sex diff in intelligence

ORGANISATION OF THE LECTURE

1. Part 1: Sex differences in general intelligence ($g$ factor)

2. Part 2: Sex differences in specific domains of intelligence; heritability of intelligence

3. Part 3: Critical perspectives on the research on sex differences in intelligence

IMPORTANCE OF STUDYING SEX DIFFERENCES IN INTELLIGENCE

• Psychologists aim to understand universal human cognition as well as individual differences to develop a comprehensive understanding of human mind and behavior.

• The study of group differences (e.g., sex, ethnicity) is sensitive and critical.

• Group differences may be misinterpreted, oversimplified, or misused to justify stereotypes, prejudice, and discrimination.

• Importance of methodological rigor and sensitivity in communicating research findings to a non-expert audience.

THE ‐NATURAL‐ GENDER GAP

• Global Gender Gap (World Economic Forum, Global Gender Gap Report 2021): Women are disadvantaged in:

  • Economic opportunity and participation

  • Political empowerment and participation

  • Educational attainment

  • Health and survival

• Brain research often provides justification for these gender differences by citing biological factors, such as brain size, suggesting a natural predisposition for men to be more intelligent.

PART 1: SEX DIFFERENCES IN GENERAL INTELLIGENCE

GENERAL INTELLIGENCE DEFINED

• General intelligence, measured by the $g$ factor, is indicative of cognitive ability, impacting problem-solving efficiency and information processing.

• The $g$ factor indicates the proportion of shared variance across a wide range of cognitive tasks and is a predictor for:

  • Educational attainment

  • Job performance

  • Health (e.g., Gottfredson, 1997).

EVIDENCE FROM SYSTEMATIC REVIEWS

• Court (1984): First systematic review on sex differences using approximately 120 studies focused on Raven’s Progressive Matrices, resulting in inconsistent findings:

  • Some studies showed higher female performance, others indicated male advantage, and some found no difference.

• Anderson (2004): Confirmed no sex differences found in Raven’s Progressive Matrices or Wechsler intelligence tests.

META-ANALYSIS (LYNN & IRWING, 2004; IRWING & LYNN, 2005)

• Critique of narrative analyses:

  • Lacks statistical rigor regarding sample size and effect sizes.

• Meta-analysis conducted using data from large population samples (N > 500) from 1939-2002:

  • 57 studies from 30 countries, 195 samples, total N > 80,000.

FINDINGS OF LYNN AND IRWING (2004, 2005)

• Children (up to age 15): No sex differences.

• Teenagers (ages 15-19): Men scored approximately $2$ IQ points higher than women.

• Undergraduate students: Men scored $3-5$ IQ points higher than women.

• Adults: Men scored $5$ IQ points higher than women.

• Effect sizes were small: $d = 0.16-0.30$.

STUDY BY VAN DER LINDEN, DUNKEL AND MADISON (2017)

• Sample: 896 participants (393 males, 503 females); Age: 22-37 years (Mean = 28.82, SD = 3.66).

• ‘g’ Measurement: 10 cognitive measures including:

  • Penn Progressive Matrices

  • Vocabulary size

  • Oral reading recognition test

  • Working memory

  • Episodic memory

  • Spatial orientation

  • Card sorting

  • Verbal episodic memory

  • Processing speed

  • Flanker task.

• Measures were positively correlated on average ($r = 0.243$).

METHODS USED (VAN DER LINDEN ET AL., 2017)

• MRI utilized to evaluate 7 measures of brain size:

  • Total brain size

  • Intracranial volume

  • Grey matter (GM) volume

  • White matter (WM) volume

  • GM/WM ratio

  • Cortical thickness

  • Cortical area.

• Body height controlled before analyzing correlations between $g$ and brain size measures.

RESULTS (VAN DER LINDEN ET AL., 2017)

• Males exhibited higher general intelligence ($g$): approximately $3.75$ IQ points higher than females (small effect size $d = 0.25$).

• Significant sex differences in 9 out of 10 measures:

  • Males scored higher on 6: progressive matrices, vocabulary, reading, working memory, spatial orientation, Flanker task.

  • Females scored higher on 2: episodic and verbal memory tests.

• Processing speed showed no sex difference.

SPECIFIC COGNITIVE ABILITIES (VAN DER LINDEN ET AL., 2017)

• Following removal of $g$, significant sex differences remained in 4 measures:

  • Males: higher on spatial orientation and executive function.

  • Females: higher on episodic and verbal memory tests.

• No other measures showed differences beyond what was explained by $g$.

