GENETICS & PERSONALITY

PSYC 2600

Lecture 8

Dr. Kira McCabe

Test 1 Comments

■ There are students still taking make-up exams (including some people who are likely here today), so please do not discuss the exam content

■ I will post grades in Brightspace when they are available, but it likely won’t be until the week of Oct. 16 at the earliest

■ You’ll get 2 values – Multiple-choice grade and Short-answer grade, combined together for the overall

• If you want specific feedback about your exams, you can schedule a meeting with the TAs to review your exams
• I can answer general questions

■ Review the syllabus for more info (e.g., “Grading Questions”)

Learning Objectives

■ Introduce the topic of behavioural genetics

■ Define heritability and heritability estimates

■ Explore how to find genetic and environmental variance

■ Explain findings of behavioural genetics research

Controversy about Genes and Personality

■ Behavioural geneticists attempt to determine the degree to which individual differences, including personality, are caused by genetic and environmental differences.

■ Highly controversial

• Ideological concerns

■ Research used/misused to make ideological point

• Concerns about renewed interest in eugenics

■ Eugenics: Is the notion that we can design the future of the human species by

• Fostering the reproduction of people with certain traits
• Discouraging the reproduction of people without those traits

Controversy about Genes and Personality

■ Modern behavioural geneticists who study personality are

• Careful about addressing implications of work
• Are sensitive to ideological concerns

■ However, knowledge about behavioural genetics is better than ignorance

■ Finding that a personality trait has a genetic component does not mean the environment is powerless to modify the trait.

■ My additional take: If you find genetic differences between group, these are differences in group means. It cannot tell you anything about where an individual will fall within a group.

People can still vary within a genetic trait.

Goals of Behavioural Genetics

■ Determine the percentage of individual differences in a trait that can be attributed to genetic differences and percentage that can be attributed to environmental differences

■ Determine the ways in which genes and environment interact and correlate with each other to produce individual differences.

■ Determine precisely where in the “environment” environmental effects exist—e.g., parental socialization, different teachers.

Role of Genes

■ How significant of a role do your genes play in your life?

• Influences height, eye colour, facial features, etc.
• Influences your personality?
• Influences your attitudes?
• Influences your behaviour?

Highlights of Genetic History

■ 1856-1863: Gregor Mendel’s work with pea plants

■ 1951: Rosalind Franklin images of DNA (image on this slide)

■ 1953: Watson & Crick publish article on double-helix structure of DNA

■ 2000: Human Genome Project completed as first draft to map all the genes of the human genome

■ My point: This is science is quite young and developing

What is meant by “Influence”?

■ Heritability: extent to which genetics predict behaviour or trait.

• How much do genetic differences among individuals cause differences in something, such as height, extraversion, or sensation seeking.

■ Inherited: behaviour or trait is determined by genes alone.

■ We inherit genes from parents

• our genes are determined at conception

Heritable vs. Inherited

■ Physical features (such as height) are largely heritable, but not inherited.

■ Example: If both your parents are 5’2”, it doesn't mean you will be exactly 5’2”

• If average height for a population is 5’11”, then you are likely to be shorter than most (to the extent height is

“heritable”).

• Inherited would mean that you absolutely would be 5’2”

Our Genes

■ 90%+ of genes inherited are same across species (all humans share these).

■ < 10% differ across species

■ Because focus is on how we differ from each other, we look only at small percentage of genes that varies from one person to next

■ Variance: How much something varies from on an average (mean) level.

• Similar to standard deviation

■ Phenotypic variance: Observed Individual difference of interest (e.g., height, personality, etc.)

• How did we get this phenotypic variance?

■ Genes + Environment

■ Heritability (in terms of behaviour): The proportion of observed variance in a group of individuals that can be “accounted for” by genetic variance.

• What percentage of the phenotypic variance be attributed to genes?

■ Heritability focuses on genes, but there are two sources of variance in the phenotype:

■ Genotypic variance: Genetic variance that is responsible for individual differences in the phenotypic expression of specific traits.

■ Environmentality: Percentage of observed variance in a group of

individuals that can be attributed to environment.

■ Behavioural geneticist asks: “What is estimate of influence of genotype on phenotype?”

• Is it due more to genes? (more phenotypic variance attributed to the genotypic variance than environmentality)
• Is it due more to environment? (less phenotypic variance attributed to genotypic variance)
• Or is about equal parts of both genes and environment?

Phenotypic Variance of a Trait

Genotypic Variance

Environmentality

Cake Analogy (Why Not the Individual Level)

■ Need to look at the population level, not at the individual level

■ Tease apart differences due to genes and differences due to environments

• Nature-nurture debate

■ Cake analogy: You bake a cake with flour, sugar, eggs, and water

■ Is the finished cake caused more by the flour or the eggs?

