Nature & Nurture

Heritability Estimate: Extraversion

0.53

Heritability Estimate: Neuroticism

0.41

Heritability Estimate: Conscientiousness

0.44

Heritability Estimate: Agreeableness

0.41

Heritability Estimate: Openness

0.61

Jang et al. (1996)

• Twin study (MZ vs DZ)

• Heritability of The Big Five

Johnson et al. (2008); van den Berg et al. (2014); Polderman et al. (2015); Vukasović and Bratko (2015)

• Twin studies

• Resemblance between MZ twins compared to DZ twins

• Heritability estimates = 40%

Bouchard and Loehlin (2001); Rijsdijk and Sham (2002)

• Family studies

• Resemblance between parent/biological child compared to relatives separated by adoption

• Heritability estimate ~30%

van den Berg et al., 2014

• Meta-analysis of data from over 29,000 twin pairs

• Heritability estimates do not appear to vary by sex

Definition of Heritability

Denotes the proportion of variation in the characteristic that is associated with variation in genetic material.

Estimates of heritability

• Heritability is a number between 0 and 1, like a correlation.

• Care is needed when interpreting heritability estimates.

Candidate Gene Approach

• Late 1990s and early 2000s

• Hypothesis driven

• Constrained by prior knowledge

• Allows systematic scanning

Ebstein et al. (2014)

DRD4 dopamine receptor gene

• Polymorphic gene (i.e. occurs in several forms) including 2 to 11 repeats

• 3 most common variants: DRD4 2R, DRD4 4R, DRD4 7R

• DRD4 7R = less dopamine binding in the expressed dopamine receptor

Candidate genes approach: Extraversion

• Linked to DRD4 7R (low dopamine binding)

• People with at least one 7-repeat allele have higher Novelty Seeking than other genotypes.

Garcia et al. (2010)

Dopamine D4 Receptor Gene Variation: Infidelity and Sexual Promiscuity

• Infidelity in people with vs without 7R gene = 50% vs 20%

• Promiscuity in people with vs without 7R gene = 45% vs 25%

Serotonin transporter gene (5-HTT)

• Terminates the action of serotonin by facilitating its reuptake from the synapse.

• 5-HTTLPR: a polymorphism in the promoter region of the 5-HTT gene

• A 44bp deletion/insertion generated two alleles of 5-HTTLPR: Short (14-repeat) and Long (16-repeat).

5-HTTLPR

A polymorphism in the promoter region of the 5-HTT (serotonin transporter) gene

44bp deletion/insertion

Generated two alleles of 5-HTTLPR

• Short (14-repeat)

• Long (16-repeat).

Candidate genes approach:Neuroticism & the serotonin transporter gene

Lesch et al. (1996)

• 1-2 copies of Short 5-HTTLPR allele = higher neuroticism score

• The 5-HTT polymorphism accounted for:

  • 3-4% of total variation (and 7-9% of inherited variance) in anxiety-related personality traits in individuals.

Munafo et al. (2003)

Counterargument for candidate gene studies

• 79 studies: evidence is strongest for HTTLPR, some indications for DRD4.

• However, the effects are small

  • A few percent of variance is explained; may also vary with sex and ethnicity.

• Would require 1000s of Ps to reliably detect these associations

Even if they are real, they don’t explain much of the genetic basis of personality.

Genome-Wide Association Studies (GWAS) Approach

Instead of having a candidate gene in mind, you set markers across the whole genome.

• Should find all variants associated with the phenotype of interest.

1. Hypothesis-free

2. Exploratory

3. Lower power

4. Large number of variants

GWAS and the problem of the missing heritability

For personality, each GWAS finds some associations, but:

• These initially explain <1% of the variance in personality

• Usually did not replicate across studies.

Moutinho (2021)

Identical twins are not genetically identical

• Starts before embryos form a mass of cells

• Mutations during cell replication resulted in 5.2% of genetic differences.

Caspi et al (2003)

Number of stressful live events moderates expression of 5-HTT genotype (short/short, short/long, long/long)

Caspi et al (2010)

• Environmental exposure frequency affects researchers' ability to detect genetic effects and gene-environment (G×E) interactions

• High and low exposure (<10%, >90%) significantly reduce ability to detect interactions

Explains replication failures of previous studies

Epigenome

A secondary layer of biochemical information that modifies or marks the genome in a way that alters the instructions given by the DNA.

Limitation of Heritability Studies - Epigenetics

Not part of the DNA

• Influence which genes are turned active and inactive at different times and places in the body.

Genome → largely static within an individual

Epigenome → altered by the environment.

• Epigenetic changes can be passed from generation to generation via transgenerational epigenetic inheritance.

Heritability and Personality: Summary

• Candidate genes for involvement in personality variation, but their effects are small.

• GWAS studies had not found a genetic basis of personality and raised the problem of the missing heritability.

• This is improving in recent years, and is aided by modern theoretical and methodological advances

  • e.g. machine learning methods

• Massive polygenicity.

• G x G and G x E interactions/epigenetic factors may be important in personality.

Early experience and personality

• Childhood experience could explain the variation in personality

  • Everyone agrees that heritability is not 100%.

• Previously measured personality with broad, explicit self-rating scales

  • May not be true with more specific or behavioural measures.

