Genetics of Intelligence

Bouchard & McGue (1981)

IQ correlations

• Parent-child = 0.42

• Siblings = 0.47

Explanation for Bouchard & McGue’s (1981) Findings

• Heredity: IQ has a genetic component

• Nurture: High IQ parents transmit high IQ by the way they nurture their children

• Shared Environment: Families share environmental factors that determines variation in IQ

• Some combination of the above

IQ Correlations of MZ twins

Bouchard & McGue (1981)

4672 pairs in 34 studies

• Expected correlation = 1.00 (if IQ is genetic)

• Actual correlations = between 0.58 and 0.96

• Weighted Average = 0.86

IQ Correlations of DZ twins

Bouchard & McGue (1981)

992 pairs in 8 studies

• Expected correlation = 0.50 (If IQ genetic)

• Actual correlations = between 0.21 and 0.87

• Weighted average = 0.60

Suggests environmental influence

IQ Correlations in Adoption Studies - Siblings

Bouchard & McGue (1981)

• DZ twins ~0.60

• Siblings ~0.47

Twins have more similar environment (in utero, age), indicates envrionmental influence

IQ Correlations in Adoption Studies - Parents

Bouchard & McGue (1981)

• Parent-Child ~0.42

• Parent-Adopted Child ~0.19 (environmental influence)

IQ Correlations in Adoption Studies - MZ Twins

65 pairs in 3 studies

• Expected correlation = 1.00 (if IQ genetic)

• Actual correlations = between 0.62 and 0.76

• Weighted Average = 0.72

Estimating Heritability

2 x (MZ correlation – DZ correlation) = environmental influence

Haworth et al. (2010) - Methods

• USA (Ohio; Minnesota; Colorado), UK, Australia, Netherlands

• Child samples (mean ages 5-18y)

  • 4809 MZ pairs of twins

  • 5880 DZ pairs of twins

• Web-based cognitive tasks (UK)

  • Ravens Matrices

  • Weschler Intelligence Scale for children

Haworth et al. (2010) - Correlations

• MZ = 0.76

• DZ = 0.49

Double the difference → 2 x (0.76 – 0.49) = 0.54

Haworth et al. (2010) - Heritability

• 54% of the variance seen in IQ is attributable to genetic variance (‘a’)

• 24% to non-shared environment ‘e’

  • (1- monozygotic correlation: 1-0.76)

• 22% to shared environment ‘c’

  • (monozygotic correlation - genetic effect: 0.76-0.54)

• HOWEVER…

  • Only shows us heritability estimates for these specific populations.

Turkheimer et al. (2003) - Method

Heritability of IQ Varies Across Social Groups

• Analysed WISC scores from several hundred 7-year-old monozygotic and dizygotic twins.

• Explored how the additive effects of genotype, shared environment, and nonshared environment interacted with SES.

Turkheimer et al. (2003) - Findings

Results demonstrate that the proportions of IQ variance attributable to genes and environment vary nonlinearly with SES.

• Low SES = 60% of the variance in IQ scores was accounted for by shared environment.

  • The contribution of genes was close to zero.

• In higher-income families, the reverse was true.

Turkheimer et al. (2003) - Summary

• SES and heritability = positive relationship

• SES and environmental influence = negative relationship

• Genes can have more or less of an impact depending on the environment.

• However

  • Some studies may lack generalisability

Herrnstein & Murray (1994)

“The Bell Curve” (1994)

• Controversial argument concerning group differences.

• African Americans' average score was 15 points lower than that of White Americans.

The argument:

• African Americans have lower IQ scores, which is highly heritable.

Therefore…

• Lower attainment of African Americans is due to genetic differences between the races.

Limitations of Herrnstein & Murray (1994)

Heritability estimates:

• Do not tell you how much of a trait is due to genetics

• Estimate % of difference in trait in that specific population

Inappropriate to make the assumptions made in “The Bell Curve”

• Significant evidence that differences in environmental factors can explain differences in IQ.

• Does not acknowledge how environmental factors, such as SES, which would explain the difference in intelligence between whites and African Americans.

IQ and Heritability Across the Lifespan - Haworth et al. (2010

Childhood → Adolescence → Adulthood

• The effect of genes increases

• The effects of shared environment decrease.