• Both needed, combined and cannot be separated from finished cake.

Heritability Estimates

■ How can we quantify heritability?

• Heritability estimates

■ Heritability estimates can range from 0.0 to 1.0 (variance can only be a positive value)

• if 0.0, means no influence of genotype on phenotype
• if 1.0, means variation in phenotypic scores in a given population is entirely predicted by variation in genotypes
• If 0.50, means 50% of variability in phenotype in a given population is predicted by variation in genotypes.

What Heritability Means

■ Heritability estimates are tied to a place and time

• If everyone in a population has exactly same environment, then influence of genotypes will tend to be greater.
• if environment differs greatly across people in a population, then influence of genotypes will be less

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Traits are influenced by genotypes

■ Does not mean that genes influence behaviour directly

• Perhaps genes influence physical or physiological characteristics, which influence behaviour & personality

■ Example: Norepinephrine

• Heritability is .80
• Impulsivity Is related to sensitivity to norepinephrine
• Genes don’t make you impulsive, but it may explain different biochemical states and processes

Behavioural Genetics Methods

■ Selective Breeding— Studies of Humans’ Best Friend

■ Family Studies

■ Twin Studies

■ Adoption Studies

• Adoption Twin Studies

Selective Breeding

■ Can only occur if a desired trait is heritable.

■ Selective breeding studies of dogs (or other animals)

■ Cannot be ethically conducted with humans.

■ You can think of different personality traits of dogs and different dog breeds (or other animals).

– Dog Personality Questionnaire (link)

Family Studies

■ Correlates the degree of genetic overlap among family members with the degree of similarity in personality trait.

■ If a trait is highly heritable

– Family members with greater genetic relatedness should be more similar to one another on the trait than family members who are less closely genetically related.

Family Studies Example: Garrison & Rodgers (2019)

■ Looking at kinship pairs in longitudinal data (NLSY79) to explore whether socioeconomic status (SES) and health relationship is more environmental or genetic.

• Kinship pairs: Full siblings (n = 4,009), half-siblings (n = 297), and cousins (n = 96)
• Health: Physical and Mental Health at Age 40

■ Main findings:

• Physical health & SES: Predicted more by genes
• Mental health & SES: Predicted more by environment

Family Studies Limitations

■ Members of a family who share the same genes also usually share the same environment

• confounds genetic with environmental influences
• This doesn’t mean people can’t have different environmental influences within a family.

■ Thus, family studies are never definitive

Twin Studies

■ Twin Studies

• Monozygotic (MZ) twins are identical twins
• Dizygotic (DZ) twins are fraternal twins

■ MZ share 100% of genes

■ DZ share 50% of genes that vary across species

• Same as siblings of same parentage

Twin Studies Heritability Estimates

■ If MZ twins are more similar than DZ twins, this provides evidence of heritability.

■ Calculating heritability estimates

• Different ways to do it
• For class, I’m going to keep the math fairly simple, but there are more complex statistical models (e.g., ACE models) that tease apart this in more depth.

Extraversion Example

Doing the Math…

■ Since MZ twins share 100% of genes, and DZ twins share 50% of genes, the difference in correlations is attributable to difference of genes.

■ The formula below simply subtracts the two correlations and multiplies it by 2

■ Heritability (h) = 2(r_MZ - r_DZ)

= 2(.60 - .35)

= .50

Twin Studies Heritability Estimates

■ Because MZs share same genes, differences in phenotypes (traits) can only be attributable to “environment”.

• Differences between Twin 1 and Twin 2 not due to genes; therefore, due to “environment”

■ Differences in phenotypes between pairs of DZ twins can be attributable to different genes or different “environment”

• Some of the genes are different between DZ twins

Twin Study Limitations

■ Equal environments assumption: The twin method assumes that the environments experienced by identical twins are no more similar to each other than are the environments experienced by fraternal twins.

– General support for this assumption, but it is an ongoing concern with this kind of research

■ Representativeness: The experience of twins is not representative of the general population.

Adoption Studies

■ Positive correlations on traits between adopted children and adoptive parents provide evidence of environmental influence.

■ Positive correlations on traits between adopted children and genetic parents provide evidence of genetic influence

■ Adoption studies are powerful because they get around the equal environments assumption

■ Still have a representativeness issue with adoption generalizing to the broader population

– Problem of selective placement of adopted children

Adoption Twin Studies

■ Combine twin & adoption studies

• MZ Twins raised in the same environment (reared together)
• MZ Twins raised in different environments (reared apart)

■ MZ reared apart unlikely to have same environments as MZ reared together

■ Minnesota Study of Twins Reared Apart

• 1979 by Tom Bouchard
• Newer twin studies continue at University of Minnesota today

Big Five Heritability

■ A recent meta-analysis, for instance, identified an average heritability estimate of 48 percent across the Big Five traits (Vukasović & Bratko, 2015)

• This is in large part because personality is relatively stable

■ Several studies show neuroticism to be right around .47

• Australian twin studies, large Dutch family studies, etc.
• Adoption studies show lower heapabilities

Is Sexual Orientation Heritable?

