PSY1202 Introduction to Biological Psychology - Sex Differences

Sex Differences

Sex, Gender, and Sexuality

  • This lecture focuses on biological sex, not gender identity or sexual orientation.
  • Gender Identity:
    • Personal identification as male, female, non-binary, etc.
    • A sociocultural construct correlated with biological sex.
  • Sexual Orientation:
    • Romantic/sexual attraction to people of the same/other genders and sexes.
    • Heterosexual, homosexual, bisexual, asexual, etc.
    • Moderately heritable, but genetic basis unclear.
    • Same-sex sexual behavior is common across animal species.

Biological Sex

  • Biological sex refers to differences due to sex chromosomes (XX vs. XY) and subsequent developmental changes.
  • Exceptions:
    • People with other genotypes (e.g., Turner syndrome (XO), Klinefelter syndrome (XXY)).
    • People with XX/XY genotype but atypical development due to mutations (e.g., androgen insensitivity syndrome, 5α-reductase deficiency).
    • People who have undergone medical procedures to alter sex/gender phenotype (e.g., hormone therapy, gender reassignment surgery).

Sexual Dimorphism

  • Sexual dimorphism refers to differences between sexes in anatomy, physiology, psychology, and behavior.
  • The lecture discusses differences between 'women' (XX) and 'men' (XY), but these are averages.
  • There is more variation within each sex than between them, especially for psychological and behavioral traits.

Types of Hormone Action and Sex Differences

  • Sexual differentiation involves genetic, gonadal, and phenotypic aspects.
  • Sex differences are explored in behavior and the brain.

Organisational vs. Activational Effects

  • Organisational Effects:
    • Effects on tissue differentiation and development.
    • Permanent.
  • Activational Effects:
    • Effects that occur in a fully developed organism.
    • May depend on prior exposure to organisational effects of hormones.
    • Transient.
  • Sexual differentiation in mammals depends on organisational effects of hormones during development.

Genetic Basis of Sex Differentiation

  • DNA is present in the nucleus of every cell (except red blood cells).
  • DNA is packaged in chromosomes.
  • Diploid human cells contain 46 chromosomes (23 matched pairs):
    • 22 pairs of autosomes.
    • 1 pair of sex chromosomes.
  • Biological sex is determined randomly by the type of male gametes (sperm cells).
  • Gametes are haploid: just one copy of each chromosome.
  • Gametes fuse to create diploid offspring.

Sex Determination

  • Male Heterogamety: XY
  • Female Heterogamety: ZW
  • Other methods include polygenic sex determination, haplodiploidy, cytoplasmic sex determination, paternal genome elimination, monogeny, temperature-dependent, and social factors.
  • Hermaphrodites: simultaneous and sequential.
  • Examples of environmental sex determination include temperature and social factors.

Role of SRY Gene

  • SRY (sex-determining region Y) gene: also known as testis-determining factor (TDF) gene.
  • Biological sex is determined by the presence or absence of the Y chromosome in the sperm cell.

Gonadal Sex Differentiation

  • Undifferentiated 'primordial' gonads develop into ovaries or testes.
  • At 6 weeks, the baby is undifferentiated.
  • During early embryonic development:
    • In the absence of the SRY gene, the gonad develops into an ovary ('default' sex).
    • In the presence of the SRY gene, the gonad develops into a testis.
  • The testis produces various androgens, including testosterone, and anti-Müllerian hormone (AMH).
    • Testosterone masculinises many other tissues, including effects on the brain.
    • AMH masculinises internal genitalia (Müllerian regression factor).
  • Ovaries do not produce significant amounts of steroid hormones during embryonic development.
  • Each part of the body develops according to its own intrinsic program.
  • Testosterone is a steroid hormone, fat-soluble, and passes readily through the cell membrane, synthesized by Leydig cells in the testes.
  • Sertoli cells produce AMH.

Sex-Determining Cascade

  • XX: no SRY gene → ovaries → feminisation (default).
  • XY: SRY gene → SRY protein → testes → masculinising hormones (AMH + androgens (T, DHT)).
  • Genotypic sex determines gonadal sex, which determines phenotypic sex.

