Lecture 11: Personality & Genetics: Quantitative

The Jim Twins

• Jim Springer and Jim Lewis, identical twins separated four weeks after birth, reunited at age 39, sharing striking coincidences.
  • Both named "Jim" by adoptive parents.
  • Shared similar physical characteristics: weight of 180 pounds, 6 feet tall.
  • Shared interests: mechanical drawing and carpentry.
  • Both had police training and worked part-time with law enforcement.
  • Marital similarities: Divorced women named Linda, remarried women named Betty.
  • Love notes to wives, similar names for firstborn sons (James Alan/Allan).
  • Shared habits: chain-smoking Salem cigarettes, nail-biting, liking Miller Lite beer.
  • Similar health issues: high blood pressure, perceived heart attacks, vasectomies, tension headaches starting at eighteen, and weight gain.
  • Vacationed on the same beach in Florida and drove the same model light blue Chevrolet.
• Dr. Thomas Bouchard administered psychometric tests revealing similar personality scores (tolerance, conformity, flexibility) and intelligence test scores.
• The Jim twins were the first participants in the Minnesota Study of Twins Reared Apart (MISTRA).

Limitations of the Jim Twins Case Study

• Case studies may be subject to confirmatory bias, where similarities can be found even in randomly selected individuals if one looks hard enough.
• Despite limitations, coincidences in the Jim twins' lives remained remarkable compared to other MISTRA twins.
• Genes do not code for specific preferences like marrying someone named "Betty."
• The case highlights the question of genetics' role in shaping personality.

Basic Concepts in Behavioural Genetics

• Genes are small parts of DNA (Deoxyribonucleic Acid).
• DNA's double helix consists of deoxyribose (a sugar), a phosphate group, and a base.
  • Four types of base: Adenine, Thymine, Cytosine, Guanine.
• Specific base pairing allows DNA to replicate and direct protein synthesis.
  • DNA replication occurs during cell division, creating two identical DNA double helices.
  • Humans begin as a single cell replicating DNA into trillions of cells.
  • Approximately 50 trillion cells make up the human body.
• DNA directs protein synthesis, with base sequences coding for amino acid sequences.
  • A gene is a base sequence coding for one protein chain.
  • Examples of Amino Acid & DNA Code: Alanine (CGA, CGG, CGT, CGC), Glutamine (GTT, GTC), Phenylalanine (AAA, AAG)
  • Gene regulators control protein synthesis by turning genes on or off in different cells.
• DNA containing genes is organized into chromosomes.
  • Humans usually have 46 chromosomes (two sets of 23).
  • A genome is an entire set of chromosomes; the human genome has about 3.3 billion base pairs.
  • These base pairs contain about 25,000 protein-coding genes, ranging in size from about 1000 bases to 2 million bases.
• New DNA differences arise from mutations during DNA copying.
  • Single-base mutations: a base pair is left out, added, or substituted, changing the protein a gene produces.
  • Alternative forms of a gene are called alleles, differing by one or more bases but occupying the same locus on the chromosome.
  • Mutations in egg or sperm cells (gametes) can be passed to the next generation, leading to different characteristics.
• Genotype: an individual's unique combination of alleles.
• Phenotype: an observed characteristic resulting from the interaction between genotype and environment.
  • Phenotypes include Huntington's disease, height, weight, or personality traits.

Quantitative Genetics goals

• Estimating the heritability of quantitative traits.
• Estimating the influence of shared and non-shared environment on quantitative traits

Goal 1: Heritability

• Heritability can be defined as the proportion of the total variance in the phenotype which is due to variance in the genotype.
• Heritability is a population statistic, meaning:
  • Heritability estimates are not constant or immutable.
  • Heritability estimates cannot be applied to a single individual.

Goal 2: Environmental Influence

• Shared factors: environmental factors experienced the same by siblings in the same family.
  • Examples: number of books in the home, some parenting practices, SES, religion.
• Non-shared factors: environmental factors experienced differently by siblings in the same family.
  • Examples: differential treatment from parents, different teachers, friends, hobbies, life events.

