A Systematic Review of the Human Neuroimaging Literature. Arch Sex Behav (2021). https://doi.org/10.1007/s10508-021-02005-9

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This article originally ran on realclearscience.com.

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