Belsky et al. (1991); Belsky (2012)

Psychosocial Acceleration Theory

• Theory of the development of reproductive strategies

The Belsky, Steinberg and Draper Hypothesis (1991)

Individuals differ in sex and relationship behaviours, like fidelity, age at onset of sexual activity, sociosexuality, etc.

• Promiscuous sexuality may be an adaptive response to certain types of early experience

Unsupportive care from caregivers in the first 5-7 years of life is a cue of harsh ecological conditions.

• A fast life history strategy may be an adaptive response to ensure reproductive fitness in an unpredictable/harsh environment

Consequence of Harsh Development Context

According to the Belsky, Steinberg and Draper Hypothesis (1991):

• Earlier pubertal maturation and initiation of sexual activity,

• Short-term and unstable pair-bonds

• Limited parental investment as individuals sought to bear more children but not care for them intensively.

Nettle et al., 2011 - Method

Prospective study of about 3000 women born in 1958.

• The outcome was age at first pregnancy.

• Predictors of four measures of parental investment/stability in the first 7 years of life:

  • Being breastfed

  • Longest separation from mum

  • Dad’s role

  • Residential moves.

• Belsky, Steinberg, and Draper's hypothesis predicts that the earlier the first pregnancy, the more adversities there are.

Nettle et al., 2011 - Findings

No difference:

• Breastfeeding

Younger pregnancy for:

• Maternal separation for 6-24 months

• Absence of father

• 2 or more residential moves

Number of exposures to adversity (0-4) correlated with age of pregnancy

Alvergne et al. (2008)

Menarche and sexual behaviour are different in girls from one-parent households

• Living with single mother:

  • First had sex at a younger age

  • Increased no. of sexual partners

Koehler & Chisholm (2007)

Childhood adversity and infidelity in adulthood

• Retrospective Study (participants reflect on their childhoods to provide an estimate of early psychosocial stress).

• EPC: extra-pair copulations

Early psycho social stress → Higher no. of EPC-S&O (Cheating and Homewrecking!)

Nurture and Personality: Helsinki Study

(Pesonen, 2008)

Children in 1940-4 who were evacuated away from their families (random selection)

• More likely to start menstruation early (2.1) and have more than 3 children (1.9)

• Less likely to get divorced (0.7)

Puberty Prediction: Other findings

• Maltreated girls reached pubertal maturity 8 months earlier than non-maltreated girls (Costello et al., 2007).

• Harsh parenting at age 4.5 years predicted earlier menarche

  • Indirectly fostered greater sexual risk-taking in adolescence (Belsky et al., 2007, 2010).

• Does not appear to apply to males (James et al., 2012).

Maestripieri (2005)

Effects of early experience on female behavioural and reproductive development in rhesus macaques

• Responsiveness to stress mediates the relationship between abuse and infant handling

Nature vs Nurture: Mortality and Age-related diseases

Argentieri et al. (2025)

• Exposome (Environmental Exposures) → 17% of the variation in mortality risk

• PRS (Polygenetic Risk Score) → less than 2% of the variation in mortality risk

RBFOX1

A RNA-binding protein that regulates splicing in neurons

Been linked to:

• Alzheimer's disease

• Autism spectrum disorders

• Bipolar disorder

• Tourette's syndrome.

The Puzzle of Causality

• Candidate genes are, at least, complete genes.

• SNPs are parts of genes, markers, modifiers, etc…

• But the basic mechanism of organismal biology requires genes!

Fundamentals of “Evo Devo” biology

20,000 genes that code for proteins.

An estimated 100,000 and over 1,000,000 unique types of protein.

• Alternative splicing

  • ABCDE

  • ABD

  • ABCE

• Post-translational modification

• Protein folding

• Epigenetics

• … and literally all of organismal biology

Development encompasses a lot of things!

• Deep homology

• Toolkit genes

• Internal regulatory networks

Developmental systems theory

All biological processes operate by continually assembling new structures.

• Each structure transcends the structures that made it

  • Has its own systematic characteristics, information, functions and laws.

• Each structure is irreducible to any lower (or higher) level of structure

  • It can be described and explained only on its own terms.

• Many more layers of structure would be required to explain the complexity of processes such as evolution and heredity

  • Conventional concepts of ‘gene’ and ‘environment’ may be too simplistic

Developmental Systems Theory - Genetics

• A developmental system can be thought of as similar to “species”.

• So we are, each, developmental systems.

• Genetic effects are impossible to interpret outside of the developmental system.

• Genetics are inseparable from the environment.

Zwir et al. (2020)

Machine learning method for GWAS to uncover complex genotypic–phenotypic networks and environmental interactions.

• Identified:

  • “clusters of SNPs” within individuals (GxG interactions)

  • clusters of people with “distinct profiles of character traits”.

• Found 42 SNP sets that identified 727 gene loci and were significantly associated with one or more of the character profiles.

Zwir et al. (2020) - Big Five

Gene sets modulate brain processes such as:

• Intentional goal-setting → Conscientiousness

• Self-reflection → Neuroticism

• Empathy → Agreeableness

• Episodic learning and memory → Openness/Intellect/IQ

Missing ‘plasticity’ traits

Zwir et al. (2020) - Conclusion

Gene-character modulations appeared consistent across cultures and environments

• Explained nearly all the heritability expected for a character in each sample (50 to 58%).

Personality heritability is not missing, but is distributed in multiple networks of interacting genetic and environmental variables that influence different people