Estimating Heritability

• Twin studies
• Adoption studies

Twin Studies

• If genetic factors affect a quantitative trait, phenotypic resemblance of identical twins should be greater than fraternal twins.
  • Identical twins (monozygotic or MZ) share 100% of their genes.
  • Fraternal twins (dizygotic or DZ) share approximately 50% of their genes.
• A rough estimate of heritability can be obtained in the following manner: $h^2 = 2(r<em>{mz} – r</em>{dz})$
  • MZ twins share 100% of variable genes, DZ twins roughly 50% of variable genes – the difference in their correlations then reflects half of the genetic effect and is doubled to estimate heritability:
  • Example: Loehlin & Nichols (1976) administered a Dominance Scale to twins $r<em>{mz} = +.57$ $r</em>{dz} = +.12$ $2(.57-.12) = .90$
• The variance not explained by genetics is attributed to the environment and measurement error.
• Less reliable scales produce lower heritability estimates.
• Results for adolescent or adult twins tested using questionnaire or other self-report methods (Zuckerman, 2005).
  • Extraversion
  • Neuroticism
  • Adolescent or Adult Twins: Correlations and Heritabilities ($h^2$) on Extraversion
    • Study: Eaves et al. 1989
      • Population: 36 studies before 1976
      • Test/Subjects: EPI
      • ${r_{mz}}$: .53
      • ${r_{dz}}$: .24
      • $h^2$: .58
    • Study: Zuckerman 1991
      • Population: 7 studies 1976-1988
      • Test/Subjects: Various countries, ages
      • ${r_{mz}}$: .54
      • ${r_{dz}}$: .19
      • $h^2$: .54
  • Adolescent or Adult Twins: Correlations and Heritabilities ($h^2$) on Neuroticism
    • Study: Eaves et al. 1989
      • Population: 22 studies before 1976
      • Test/Subjects: EPI
      • ${r_{mz}}$: .44
      • ${r_{dz}}$: .22
      • $h^2$: .44
    • Study: Zuckerman 1991
      • Population: 7 studies 1976-1988
      • Test/Subjects: Various countries, ages
      • ${r_{mz}}$: .46
      • ${r_{dz}}$: .22
      • $h^2$: .46

Limitations of Twin Studies

• Twin studies compare identical (MZ) and same-sex fraternal (DZ) twins to control for environmental factors.
• The twin method assumes that environments experienced by identical twins are no more similar than those of fraternal twins (equal environments assumption).
• If MZ twins are more alike than DZ twins, the higher concordance is attributed to their higher genetic overlap.
• The equal environments assumption may be violated in two ways:
  • Equal prenatal environments?
    • Roughly three-quarters of MZ twins share a common placenta and chorionic sac (monochorionic), while DZ twins are always dichorionic
    • Maternal effects may contribute to greater personality concordance in MZ twins.
  • Equal postnatal environments?
    • Parents may treat identical twins more similarly because they look more alike.
    • The postnatal equal environments assumption appears reasonable for most traits.
    • MZ twins don't seem to resemble each other more in personality if treated alike.
    • MZ twins believed to be DZ are as concordant as correctly classified MZ twins for personality traits.

Adoption Studies

• Adoption studies disentangle genetic and environmental sources of family resemblance.
• Adoption creates pairs of genetically related individuals who do not share a common family environment.
• Types of adoption studies:
  • Non-twin adoption studies (biological parents and adopted-away offspring; siblings adopted apart).
  • Separated twin studies (MZ or DZ twins adopted apart).
  • Studies of identical twins raised apart (MZ-A) without contact during formative years are most powerful.
• The correlation on a trait between separated identical twins directly estimates heritability ($h^2 = r_{MZ-A}$).
• Results of adult monozygotic twins raised apart (MZ-A) (Zuckerman, 2005).
  • Extraversion
  • Neuroticism
  • Correlations on Extraversion - Twins Raised Apart
    • Study/Test: Shields 1962/MPI
      • Population: UK
      • MZ-A: .61
    • Study/Test: Pederson et al. 1988/EPI Short form
      • Population: Sweden
      • MZ-A: .30
    • Study/Test: Langinvainio et al. 1984/EPI Short form
      • Population: Finland
      • MZ-A: .38
    • Study/Test: Bouchard 1993/MPQ
      • Population: USA
      • MZ-A: .40
    • Study/Test: Bouchard 1993/CPI
      • Population: USA
      • MZ-A: .60
    • Study/Test: Bouchard & Hur 1998/MBTI
      • Population: USA
      • MZ-A: .60
  • Correlations on Neuroticism - Twins Raised Apart
    • Study/Test: Shields 1962/MPI
      • Population: UK
      • MZ-A: .53
    • Study/Test: Pederson et al. 1988/EPI Short form
      • Population: Sweden
      • MZ-A: .25
    • Study/Test: Langinvainio et al. 1984/EPI Short form
      • Population: Finland
      • MZ-A: .25
    • Study/Test: Bouchard 1993/MPQ
      • Population: USA
      • MZ-A: .53
    • Study/Test: Bouchard 1993/CPI
      • Population: USA
      • MZ-A: .55

Limitations of Adoption Studies

• Selective placement: foster homes vetted by social service agencies may be more similar than families at large.
• If foster families are more homogenous, heritability estimates may underestimate the effect of family environment.

Concluding Comments

• Genetic research on personality has focused on extraversion and neuroticism.
• Estimates of heritability differ due to age, tests, reliability, countries, and so on.
• Advanced model-fitting analyses across twin and adoption designs produce heritability estimates of about 50% for extraversion and about 40% for neuroticism (Loehlin, 1992).
• These genetic effect sizes are amongst the largest effect sizes found anywhere in the behavioural sciences!
• Other dimensions in the FFM
  • Openness to experience: 45%
  • Conscientiousness: 38%
  • Agreeableness: 35%

Importance of Environment

• Despite its name, behavioural genetics is as useful in the study of environment as it is in the study of genetics.
• Indeed genetic research provides the best available evidence yet for the importance of the environment.
• Heritability estimates rarely exceed 50% and thus the environmental influence is rarely less than 50%.