RELATIONSHIP BETWEEN GENERAL INTELLIGENCE AND BRAIN SIZE (VAN DER LINDEN ET AL., 2017)

• Positive correlation found between $g$ and:

  • Total brain size

  • Intracranial volume

  • Grey matter volume

  • White matter volume

  • Cortical area.

• No correlation observed for GM/WM ratio and cortical thickness.

• Conclusion: Bigger brains are associated with higher $g$ for both sexes.

SEX DIFFERENCES IN BRAIN SIZE (VAN DER LINDEN ET AL., 2017)

• Males: greater total brain volume, intracranial volume, grey matter, and white matter.

• Females: higher ratio of grey to white matter.

• No significant differences in overall cortical thickness.

• Mediation analysis indicated brain size measures could account for approximately half of the sex differences in $g$.

• Higher $g$ observed in men may partly result from larger brain sizes.

EVIDENCE FROM NAVE ET AL. (2018)

• Method: Examined relationship between brain size and fluid intelligence in a large sample ($N = 13,608$), 47% males, 53% females; average age = 61.6 years (SD = 6.98).

• Controls for sex, age, body height, SES, population structure.

• Measured fluid intelligence with a 13-item verbal-numerical reasoning task.

• Results:

  • Positive relationship between total brain volume and fluid intelligence.

  • Total brain volume accounts for a small proportion of intelligence variation (\Delta R\u00b2 \approx 2%).

  • Small correlation between sex and general intelligence: fluid intelligence ($r = 0.08$), $g$ ($r = 0.10$).

RECENT EVIDENCE FROM GIOFRE ET AL. (2022) - META-ANALYSIS

• 79 studies ($N = 46,605$) on intelligence in school-aged children indicated a male advantage (negligible magnitude: 1.395 IQ points).

• Newer versions of intelligence tests (e.g., WISC) resulted in smaller, non-significant differences (0.81 IQ points).

PART 2: SEX DIFFERENCES IN SPECIFIC DOMAINS OF INTELLIGENCE AND HERITABILITY

SPECIFIC INTELLIGENCES

• Men typically outperform women in spatial intelligence:

  • Ability to generate, retain, retrieve, and transform visual images.

  • High heritability of spatial intelligence (approximately $84\%$), with $45\%$ independent of general intelligence ($g$).

BIOLOGICAL EXPLANATIONS OF SEX DIFFERENCES IN INTELLIGENCE

• Developmental Theory: Brain maturation rates and size effects on general intelligence (Lynn, 1994).

• Evolutionary Explanations: Tied to historical roles in foraging, mating, and warfare.

• Hormonal Influences: Testosterone linked to performance in mental rotation tasks.

EMPATHIZING-SYSTEMIZING THEORY (BARON-COHEN, 2002)

• Empathizing (E): Recognition and response to emotions of others. Females typically score higher ($d = 0.39$).

• Systemizing (S): Understanding systems and deriving rules. Males typically score higher ($d = 0.31$).

• Extreme Male Brain (EMB) Theory of Autism: Suggests individuals with autism show high systemizing and low empathizing traits.

PART 3: HERITABILITY OF INTELLIGENCE

DEFINITION OF HERITABILITY

• Heritability: A statistical estimate indicating the proportion of a characteristic's variance due to genetic factors within a population ($0$ to $100\%$).

HERITABILITY OF INTELLIGENCE OVER TIME

• About $50\%$ of variance in intelligence is attributed to heredity.

• Heritability increases with age:

  • Infancy: $20\%$

  • Childhood: $40\%$

  • Adulthood: $60\%$

ASSESSING GENETIC HERITABILITY

1. Family Studies: Parents to biological children ($40\%$); biological siblings ($47\%$).

2. Twin Studies: MZ twins together ($86\%$); MZ twins apart ($76\%$); DZ twins together ($55\%$\ shared genes).

3. Adoption Studies: Biological parents living apart ($31\%$); non-related adopted children together ($0\%$).

CRITICAL PERSPECTIVES ON SEX DIFFERENCES IN INTELLIGENCE

DISTINCTIONS BETWEEN SEX AND GENDER

• Sex: Biological classification (chromosomes and hormones).

• Gender: Culturally and socially rooted constructs influencing behavior.

• Neuroplasticity: Experiences can reshape brain structures over a lifetime, challenging the idea that sex differences are strictly prenatal.

• Brain Diversity: Research suggests individual brains exhibit arrays of characteristics from both sexes (‐mosaic‐ structure) rather than binary male or female types.

CONCLUSION

• Meta-analyses indicate small IQ differences favoring males with considerable overlap.

• Brain size explains a significant portion of observed differences in $g$.

• Interactions between genes, environment, and lived experiences are critical in shaping cognitive outcomes.