Biological accounts suggest that variations in genes, prenatal hormone exposure, and brain development may help explain why some people’s experienced gender differs from their sex assigned at birth.

Gender incongruence and the biological approach

Gender incongruence is when a person’s experienced gender does not match the sex assigned at birth.

Biological explanations assume gender identity is influenced partly by innate factors. They do not claim one single cause. Instead, they suggest that several biological processes may interact across development.

Genetic explanations

Some researchers argue that genetic factors contribute to the likelihood of gender incongruence. If genes are involved, people who share more genes should show more similarity in gender identity.

Twin evidence

Studies comparing monozygotic and dizygotic twins are central.

This diagram contrasts how monozygotic (identical) twins arise from a single fertilized egg that splits, whereas dizygotic (fraternal) twins arise from two separate eggs fertilized by two different sperm. It reinforces why monozygotic twins share ~100% of their genes while dizygotic twins share ~50% on average, which is the logic behind using concordance rates to estimate genetic influence. Source

Monozygotic twins share all of their genes, whereas dizygotic twins share about half on average. If monozygotic twins show higher concordance, this suggests a genetic contribution.

One often-cited study is Heylens et al. (2012), which found much higher concordance for gender incongruence in identical than non-identical twins. This supports the idea that inherited factors may be involved in differences in gender identity.

However, concordance in identical twins is not 100%. This means genes cannot be the complete explanation. If gender incongruence were caused only by genes, monozygotic twins would always both show it.

Candidate genes

Researchers have also looked for specific genes linked to the way sex hormones act on the developing brain. For example, genes affecting androgen receptors or estrogen pathways may alter sensitivity to hormones before birth. This could influence later gender identity.

The main implication is that gender incongruence may be partly polygenic, meaning influenced by many genes with small effects rather than by one single gene. However, findings are mixed, and no clear set of genes has been established.

Hormonal explanations

Hormonal accounts focus on the effects of prenatal hormones, especially androgens, during early development. The basic idea is that exposure to different hormonal levels, or unusual sensitivity to those hormones, may influence the organization of brain systems linked to gender identity.

Prenatal hormone exposure

During fetal development, sex hormones help shape parts of the brain.

This schematic summarizes key fetal sex-differentiation pathways, illustrating how the presence of testes and their hormone output steers development along a typically male pathway, while their absence leads to a typically female pathway. It provides concrete biological context for why researchers focus on prenatal hormone exposure (especially androgens) as a plausible mechanism influencing later development. Source

Biological explanations suggest that if hormone exposure does not match typical patterns associated with the assigned sex, this may contribute to later gender incongruence.

For example, atypical androgen exposure may affect neural development in ways that are more consistent with the person’s experienced gender than with their assigned sex. This explanation focuses on very early development, before conscious awareness of gender emerges.

A strength of this account is that it provides a clear mechanism: hormones can influence brain differentiation during fetal life. A limitation is that direct evidence is difficult to obtain because researchers cannot ethically manipulate prenatal hormone levels in humans.

Brain structure and function

Another biological explanation is that brain differences underpin gender incongruence. Researchers have examined whether certain sexually dimorphic brain areas in transgender individuals are more similar to those typically found in people who share their gender identity.

BSTc research

A well-known line of research focuses on the bed nucleus of the stria terminalis or BSTc, an area linked to sexual behavior and gender-related processing.

This labeled brain dissection image highlights the stria terminalis, the fiber pathway associated with the bed nucleus of the stria terminalis (BNST/BST region). Using a concrete anatomical reference can help you visualize where BST-related structures sit within the limbic system when evaluating claims about sexually dimorphic brain areas. Source

Zhou et al. reported that the BSTc of transgender women was more similar in size to that of cisgender women than cisgender men.

This finding suggests that gender identity may relate to brain development rather than only external anatomy. It supports the view that the brain can develop in a pattern different from the rest of the body.

However, many brain studies use very small samples, sometimes rely on post-mortem tissue, and may be affected by adult hormone treatment. This makes it difficult to know whether the brain difference caused gender incongruence or developed later through experience and transition-related treatment.

How convincing are biological explanations?

Biological explanations have several strengths.

• They are supported by scientific methods, including twin studies, genetic analysis, brain scanning, and post-mortem research.

• They move away from the outdated assumption that gender incongruence is simply a matter of personal choice.

• They recognize that gender identity may have a genuine biological basis.

There are also important limitations.

• Much of the evidence is correlational. A biological difference may be associated with gender incongruence without directly causing it.

• Samples are often small because gender incongruence is relatively uncommon and difficult to study in large representative groups.

• Findings are not always consistent across studies, especially in gene research.

• A purely biological account can become deterministic, suggesting biology completely fixes gender identity. In practice, biological factors may interact with other influences across development.

Exam focus

For AQA, the key idea is that biological explanations look for innate influences on gender identity. The three main strands are:

• genetics, shown by higher concordance in identical twins

• prenatal hormones, which may shape early brain development

• brain structure, where some regions appear to resemble the person’s experienced gender

Strong answers usually explain the proposed biological mechanism and then evaluate the evidence in terms of scientific support, causation, sample size, and determinism.