Biological Foundations of Gender: Genes, Hormones, and Behavior

When we talk about gender, biology is almost always the first place the conversation starts – and for good reason. Chromosomes, hormones, and genes lay the groundwork for sex differentiation and, to varying degrees, influence behavior and psychology. But the more scientists study these biological mechanisms, the clearer it becomes that the story is far more complex than a simple XX-equals-female, XY-equals-male equation. Modern research in genetics, endocrinology, and epigenetics has fundamentally reshaped how we understand the biological foundations of gender – revealing a dynamic interplay between our DNA, our hormones, and the environments we live in.

How chromosomes set the stage for sex and gender 🔗

Humans have 23 pairs of chromosomes, and it is the 23rd pair – the sex chromosomes – that determines biological sex. Females typically carry two X chromosomes (XX), while males carry one X and one Y chromosome (XY). Every egg a female produces contains an X chromosome, while male sperm can carry either an X or a Y. This means the sperm cell is what determines the sex of the child.

The key player on the Y chromosome is the SRY gene (Sex-determining Region Y). Around six weeks into fetal development, the SRY gene triggers the embryonic gonads to develop into testes rather than ovaries. The testes then begin producing testosterone, which drives further male development – including genital formation and, later, subtle differences in brain organization. Without the SRY gene, the default developmental pathway leads to ovarian development and a female phenotype.

It’s important to note that genital differentiation occurs in the first trimester of pregnancy, whereas brain differentiation is considered to begin in the second trimester. These two processes are usually coordinated, but research suggests they can occasionally be influenced independently – which has implications for understanding gender identity that diverges from biological sex.

What atypical chromosomal patterns reveal 🔗

Some of the most instructive evidence for how chromosomes shape gender development comes from conditions involving atypical chromosome configurations. Two well-studied examples are Turner syndrome and Klinefelter syndrome.

Turner syndrome occurs when a person has only one X chromosome (XO) instead of the typical XX. Individuals with Turner syndrome typically present as female but may have underdeveloped ovaries, leading to infertility. They tend to show poorer spatial ability and perform below average in mathematics, suggesting these cognitive patterns may have a chromosomal component. They require hormone replacement therapy to go through puberty.

Klinefelter syndrome, on the other hand, occurs when a male is born with an extra X chromosome (XXY). This results in the individual being taller, less muscular, and having reduced body and facial hair. They are generally infertile and may develop health complications including osteoporosis or breast cancer. Studies also note that boys with Klinefelter syndrome tend to show a more passive, cooperative temperament – a behavioral profile some researchers argue reflects the influence of their atypical hormone environment rather than purely social factors.

These syndromes demonstrate that the relationship between chromosomes, hormones, and behavior is real – but not deterministic. They also highlight that biological sex is far more varied than a binary model suggests.

Hormones: the chemical messengers that shape behavior 🔗

Chromosomes set the initial direction, but it is hormones that do much of the ongoing work in shaping biological sex differences – and potentially gendered behaviors. Both males and females produce the same core sex hormones: testosterone, estrogen, and progesterone. What differs is the quantity and the effect those hormones have on specific tissues, including the brain.

Testosterone and its behavioral effects 🔗

During early fetal development and in a surge period shortly after birth known as “minipuberty,” testosterone drives organizational changes in the brain that shape how males and females think, feel, and behave later in life. In males, prenatal testosterone exposure is linked to traits such as higher aggression, stronger visuospatial abilities, and greater sexual drive.

Research by Van Goozen et al. (1995) found that transgender men given testosterone showed increases in aggression and visuospatial skills, while transgender women given estrogen showed decreases in aggression – providing direct evidence that hormones actively regulate gendered behavioral patterns. Similarly, research by Dabbs et al. (1995) found that male offenders with the highest testosterone levels were more likely to have committed violent or sexually motivated crimes, though this association is probabilistic, not absolute.

Testosterone’s influence is not limited to males. Females with Congenital Adrenal Hyperplasia (CAH) – a condition in which the adrenal glands produce elevated androgens, including testosterone – offer a natural experiment. Females with CAH who were exposed to higher-than-typical levels of testosterone in the womb show a greater degree of dissatisfaction in their gender assignment and report above-average levels of same-sex attraction, suggesting that prenatal testosterone plays a meaningful role in shaping gender identity and sexual orientation – even when chromosomal sex is XX.

Estrogen, oxytocin, and female-typical behaviors 🔗

On the other side of the hormonal spectrum, estrogen plays a key role in female brain development and behavior. Research by Schultheiss et al. (2020) suggests that estrogen exposure explains why women generally show greater verbal fluency than men. Women also produce higher levels of oxytocin – a hormone amplified by estrogen – which promotes bonding, nurturing behavior, and a stress response pattern called “tend and befriend” rather than the fight-or-flight response more commonly associated with testosterone-dominant systems.

It is critical to remember, as researchers consistently note, that these hormones are secreted by both males and females – no single hormone belongs exclusively to one sex. The behavioral differences we observe are a product of dosage, timing, and receptor sensitivity, not simply presence or absence.

Epigenetics: when environment rewrites the biological script 🔗

For a long time, the dominant view was that your chromosomes and hormones were a fixed biological blueprint. Epigenetics has fundamentally challenged that view. Epigenetics refers to changes in gene expression that do not alter the underlying DNA sequence but determine whether specific genes are switched on or off. These changes can be triggered by diet, stress, toxins, social experiences, and more – and some can persist across generations.

