Sexual Behaviour – Lecture 5

Week 4 Revision

Following last week, you should be able to discuss:
- Basic processes from sensation ➔ perception.
- Relationship between electromagnetic radiation and vision.
- Basic eye anatomy (retina & photoreceptors).
- Transduction & retinal processing of light.
- Pathway of visual information from eye ➔ striate & extrastriate cortices.
- Higher‐level visual processing & perception.

Defining Sex & Gender

Sex = biological status.
Gender = social, learned, personal aspects of sex.
Phenotype = biology × environment.

Sex Differences in the Body

Reproductive anatomy, shoulders, height, total skeletal muscle mass, immune activation, taste & touch sensitivity, liver metabolism.

Mediators of Sex Differences

Continued gene expression on X & Y chromosomes (especially in brain).
Sex hormones.
Epigenetic modifications of gene expression.

The Human Genome

Humans possess $23$ chromosome pairs; the $23^{\text{rd}}$ pair = sex chromosomes.
Karyotype = visual chromosome profile.
Eggs always carry an $X$ ; sperm carry $X$ or $Y$ ⟹
- $XX$ ➔ biological female.
- $XY$ ➔ biological male.

Sexual Chromosome Abnormalities

Turner Syndrome $\,(45,\,XO)$
- Incidence $1\text{/}2000$ .
- Normal female external genitalia, atypical ovaries → infertility; short stature; neck skin folds; normal IQ with mild spatial/memory deficits.
Klinefelter Syndrome $\,(47,\,XXY)$
- Incidence $1.79\text{/}1000$ male births.
- Male, reduced fertility, need puberty hormone therapy; normal IQ, social awkwardness, delayed/reduced verbal skills.
Jacobs Syndrome $\,(47,\,XYY)$
- Incidence $1\text{/}1000$ male births.
- Taller, leaner, acne, minor anomalies, slightly lower IQ, higher language difficulty & ASD risk.
Androgen‐Insensitivity Syndrome (AIS)
- Genotype $46,XY$ with defective androgen receptor.
- Fetus cannot respond to testosterone ➔ phenotypic female with female gender identity.
Congenital Adrenal Hyperplasia (CAH)
- Excess fetal androgens from adrenal gland.
- Masculinises female fetuses (ambiguous genitalia).
- Few observable effects in males.

Three Stages of Prenatal Development

Gonadal Development (≈ $6$ weeks)
- SRY gene on $Y$ expressed → testis‐determining factor + testosterone.
Differentiation of Internal Organs (≈ $8$ weeks)
- Testes secrete:
  - Testosterone → development of Wolffian system (seminal vesicles, vas deferens).
  - Anti‐Müllerian hormone → degeneration of Müllerian system.
- In absence of these, Müllerian system forms uterus, upper vagina, fallopian tubes.
Development of External Genitalia
- Requires androgen $5\text{α}$ -dihydrotestosterone.

Development at Puberty

Activation of the hypothalamic–pituitary–gonadal (HPG) axis:
- GnRH ➔ FSH & LH.
- Testes produce testosterone (+ estrogens); ovaries produce estradiol (+ androgens).
Hormonal effects:
- Males: muscle growth, facial hair, laryngeal enlargement, voice deepening.
- Females: breast & uterine growth, fat redistribution.
- Both: maturation of external genitalia.

Organisational vs. Activational Roles of Sex Hormones

Three sensitive periods:
\begin{aligned}
1.&\; 6\text{–}24 \text{ weeks post-conception}\
2.&\; Last prenatal weeks ➔ 6 $–$ 12 months (“mini-puberty”)\
3.&\; Puberty
\end{aligned}
Organisation = permanent structural changes (prenatal & early postnatal).
Activation = reversible, state-dependent effects (throughout life).
Organisational-Activational Hypothesis: early steroid hormones permanently organise nervous system ⇢ adult behaviour.

Synthesis of Human Sex Hormones

Sex hormones = steroid hormones derived from cholesterol.
Production sites: gonads, adrenal glands, brain, bone, fat.
Key pathway: cholesterol → progesterone → testosterone → (via aromatase) estradiol.

Sexual Dimorphism in the Brain

Estradiol + progesterone with absence of androgens ⇒ female development.
Testosterone ⇒ male-typical behaviour; estradiol can suppress female-typical behaviour.
Notable dimorphic nuclei:
- SDN-POA in rats (≈ $2.2$ × larger in males).
- INAH-2/3 in humans (≈ $2$ × larger in males; linked to sexual orientation).

