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Biological explanations of gender argue that differences in male and female behaviour, cognition, and identity are rooted in chromosomes, hormones, and brain structure. On this account, the foundations of being male or female are laid down before birth by genetic and hormonal events, and gender-typed behaviour later in life is a downstream consequence of that early biology. This is a respectful, factual area of psychology: it concerns the developmental biology of sex and the extent to which that biology shapes the psychological experience of gender. This lesson examines the role of the sex chromosomes (XX and XY) and the SRY gene, the influence of the hormones testosterone, oestrogen, and oxytocin, atypical sex-chromosome patterns (Klinefelter's and Turner's syndromes), and two illuminating natural cases — the David Reimer case and the Batista family (5-alpha-reductase deficiency) — before evaluating the strengths and limits of a biological account.
Key Definition: Sex refers to the biological status of being male or female (chromosomes, hormones, anatomy). Gender refers to the psychological and cultural attributes associated with being male or female. Biological determinism is the view that human behaviour is controlled by biological factors (genes, hormones, brain structure) and that free will plays little or no role; applied to gender it claims that gender identity and gender-typed behaviour are determined by biology.
This lesson addresses the following points from the AQA A-Level Psychology (7182) specification, Paper 3, Section B: Gender:
| Specification point | Coverage in this lesson |
|---|---|
| The role of chromosomes and hormones in sex and gender | XX/XY chromosomes; the SRY gene; testosterone, oestrogen and oxytocin |
| Atypical sex chromosome patterns: Klinefelter's syndrome and Turner's syndrome | XXY and X0 — physical and psychological characteristics, and what they reveal |
| The use of case studies and natural experiments to inform the nature–nurture debate in gender | David Reimer (Money; Diamond & Sigmundson, 1997); the Batista family (5-alpha-reductase deficiency) |
These points are assessed through short-answer AO1 questions (outline the role of chromosomes/hormones; describe an atypical pattern), AO2 application where a scenario stem is supplied, and extended-writing essays worth up to 16 marks that demand a balance of AO1 description and AO3 evaluation — typically anchored in the nature–nurture and reductionism debates.
Humans have 23 pairs of chromosomes. The 23rd pair determines biological sex:
The key gene is the SRY gene (Sex-determining Region Y), located on the Y chromosome. At approximately 6–8 weeks of prenatal development:
Key Definition: The SRY gene is the gene on the Y chromosome responsible for initiating male sexual development. It triggers the production of testosterone in the developing foetus.
The logic of prenatal sex differentiation can be summarised as a developmental cascade in which the chromosomal "switch" determines which set of hormones is released, which in turn determines anatomy and brain development:
graph TD
A["Genetic sex set at conception<br/>XX or XY"] --> B{"SRY gene present?"}
B -->|"Yes (XY)"| C["Testes develop ~6–8 weeks"]
C --> D["Testosterone released"]
D --> E["Male anatomy +<br/>masculinisation of the brain"]
B -->|"No (XX)"| F["Default pathway:<br/>ovaries develop"]
F --> G["Female anatomy<br/>(no androgen surge)"]
This cascade is important for evaluation: it shows that sex is not a single event but a sequence, and a disruption at any point (a missing chromosome, an enzyme deficiency, an unusual hormonal environment) can produce an outcome that does not follow the typical XX→female / XY→male mapping. The atypical patterns and case studies below are, in effect, naturally occurring disruptions of this cascade.
While chromosomes provide the initial biological blueprint, the relationship between chromosomes and gender identity is not straightforward. As the case of David Reimer demonstrates, having XY chromosomes does not guarantee a male gender identity if socialisation pushes in the opposite direction — although Reimer's eventual rejection of his female identity suggests biology plays a powerful role.
Hormones are chemical messengers produced by the endocrine system. Three hormones are particularly important for gender development:
| Function | Effect |
|---|---|
| Prenatal development | Masculinises the foetus: develops male genitalia, influences brain development |
| Puberty | Deepening voice, muscle development, body hair, increased aggression |
| Behaviour | Linked to aggression, dominance, and competitiveness |
| Function | Effect |
|---|---|
| Prenatal development | Feminises the foetus in the absence of testosterone |
| Puberty | Breast development, widening of hips, regulation of the menstrual cycle |
| Behaviour | Associated with mood regulation and emotional sensitivity |
| Function | Effect |
|---|---|
| Bonding | Released during childbirth, breastfeeding, and physical touch |
| Social behaviour | Promotes trust, empathy, and social bonding |
| Gender link | Women produce more oxytocin, which may explain greater emphasis on nurturing and social bonding in female gender roles |
Exam Tip: For questions on the role of hormones, always name a specific study with findings. Van Goozen et al. (1995) is the strongest evidence for testosterone's role in gendered behaviour.
