Lecture 11: Personality & Genetics - Quantitative

The Jim Twins

The central message from behavioural genetics involves genetic individuality.
Each individual, except identical twins, is a unique genetic experiment.
Jim Springer and Jim Lewis were identical twins separated four weeks after birth and reunited at age 39.
They attracted media attention due to extraordinary coincidences in their lives, including:
- Both named "Jim" by adoptive parents.
- Each weighed 180 pounds and was 6 feet tall.
- Both had a pet terrier named "Toy" in childhood.
- Shared interests in mechanical drawing and carpentry.
- Both had police training and worked part-time in law enforcement.
- Both married and divorced women named "Linda," then remarried women named "Betty."
- Both enjoyed leaving love notes for their wives.
- One named his firstborn son "James Alan," the other "James Allan."
- Both chain-smoked Salem cigarettes, chewed their fingernails, and liked Miller Lite beer.
- Both had high blood pressure, experienced what they thought were heart attacks, and underwent vasectomies.
- Both started having tension headaches at 18 and gained ten pounds simultaneously.
- Both took family vacations on the same beach in the Florida Gulf Coast.
- Both drove the same model light blue Chevrolet.
Dr. Thomas Bouchard administered psychometric tests to the Jim twins.
- Personality test scores (tolerance, conformity, flexibility) were nearly identical.
- Intelligence test scores were very similar.
The Jim twins were the first participants in the Minnesota Study of Twins Reared Apart (MISTRA).
The Jim Twins are a case study, with limitations.
- Confirmatory bias: similarities can be found even in randomly selected individuals if you look hard enough.
- Compared to other MISTRA twins, the level of coincidences in the lives of the Jims remained remarkable.
Genes do not code a preference for marrying women named “Betty.”
The case of the Jim twins highlights the role of genetics in shaping personality.

Lecture Outline

Basic concepts in behavioral genetics.
Quantitative genetics
- Estimating heritability
  - Twin studies
  - Limitations
  - Adoption studies (separated twins)
  - Limitations
- Estimating environmental influence
  - Genes and environments
- Molecular Genetics
  - Allelic Association Method
  - Selected findings

Basic Concepts

Genes are small parts of a long molecule called DNA (Deoxyribonucleic Acid).
The double helix of DNA consists of:
- Deoxyribose (a sugar).
- A phosphate group.
- Bases: Adenine, Thymine, Cytosine, Guanine.
The specific pairing of bases allows DNA to:
- Replicate itself.
- Direct the synthesis of proteins.
Replication of DNA occurs during cell division.
- The DNA molecule unzips, separating paired bases.
- Each strand attracts appropriate bases to construct its complement.
- Two identical double helices of DNA are created.
Humans begin as a single cell that reproduces DNA in trillions of cells.
Approximately 50 trillion cells make up the human body.
DNA directs the synthesis of proteins.
- Base sequences are a code for the cell to make sequences of amino acids.
- Amino acids make up the thousands of proteins in living organisms.
A gene is a length of base sequence that codes for one protein chain.
Gene regulators turn specific genes on or off in different cells to control protein synthesis.
The long molecules of DNA containing genes are organized into chromosomes.
Humans usually have 46 chromosomes (two sets of 23).
An entire set of chromosomes is called a genome.
- The human genome is composed of about 3.3 billion base pairs.
- These base pairs contain about 25,000 protein-coding genes.
- Gene size ranges from about 1,000 bases to 2 million bases.
New DNA differences occur when mistakes (mutations) are made in copying the DNA.
A single-base mutation results when a base pair is left out, added, or substituted.
- This leads to a change in the protein a gene produces.
Alternative forms of a gene are called alleles.
- Alleles differ by one or more bases but occupy the same location on the chromosome.
New mutations in egg or sperm cells (gametes) can be passed on to the next generation.
These alleles become the genetic basis for different characteristics.
Genotype: an individual’s unique combination of alleles.
Phenotype: an observed characteristic resulting from the interaction between genotype and the environment, including personality traits.
- This interaction begins in the womb and continues throughout life).

Quantitative Genetics

Research goals:
1. Estimating the heritability of quantitative traits.
2. Estimating the influence of shared and non-shared environment on quantitative traits.
Heritability: the proportion of the total variance in the phenotype due to variance in the genotype.
- Heritability is a population statistic.
- Heritability estimates are NOT constant or immutable.
- Heritability estimates CANNOT be applied to a single individual.
Behavioral geneticists try to determine the relative influence of shared and non-shared environmental factors.
Shared factors: environmental factors experienced the same by siblings growing up in the same family.
- Physical (e.g., number of books, video games).
- Psychological (e.g., parenting practices, sibling interactions).
- Social variables (e.g., SES, urban or rural setting, religion).
Non-shared factors: environmental factors experienced differently by siblings growing up in the same family.
- Differential treatment from parents.
- Different teachers, friends, hobbies.
- Different life events (accident or illness).