Markers of Prenatal Hormone Exposure

2D:4D finger ratio
- Androgen & estrogen receptors: digit $2$ < digit $4$ under high prenatal testosterone.
- Lower ratio predicts superior sports performance, competitiveness, left-handedness, ASD risk (controversial).

Gender Identity

Self-concept as male or female; partly biologically influenced.
Transgender: gender identity ≠ biological sex; brain structures often align more with identity than sex (evidence mixed).

Sexual Orientation

Stable attraction toward a sex.
Prevalence estimates: $4.9\%$ adult males & $2.1\%$ adult females identify as gay/lesbian; $5.1\%$ males & $7.2\%$ females identify as bisexual.
Kinsey Scale measures continuum hetero → homo attraction.

Hormones & Sexual Orientation

Animal studies: prenatal androgen exposure reorganises SDN-POA affecting partner preference.
Humans: majority of CAH individuals are heterosexual.
Fraternal birth-order effect: each older brother increases odds of male homosexuality.

Additional Correlates

2D:4D ratio: homosexual women intermediate between heterosexual women & men; no effect in men.
Otoacoustic emissions: heterosexual females > bisexual females > homosexual females ≈ males.
LeVay (1991): INAH-3 $2$ – $3$ × larger in heterosexual vs. homosexual men (correlational).
Cognition: mental-rotation task shows homosexual men between heterosexual men & women; other tasks show no simple pattern.
Self-rated masculinity/femininity varies widely, arguing against simple “inversion” models.

Sex Differences in Behaviour

Toy preferences emerge $12$ – $18$ months; parallel findings in non-human primates.
CAH girls play more with “boys’ toys” than unaffected girls but less than boys; spend less time with “girls’ toys”.

Sex Differences in Cognition

Prenatal androgens: ↓ empathy, ↑ aggression.
Typical adult patterns (small effects):
- Males: slight visuospatial advantage.
- Females: slight verbal advantage.
Hormone manipulation:
- Testosterone supplementation (older men) ↑ spatial performance.
- Women: high testosterone → ↑ spatial; high estrogen → ↓ spatial but ↑ verbal fluency & manual dexterity.

Sex Differences in Personality

Females score higher on neuroticism, agreeableness, openness.
Higher empathy in women; higher physical aggression in men.
Fetal testosterone inversely correlates with empathy.

Sex Differences in Disorders

Many brain disorders show sex bias.
Autism Spectrum Disorder ratio ≈ $4:1$ (male:female); females likely underdiagnosed.

Attraction

Beauty preferences exist early, preceding cultural/media influence.
Facial symmetry linked to perceived health & attractiveness.
Female mate preferences:
- More masculine faces for short-term mating; less masculine for long-term.
- Masculine traits imply dominance, less fidelity, poorer parenting but stronger immunity genes.
Other cues: youthful female features; scent preference for complementary immune systems (MHC).

Romantic Love vs. Sexual Desire

Distinct yet overlapping neural systems.
- Caudal insula (posterior) ↔ sexual desire.
- Rostral insula (anterior) ↔ romantic love.
fMRI (Bartels & Zeki $2000$ ): viewing a lover activates reward regions, suppresses negative-emotion & social-judgment areas.
Pituitary hormones:
- Oxytocin (higher in women): released during hugging, intercourse, childbirth, lactation; enhances bonding.
- Vasopressin (higher in men): linked to pair bonding.

Reproduction & Parenting

Goal of sexual behaviour = offspring survival.
Males can produce many offspring → higher promiscuity, lower selectivity.
Females invest gestation & care → higher selectivity, lower promiscuity.
Monogamy favoured for offspring safety; elevated oxytocin & vasopressin promote monogamous behaviour (e.g., prairie voles).
Human study: intranasal oxytocin ↑ preferred distance from an attractive stranger & ↑ perceived attractiveness of one’s own partner only.

Lecture Review – Key Learning Outcomes

Basics of sexual development & genome.
Three prenatal development stages.
Pubertal development mechanisms.
Biological sex differences & sexual orientation.
Sex differences in behaviour, cognition, personality, disorders.
Mechanisms of attraction, romantic love, sexual desire, reproduction & parenting.