Lecture 8 Slides - Sexual Dimorphism

Historical Perspective

• Quote by Paul Broca (1824 – 1880):

  • "We are therefore permitted to suppose that the relatively small size of the female brain depends in part upon her physical inferiority and in part upon her intellectual inferiority."

Scientific Method in Psychology

• Emphasizes importance of observation, value judgment, and moral (ethical) judgment.

• Distinctions made:

  • Greater/More than (>) - Less than (<)

  • Better than - Worse than

  • Good - Bad

  • Not equal to/Different from (≠)

• Karl Popper:

  • Advocated for empirical falsification as a criterion of scientific validity.

• Objective of science: To observe and explain phenomena and build theories with testable predictions.

Definition of Sexual Dimorphism

• Sex: Biological qualities distinguishing males from females.

• Dimorphism: From Greek "dimorphos" meaning "of two forms".

• Sexual Dimorphism: Differences in form between individuals of different sexes in the same species.

Ways to Differentiate Individuals by Sex

• Chromosomal/Genetic sex

• Hormonal sex

• Gonadal sex

• Morphological sex: Includes genitals, body size, and shape.

• Brain type

• Sexual orientation

• Gender identity

• Sex-typical behaviour

• References for parameters:

  • a. Migeon & Wisniewski (1998)

  • b. Nieve (2008)

  • c. Baron-Cohen (2005)

Sexual Differentiation

• Begins with chromosomes:

  • Typical humans share 22 common chromosomes; 23rd pair differentiates sexes.

• Typical Female: XX; Typical Male: XY.

Mechanisms of Sexual Differentiation

Primordial Gonads:

• Definition: The primordial gonads are the initial reproductive structures in embryonic development that can develop into either ovaries or testes depending on genetic and hormonal influences.

• Development: Present in the early stages of embryonic development, primordial gonads appear around the 6th week of gestation.

• Differences: If the SRY gene on the Y chromosome is active, the primordial gonads develop into testes; in the absence of this gene (typically in XX embryos), they develop into ovaries.

• Function: The primordial gonads eventually produce sex hormones and gametes (sperm in males and eggs in females), playing a crucial role in sexual differentiation and reproduction

• Influenced by interaction of genes, proteins, and hormones:

  • SRY gene on Y chromosome produces testis determining factor at 6 weeks gestation.

  • Activation of sex-limited genes by androgens/oestrogens influences sex characteristics (e.g., breast growth, facial hair).

Role of Sex Hormones

• Released by gonads and adrenal glands.

• Affect brain, genitals, and other organs.

  • Types of sex hormones:

    • Androgens

    • Oestrogens

• Both sexes possess both hormones but in different quantities.

Organizing vs. Activating Effects of Hormones

• Organizing effects: Occur during critical developmental phases, influencing sexual characteristics.

• Activating effects: Occur throughout life, influencing behaviors such as sexual activity.

Chromosomal Conditions

• Turner Syndrome (XO): Female with abnormal ovaries, intact external genitalia, normal intelligence, but spatial relationship deficits.

• Klinefelter Syndrome (XXY): Male with reduced fertility, hormone treatment needed for secondary sex characteristics, normal intelligence but social awkwardness.

• Jacob’s Syndrome (XYY): Associated with increased risk of learning disabilities, delayed speech/motor skills, controversial link to antisocial behavior.

Sexual Dimorphism in Brain Imaging Studies

• Male brains: Larger grey matter volume in specific areas (cerebellum, amygdala).

• Female brains: Increased grey matter volume in language-related regions and other cortical areas.

• Global size comparison: Male brains are generally larger even when adjusted for body size.

Neuronal Connectivity Patterns

• Female brains: Enhanced left-right (inter) hemispheric connectivity.

• Male brains: Increased intra-hemispheric connectivity.

Do Biological Differences Lead to Psychological and Behavioural Differences?

• Geschwind and Baron-Cohen's theories: Links between testosterone levels and functions; implications for lateralization and systematizing vs. empathizing traits.

Critiques on Neuro-Sexism

• Fine (2010): Cautions against over-interpretation of biological differences, advocating for a more nuanced view on cognitive abilities and brain structure functioning independent of sex.

Big-5 Personality Trait Differences

• Variability in scores across sexes, with females often scoring higher in agreeableness and emotional stability traits.

Intelligence (IQ) Differences

• Studies indicate comparable mean IQ averages across sexes, but variance in male scores tends to be greater.

Conclusions from Evidence

• Some biological differences exist (e.g., brain structure), but psychological and behavioral differences are subtle and context-dependent, influenced by both biology and environment.

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