Estimating Heritability

Two main methods:
- Twin studies.
- Adoption studies.

Twin Studies

If genetic factors affect a quantitative trait, phenotypic resemblance should be greater in identical twins 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 as follows: $h^2 = 2 (r{mz} – r{dz})$
- MZ twins share 100% of variable genes, DZ twins roughly 50%.
- The difference in their correlations reflects half of the genetic effect and is doubled to estimate heritability.
Example: Loehlin & Nichols (1976) administered a Dominance Scale to twins:
- $r_{mz} = +.57$
- $r_{dz} = +.12$
- $2(.57-.12) = .90$
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

Eaves et al. 1989 (36 studies before 1976, EPI): $r{mz} = .53$ , $r{dz} = .24$ , $h^2 = .58$
Zuckerman 1991 (7 studies 1976-1988, Various countries, ages): $r{mz} = .54$ , $r{dz} = .19$ , $h^2 = .54$

Neuroticism

Eaves et al. 1989 (22 studies before 1976, EPI): $r{mz} = .44$ , $r{dz} = .22$ , $h^2 = .44$
Zuckerman 1991 (7 studies 1976-1988, 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.
Both types of twins share broad environmental features.
The twin method assumes that environments experienced by identical twins are no more similar than those experienced by fraternal twins (equal environments assumption).
If MZ twins are more alike than DZ twins, the higher concordance is attributed to their higher genetic overlap.
However, the equal environments assumption may be violated:
- Equal prenatal environments? Roughly three-quarters of MZ twins share a common placenta and chorionic sac (monochorionic), whereas DZ twins are always dichorionic.
  - These "maternal" effects may contribute to greater personality concordance in MZ twins.
- Equal postnatal environments? Parents may treat identical twins more similarly than fraternal twins 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 terms of personality if they were treated alike.
  - MZ twins believed to be DZ are just as concordant as correctly classified MZ twins for personality traits.

Adoption Studies

Adoption studies are the most direct way to 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 (e.g., biological parents and their adopted-away offspring; siblings who have been adopted apart).
- Separated twin studies (MZ or DZ twins adopted apart).
- Studies of identical twins raised apart (MZ-A) without any contact during their formative years are most powerful.
  - The correlation on a trait between sets of separated identical twins is a direct estimate of heritability, where $h^2 = r_{MZ-A}$ .

Adoption Studies: Results of Adult Monozygotic Twins Raised Apart (MZ-A) (Zuckerman, 2005)

Correlations on Extraversion - Twins Raised Apart

Shields 1962/MPI (UK): $r_{MZ-A} = .61$
Pederson et al. 1988/EPI Short form (Sweden): $r_{MZ-A} = .30$
Langinvainio et al. 1984/EPI Short form (Finland): $r_{MZ-A} = .38$
Bouchard 1993/MPQ (USA): $r_{MZ-A} = .40$
Bouchard 1993/CPI (USA): $r_{MZ-A} = .60$
Bouchard & Hur 1998/MBTI (USA): $r_{MZ-A} = .60$

Correlations on Neuroticism - Twins Raised Apart

Shields 1962/MPI (UK): $r_{MZ-A} = .53$
Pederson et al. 1988/EPI Short form (Sweden): $r_{MZ-A} = .25$
Langinvainio et al. 1984/EPI Short form (Finland): $r_{MZ-A} = .25$
Bouchard 1993/MPQ (USA): $r_{MZ-A} = .53$
Bouchard 1993/CPI (USA): $r_{MZ-A} = .55$

Limitations of Adoption Studies

Selective placement: foster homes may be more similar to each other than the environments encountered in families at large.
If foster families are more homogenous, heritability estimates based on adoption studies will underestimate the effect of family environment.

Concluding Comments

Genetic research on personality has focused primarily on extraversion and neuroticism.
Estimates of the heritability of these two dimensions differ from study to study due to variations in:
- Age of participants.
- Tests used.
- Test reliabilities.
- Countries in which studies were conducted.
Advanced model-fitting analyses across twin and adoption designs produce heritability estimates of:
- About 50% for extraversion.
- About 40% for neuroticism.
These genetic effect sizes are amongst the largest effect sizes found anywhere in the behavioral sciences!
Heritability estimates:
- Openness to experience: 45%.
- Conscientiousness: 38%.
- Agreeableness: 35%.
- One study (Bergeman et al, 1993) found a much lower heritability estimate for Agreeableness (12%).
Despite its name, behavioral genetics is as useful in the study of environment as it is in the study of genetics.
Genetic research provides the best available evidence yet for the importance of the environment.
Heritability estimates rarely exceed 50%, so the environmental influence is rarely less than 50%.