■ Sexual orientation: Refers to one’s sexual and/or emotional attraction to others based on their sex or gender. Identities associated with sexual orientation include gay, lesbian, bisexual, pansexual, asexual, etc.

■ Tremendous animosity toward LGBTQ+ community gets as this heritability question

• Current evidence suggests that genes provide modest and indirect influence (via childhood gender nonconformity) on adult sexual orientation
• Estimates around .30, but estimates vary widely
• More research is needed

Where does your personality come from?

■ Can we go beyond just genes and environment?

• Yes

■ These twins-reared apart studies allow you to split up the variance in phenotypes (traits) due to

• Genetic similarity
• Shared family environments
• Unique environmental influences

Different Environmental Influences

■ Shared environment: In family environment, features of the environment shared by siblings.

• e.g., Kids in same family have same SES, go to same schools, share same childrearing.

■ Nonshared environment: In family environment, features of the environment that differ across siblings.

• e.g., different friends, different teachers, different relationship with parents.

Big Five: Shared & NonShared Environments

Summary of Findings

■ Substantial effect for genes (~40%)

■ No or negligible effect for common (shared) environment (i.e., non-genetic effect of being raised in same household)

■ Substantial effects for unshared (unique) environmental influences (~50%)

■ Does shared environment really not matter?

• How shared are “shared experiences”?
• How variable are shared experiences in a given sample/population?

Are attitudes and behaviours heritable?

■ Eaves, Eysenck, & Martin (1989)

• Attitudes toward religion

■ h = .30

■ shared environment = .35

■ Kessler et al., (1992)

• Perceived social support

■ h = .28

■ shared environment = .18

Are attitudes and behaviours heritable?

■ Jocklin et al., (1992)

• Divorce

■ h = .52

■ shared environment = .00

■ Plomin et al., (1990)

• TV watching (adoption study)

■ 3 yr olds: h = .54; shared environment = .18

■ 4 yr olds: h = .62; shared environment = .26

■ 5 yr olds: h = .19; shared environment = .34

Heritability of Happiness

■ Explaining individual differences in happiness – Why are some people happier than others?

• Lykken & Tellegen (1996):

■ 2000+ adult twins report Subjective Well-Being (SWB) in 1980s, then re-tested 4.5 & 10 years later

■ Heritability of SWB: h = .40

■ Stability: T1 -> T2 test-retest correlation, r = .50

Lykken, D., & Tellegen, A. (1996). Happiness is a stochastic phenomenon. Psychological Science, 7, 186-189.

Is Everything Heritable?

■ So what?

• Are there individual genes for attitudes, social support, TV watching, and divorce?

■ Not quite…

Wed, 11 Oct.

Genotype-Environment Interaction

■ People with different genotypes can respond differently to the same environments

• Example: Task performance of introverts versus extraverts in loud versus noisy conditions

■ People with different genotypes exposed to different environments

• Example: How often do you go to parties or not? Extraverts may opt into these environments more often.

Genotype-Environment Correlation

■ Genotype-environment correlations can be positive or negative

■ Three types of genotype-environment correlations

• Passive: Parents provide genes and environment, kid does nothing

■ Parents choosing number of books in home

• Reactive: Kid behavior, parent reacts to specific genotype

■ Kid likes reading, parents give kid more books or take the kid to the library more often

• Active: Kid with particular genotype seeks out environments

■ Kid chooses to spend time in libraries

If you want to learn more…

■ This is totally optional!

■ This is an article I usually assign to my advanced undergraduate & graduate course on individual differences

– Also emphasizes replication of findings in this area, which I like to share when possible

■ Plomin, R., DeFries, J. C., Knopik, V. S., & Neiderhiser, J. M.

(2016). Top 10 replicated findings from behavioral genetics. Perspectives on Psychological Science, 11(1), 3-23.

■ https://journals.sagepub.com/doi/abs/10.1177/1745691615617 439

Chapter 6 Summary and Evaluation

■ Most compelling evidence for heritability and environmentality of personality comes from findings generated across methods that do not share the same problems and limitations

■ Personality variables such as extraversion and neuroticism have moderate heritability

■ These studies suggest that these same variables have moderate to strong environmentality

■ Much of the environmental influence is due to nonshared variables—experiences unique to siblings

Next Class

■ Physiology and Personality – Chapter 7