Biological Explanations of Gender

Biological explanations of gender argue that differences in male and female behaviour, cognition, and identity are rooted in chromosomes, hormones, and brain structure. This lesson examines these biological factors, explores atypical sex chromosome patterns, and evaluates the evidence using key case studies — most notably the case of David Reimer.

Key Definition: 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, this means that gender identity and gender-typed behaviour are determined by biology.

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The Role of Chromosomes

Sex Chromosomes

Humans have 23 pairs of chromosomes. The 23rd pair determines biological sex:

• XX → female
• XY → male

The key gene is the SRY gene (Sex-determining Region Y), located on the Y chromosome. At approximately 6–8 weeks of prenatal development:

• If the SRY gene is present (XY), it triggers the development of testes, which produce testosterone. This leads to the development of male reproductive anatomy and masculinisation of the brain.
• If the SRY gene is absent (XX), the default developmental pathway is female — ovaries develop, and female reproductive anatomy forms.

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.

Chromosomes and Gender Identity

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.

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The Role of Hormones

Hormones are chemical messengers produced by the endocrine system. Three hormones are particularly important for gender development:

Testosterone

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

• Van Goozen et al. (1995) studied female-to-male transsexuals receiving testosterone and found increases in aggression and spatial ability, while male-to-female transsexuals receiving anti-androgens showed decreased aggression.
• This provides strong evidence that testosterone influences gendered behaviour.

Oestrogen

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

Oxytocin

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.

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Atypical Sex Chromosome Patterns

Studying individuals with atypical sex chromosome patterns provides natural "experiments" that illuminate the role of chromosomes and hormones in gender development.

Klinefelter's Syndrome (XXY)

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

Turner's Syndrome (X0)

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

What Do Atypical Patterns Tell Us?

• Klinefelter's (XXY): The extra X chromosome is associated with more "feminine" behaviours, supporting the idea that chromosomes contribute to gender-typed behaviour.
• Turner's (X0): Despite having only one X chromosome and no Y, individuals typically identify as female and show "feminine" behaviour, supporting the idea that the default developmental pathway is female.
• These conditions demonstrate that the relationship between chromosomes and gender is complex and not reducible to a simple XX = female / XY = male binary.