Studying individuals with atypical sex chromosome patterns provides natural "experiments" that illuminate the role of chromosomes and hormones in gender development.
| Feature | Description |
|---|---|
| Chromosomes | 47, XXY (an extra X chromosome in males) |
| Prevalence | ~1 in 660 males |
| Physical characteristics | Tall, long limbs, breast development (gynecomastia), small testes, reduced muscle mass, often infertile |
| Psychological characteristics | May have poorer language abilities, passive and shy temperament, difficulties with social interaction |
| Gender identity | Typically identify as male, but may show less stereotypically masculine behaviour |
| Feature | Description |
|---|---|
| Chromosomes | 45, X0 (only one X chromosome, no second sex chromosome) |
| Prevalence | ~1 in 2,500 females |
| Physical characteristics | Short stature, webbed neck, shield chest, ovaries do not develop (streak gonads), infertile without medical intervention |
| Psychological characteristics | Higher rates of spatial processing difficulties, typically normal verbal ability, may have difficulties with social cognition |
| Gender identity | Typically identify as female, often described as stereotypically feminine in behaviour |
Key Definition: Atypical sex chromosome patterns are conditions in which an individual has a different number or arrangement of sex chromosomes from the typical XX or XY. They provide insight into the biological basis of gender.
Simon Baron-Cohen proposed that male and female brains are systematically different:
| Brain Type | Description | Associated Sex |
|---|---|---|
| Empathising brain (Type E) | Excels at understanding and responding to others' emotions; good social cognition | More common in females |
| Systemising brain (Type S) | Excels at understanding rule-based systems (machines, mathematics, maps) | More common in males |
| Balanced brain (Type B) | Roughly equal empathising and systemising abilities | Found in both sexes |
Baron-Cohen linked this to autism spectrum conditions (ASC), which he described as an "extreme male brain" — very high systemising and very low empathising.
Strengths:
Limitations:
David Reimer (born Bruce Reimer in 1965) was one of identical twin boys. At 8 months old, his penis was accidentally destroyed during a botched circumcision.
Psychologist John Money recommended that Bruce be raised as a girl, "Brenda." Money believed that gender is entirely socially constructed and that if a child is raised consistently as one gender, they will develop that gender identity.
In reality, the case was far from successful:
| Interpretation | Explanation |
|---|---|
| Supports biological determinism | Despite being raised as female, David rejected his assigned gender — suggesting that biology (XY chromosomes, prenatal testosterone exposure) played a powerful role in gender identity |
| Challenges social constructionism | Money's theory that gender can be entirely determined by socialisation was contradicted |
| Ethical concerns | The case raises serious ethical issues — Money's intervention caused immense psychological harm |
Strengths:
Limitations:
Exam Tip: The David Reimer case is a powerful evaluation point for any question about the nature-nurture debate in gender. Use it to argue that biology plays a significant role, but acknowledge the limitations of a single case study.
A second, contrasting natural case comes from a cluster of families in the Dominican Republic studied by Imperato-McGinley and colleagues (1974). A number of children in these families have a genetic condition called 5-alpha-reductase deficiency. These children are genetically male (XY) and have functioning testes, but they lack the enzyme 5-alpha-reductase, which the body needs to convert testosterone into dihydrotestosterone (DHT) — the hormone responsible for the prenatal development of external male genitalia.
| Stage | What happens | Outcome |
|---|---|---|
| Prenatal | XY foetus produces testosterone, but cannot make DHT | External genitalia appear ambiguous or female-like; child is registered and raised as a girl |
| Childhood | Child develops and is socialised as female | Female gender role; "she" is the social identity |
| Puberty | A surge of testosterone now masculinises the body | Voice deepens, muscle develops, the phallus enlarges — the body becomes recognisably male |
Locally these children became known as "Guevedoces" (roughly, "penis at twelve"). The striking finding is that the majority of these children, despite being raised as girls, adopted a male gender identity and male gender role at puberty with relative ease, going on to live as men. Because the social environment had treated them as female throughout childhood, the shift towards a male identity at puberty is hard to explain on a purely social-learning account; it appears that the prenatal exposure to testosterone (which had still masculinised the brain) reasserted itself once puberty supplied a second androgen surge.
| Case | Genetic sex | Reared as | Eventual identity | Interpretation |
|---|---|---|---|---|
| David Reimer | XY | Female (after injury) | Male (rejected female identity) | Biology resisted socialisation |
| Batista children | XY | Female (ambiguous genitalia) | Male (at puberty) | Biology reasserted at puberty |
Read together, both cases point in the same direction — that prenatal androgen exposure exerts a powerful, possibly decisive influence on gender identity that socialisation alone cannot easily override. However, they must be handled with care: the Batista families form a culture in which the condition is known and partly anticipated, so the social "expectation" of femininity may have been weaker than in a typical Western upbringing, which is itself a confounding variable.
Exam Tip: Pairing the Reimer case with the Batista (5-alpha-reductase) families gives you two converging lines of natural-experiment evidence for the role of prenatal hormones — a far stronger AO3 argument than relying on a single case study.
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