All environmental influences of meaningful significance can epigenetically alter the brain, which means lived experience – including gender-specific experiences like socialization, discrimination, and stress – may leave a measurable biological mark. The two main mechanisms are DNA methylation (the addition of methyl groups to DNA that typically suppresses gene activity) and histone modification (changes to the proteins around which DNA is wrapped).

How early environments shape gene expression 🔗

Epigenetic modifications arising in early childhood can have lasting effects on hormone profiles and subsequently on future behavior. Animal studies show that even the quality of maternal care in the first week of life can epigenetically alter genes involved in stress regulation. These changes in gene expression then persist into adulthood and influence how an individual responds to future stressors.

Environmental exposures matter too. Prenatal exposure to endocrine-disrupting chemicals such as bisphenol A (BPA) has been shown in rodent studies to alter DNA methylation patterns in the brain, changing how gender-related genes are expressed. These findings raise serious questions about the long-term effects of widespread chemical exposure on human gender development – an area of active research.

Perhaps most strikingly, a global DNA methylation analysis found that people with gender incongruence show distinct methylation profiles compared to cisgender individuals prior to any hormone treatment. This suggests that epigenetic factors may be involved in the development of gender incongruence itself – pointing to a biological basis that is neither purely genetic nor purely social.

Transgenerational epigenetic effects 🔗

Sexual differentiation of the brain is a dynamic process that is initiated prenatally but continues throughout life, shaped by the interaction of gonadal hormones, sex chromosomes, and the environment. Epigenetic signatures can even be inherited – meaning that a parent’s environmental experiences may influence the biology of their children and grandchildren. This transgenerational dimension of epigenetics adds a profound layer of complexity to how we understand gender development across populations and across time.

Challenging the deterministic view of biology and gender 🔗

Perhaps the most important insight from modern biological research is this: biology influences gender, but it does not determine it. The classical biological approach once assumed a direct, one-to-one relationship between chromosomal sex and gendered behavior. That model has been significantly complicated – and in some respects dismantled – by the weight of evidence.

A large-scale review by Hyde (2005) examined psychological differences between males and females across dozens of studies and found that the vast majority of measured differences between the sexes were very small or negligible. The behavioral overlaps between males and females are far greater than the differences – a finding that directly contradicts strong biological determinism.

Cases of Complete Androgen Insensitivity Syndrome (CAIS) further illustrate this complexity. Individuals with CAIS are chromosomally male (XY) but cannot respond to testosterone due to a receptor mutation. They develop as females, are raised as girls and women, and almost universally report feeling comfortable in their female gender identity. Out of 156 individuals studied in CAIS research, not a single case of male gender identity was reported. Since the Y chromosome is present but testosterone cannot act on the brain, this evidence strongly suggests that it is hormonal response – not chromosomal makeup alone – that is the more powerful driver of gender identity.

Reiner and Gearhart (2004) further complicated the picture by studying 16 genetic males born without a penis. Fourteen were surgically reassigned and raised as girls. Yet eight of those 14 eventually reassigned themselves as males by age 16 – suggesting that biological factors, particularly prenatal hormone exposure, can be powerful enough to override assigned gender identity and socialization.

These findings collectively reinforce the view that the nature vs. nurture debate around gender has become increasingly irrelevant. The question is no longer whether biology or environment shapes gender – it is how they continuously interact throughout a person’s lifetime. Cross-cultural evidence adds another dimension: many societies recognize gender identities beyond the binary, including the Hijra in India, the Muxes in Mexico, and the Bugis people of Indonesia – pointing to the role culture plays in shaping gender expression even within the constraints of biology.

What this means for how we understand gender 🔗

The biological approach to gender provides an essential foundation. Chromosomes, hormones, and genes are real and measurable influences on development and behavior. The conditions of Turner syndrome and Klinefelter syndrome, the behavioral effects of testosterone and estrogen, and the epigenetic signatures linked to gender incongruence all point to a biology that is active, influential, and deeply relevant. But biology does not operate in a vacuum. It is continuously shaped by experience, environment, and context. Gender, as science now understands it, emerges from the intersection of all these forces – not from any single one of them in isolation.

Understanding this does not diminish the importance of biology. If anything, it makes the biology more interesting – because it shows us that even our most fundamental physical characteristics are part of a living, responsive system that interacts with the world around it.

What do you think? If hormones, chromosomes, and epigenetics all contribute to gender development in different ways and to different degrees, does it still make sense to talk about a biological “basis” for gender? And how should scientific research on the biology of gender be used – or kept separate – from social and political conversations about gender identity?

References

1. https://senecalearning.com/en-GB/revision-notes/a-level/psychology/aqa/10-1-4-biological-influences-on-gender
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC6677266/
3. https://www.psychologytoday.com/us/blog/natured-nurture/202311/hormones-determine-sex-more-than-genetics
4. https://www.savemyexams.com/a-level/psychology/aqa/25/revision-notes/gender-option-topic-section-b/explanations-of-gender-development-/biological-explanations-of-gender-development/
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC7301881/
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7. https://pmc.ncbi.nlm.nih.gov/articles/PMC2670935/
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