Phenotypic Sex Differentiation

  • Genotypic sex determines gonadal sex.
  • Differential exposure to sex steroids (e.g., testosterone, DHT) during critical periods of development (e.g., foetal development, puberty) causes:
    • Sexual differentiation of the body.
    • Sexual differentiation of the brain and behavior.
    • These are permanent effects.
  • Gonadal sex determines phenotypic sex.
  • In males: testosterone + 5-alpha-reductase → dihydrotestosterone (DHT) → development of the shaft and head of the penis, and scrotum.
  • In females: absence of testosterone and DHT → development of the clitoris, labia minora, and labia majora.
  • Masculinisation and de-feminisation occurs with the effect of hormones present early in development promotes later development of anatomical or behavioral characteristics typical of males.
    • AMH: internal genitalia.
    • DHT: external genitalia.
    • Testosterone: rest of body (incl. brain).

Puberty

  • Further organisational effects of hormones at puberty:
    • Testosterone released by the testis.
    • Hormones released by the anterior pituitary (growth hormone, GH, gonadotropic hormones, FSH and LH, and adrenocorticotropic hormone, ACTH) lead to the development of secondary sexual characteristics (not present at birth).

Sex Differences in Behavior

  • Examples shown include comparison of male and female typical behaviors.

Aggressive Behavior

  • Homicide rates:
    • Data from US homicide reports, 1976-1999, indicates males are more often offenders and victims.
    • Note:96.7% of same-sex homicides are male-maleNote: 96.7\% \text{ of same-sex homicides are male-male}
    • Men's severe violence to men is much more frequent than women's severe violence to women.
    • Most victims and offenders are young men.
    • This pattern is stable across cultures and over time.
    • This suggests involvement of testosterone in male-male aggressive behavior, although the evidence is not so convincing for human populations (e.g., influence of environment, social aspects).
  • Examples across different studies and countries highlight this trend.
    • The proportion of male-male homicides is shown to be higher than female-female homicides across various studies.

Mammalian Oestrous Cycle

  • Cyclic patterns of gonadotropin secretion (FSH, LH), present only in females, prompted by surge in GnRH (from hypothalamus) above tonic levels.
  • Most female mammals are only sexually active during oestrous phase ('on heat').
  • Pro-oestrus: follicular development and ovulation.
  • Oestrus: receptive period during which fertilisation is most likely to lead to pregnancy.
  • In non-human mammals, it is generally obvious due to behavioral cues.
  • Example: Female cats 'call' at night and display lordosis response.

Female Hormones and Human Menstrual Cycle

  • Menstrual Cycle:
    • Fertile window conception is only likely if sperm is present in the reproductive tract when ovulation takes place.
    • Studies have tried to find whether behavior correlates during ovulation, with not much evidence of differences in sexual activity across different phases of menstrual cycle.

Premenstrual Dysphoric Disorder

  • Severe premenstrual disorder.
  • Affects 3-8% menstruating women.
  • Occurs within one week before period starts (luteal phase).
  • Symptoms: Mood swings, depression, anxiety, irritability, difficulty concentrating, sleep problems, physical symptoms.
  • Treatment based on hormone therapy, antidepressants, surgery.

Reproductive Behavior in Humans

  • Masters and Johnson (1966) conducted detailed lab observations of physiology and behavior during intercourse.
  • 4 stages of physiological response to sexual stimulation: excitement, plateau, orgasm, resolution.
  • Similarities between males and females.

Sex Differences in the Brain

  • Sexually dimorphic brain structures:
    • Song control region in zebra finches: 5–6 times larger in males than in females and gets bigger in females given testosterone as hatchlings.
    • Rat hypothalamus (sexually dimorphic nucleus of pre-optic area, SDN-POA): smaller in males castrated at birth, bigger in females given testosterone at birth; no effect of castration/testosterone treatment in adulthood.
  • Human Brain:
    • Brain size: On average, male brains are 120–160 g (10–15%) heavier.
    • Brain lateralisation: Female brains less strongly lateralised with respect to various functions than male brains; anatomical lateralisation of the cerebral hemispheres appears to be more marked in males.

Key points

  • Hormones may have organisational effects on tissue differentiation and development and activational effects on behavior
  • Genotypic sex (XX vs XY) determines gonadal sex, which through the production of sex steroids determines phenotypic sex
  • Production of SRY protein in genotypic males initiates a cascade of processes that masculinise (and de-feminise) the developing embryo
  • Further organisational effects of hormones at puberty lead to the development of secondary sexual characteristics
  • Men and women are more similar than different, but (like in other mammals) there are some sex differences in body, brain and behavior
  • The functional significance of sex differences in the brain is still unclear