The Wilson Effect

• MZ → Concordance increases with Age

• DZ → Less concordant with age (match non-twin siblings)

Therefore, heritability is mediated by age

Bouchard (2013)

Heritability studies

Concordance Rates For:

• Shared environment decreases

• Genetic increases

• Polygenetic increases (curvilinear relationship)

Why do genetically driven differences increasingly account for differences in general cognitive ability as we get older?

• Younger – Imposed environment

• Older – Self-selected environment driven by self and own genetic predispositions

Martin et al. (1986)

• Humans are exploring organisms

• Select what is most relevant and adaptive from the range of opportunities due to innate predispositions

• The effects of mobility and learning, therefore, augment rather than eradicate the effects of the genotype on behaviour

Multiplier Effect

Genetic tendencies that guide behaviour will result in a change in the environment that magnifies the original tendency

• Example: A Child likes puzzles

  • Because the child likes puzzles, you may engage them with more puzzles them

  • Improve their already strong puzzle-solving abilities

Deary, Penke, & Johnson (2010)

GWAS have not found common variants that explain much of the genetic variance in IQ across a normal range.

• Only a few relevant genetic loci have been discovered.

• ‘Problem of the missing heritability’

• Suggested that many mutations of small effect are most likely part of the genetic architecture.

Davis et al

2010 – 2018

• The longer the study has been ongoing, the number of indicated genes increases

• Found 709 genes involved in the variation of IQ.

Definition of an SNP

• Variation at a base pair due to substitution

• Can occur in areas coding for proteins and in intergenic regions.

• Needs to occur in at least 1% of the population (not rare)

• Set of 23 chromosomes (genome) and 3 billion base pairs

Function of SNPs

They account for genetic differences between all individuals around the world

• Appearance

• Reaction to illness

Variants are passed down

• These variants can be used to establish how closely related people are

Davies et al. (2018) - Individual SNPs and intelligence

300,486 participants; 57 studies (inc. UK Biobank)

• 11,600 = interrelated

• 434 = independent

• Occurred at 148 regions along the 22 chromosomes (many outside of regions that code for genes).

Davies et al. (2018) - SNPs Across Whole Genes

• Tested 18,264 genes.

• SNP variation at 709 genes was associated with intelligence

25% of the variation observed in participants’ intelligence test scores was accounted for by DNA SNP variation.

• This is much better than previous studies

Evidence that intelligence is a polygenic trait

What do these genes and DNA variations do? (Davis et al., 2020)

Related to the development of nerve cells and the nervous system.

• Some variations appear to also be related to health

• (i.e. to height, weight, BMI, lung cancer, Crohn’s disease, bipolar disorder, schizophrenia, autism, Parkinson’s disease, and Alzheimer’s disease).

Genetic correlations - Traits and Illnesses

Positive Genetic Correlations with Intelligence

• The same genetic variants that are related to larger brains are also related to higher IQ.

• The same genetic variants that are related to autism are also related to higher IQ (note the positive direction).

Negative Genetic Correlations with Intelligence

• ADHD (r = -0.37)

• Alzheimer’s disease (r = -0.37)

• Schizophrenia (r = -0.23)

• Major Depressive Disorder (r = -0.30)

• Neuroticism (r = -0.16)

• Low health satisfaction (r = -0.26)

Explanation for Cardiovascular disease (CVD) with Intelligence

• Is linked to cognitive impairment and dementia.

• The main underlying cause of CVD is atherosclerosis.

• Atherosclerosis may induce:

  • Brain hypoperfusion,

  • Oxidative stress

  • Inflammation

• Could contribute directly to the development of the neuropathology

Explanation for Correlations with Intelligence

• The brain is not a singular organ

• Is located within the environment of the body

• If the body is a hostile environment, this would have a cost to the brain

Correlates of g: Nerve conduction velocity

• Measured by applying a burst of current through the skin over a nerve and picking up its time of arrival further up or down the nerve

Good correlations with IQ

Correlates of g: inspection time (IT)

• Pattern for a very short period of time (50 milliseconds or less), followed immediately by a mask.

• You have to identify it as one of two possible shapes

High IQ participants are faster

Correlates of g: reaction times – Der & Deary (2017)

Number in the top box

• Easy – always 0

• More difficult (still low demand) –1-4

Press the corresponding number

• In all conditions, reaction time is faster for higher IQ.