genetics and personality

Question 1

Who were the Jim twins and why are they significant to personality psychology?

Answer 1

Identical twins separated at birth and reunited at age 39. Despite growing up in different families, they displayed striking similarities in personality, habits, jobs, and preferences—highlighting the potential role of genetics in shaping personality.

Question 2

List at least five similarities between the Jim twins.

Answer 2

Same height and weight, both had wives named Linda and Betty, sons named James, smoked Salem cigarettes, drank Miller Lite, worked as part-time sheriffs, suffered same headache syndrome, bit fingernails, left love notes for wives, similar personality test scores.

Question 3

What is the human genome?

Answer 3

The complete set of genes in an organism. Humans have 20,000–25,000 genes across 23 chromosome pairs, inherited from both parents.

Question 4

What was the goal of the Human Genome Project?

Answer 4

To sequence the entire human genome—identifying the specific DNA sequence in humans.

Question 5

What percentage of the human genome codes for proteins, and what about the rest?

Answer 5

Only about 2% codes for proteins. The remaining 98% was once thought to be ‘junk DNA,’ but now is known to play important roles in human traits, including personality.

Question 6

What are pseudogenes and riboswitches?

Answer 6

Previously misunderstood non-coding DNA elements that may impact human traits, including physical characteristics and personality.

Question 7

What is behavioral genetics?

Answer 7

A field that studies the role of genetic and environmental differences in shaping individual personality traits.

Question 8

Why is behavioral genetics controversial?

Answer 8

It challenges beliefs that environment alone shapes personality, and raises concerns about misuse (e.g., eugenics, ‘designer babies’), political agendas, and limiting beliefs about personal change.

Question 9

What is eugenics?

Answer 9

The belief that humans can be improved through selective reproduction. Associated with ethical concerns and historical misuse of genetic research.

Question 10

What is heritability in behavioral genetics?

Answer 10

The proportion of observed variation in a trait among individuals that can be attributed to genetic variation.

Question 11

Give an example showing both genetic and environmental influence.

Answer 11

Height: About 90% of height variation is genetic, but nutrition (environment) also plays a role—shown by increased average height over generations in well-nourished populations.

Question 12

Do genetic influences eliminate the role of the environment?

Answer 12

No—environmental factors still play a role, and genetic findings don’t imply traits are unchangeable.

Question 13

What is percentage of variance?

Answer 13

A statistical concept in behavioral genetics referring to how much of a trait’s variation in a population is due to genetic vs. environmental factors.

Question 14

What are some methods behavioral geneticists use?

Answer 14

Twin studies, adoption studies, and family studies. These methods help identify the genetic and environmental contributions to traits.

Question 15

What kinds of traits can behavioral genetics study?

Answer 15

Height, weight, intelligence, personality traits (e.g., extraversion), attitudes (e.g., liberalism), and behaviors.

Question 17

What is heritability?

Answer 17

Heritability is the proportion of observed variance in a group of individuals that can be attributed to genetic variance

(Plomin, 2019)

Question 18

What is phenotypic variance?

Answer 18

The observed differences in traits among individuals (e.g., height, weight, extraversion).

Question 19

What is genotypic variance?

Answer 19

The individual differences in the total collection of genes possessed by each person.

Question 20

If heritability is .50, what does it mean?

Answer 20

50% of the variation in the observed trait is due to genetic differences, and the other 50% is due to environmental differences.

Question 21

What is environmentality?

Answer 21

The proportion of phenotypic variance that is attributable to environmental (non-genetic) differences.

Question 22

Can heritability apply to an individual?

Answer 22

No. Heritability applies only to variation within a population, not to individual traits.

Question 23

Is heritability constant across time or populations?

Answer 23

No. It varies depending on the population and environmental conditions.

Question 24

Is heritability a precise statistic?

Answer 24

No. It’s an estimate that can be affected by measurement error and sample variation.

Question 25

What is the nature–nurture debate at the individual level?

Answer 25

It's meaningless—genes and environment are inseparable, like ingredients in a cake.

Question 26

What is the nature–nurture debate at the population level?

Answer 26

It is meaningful—we can compare the percentage of variance in traits due to genetic vs. environmental causes.

Question 27

Give examples of traits and their heritability.

Answer 27

Height: ~.90 (mostly genetic) Weight: ~.50 (both genetic and environmental) Mate preferences: ~.10 (mostly environmental)

Question 28

What is selective breeding and what does it show?

Answer 28

Breeding animals with desired traits; if successful, it shows the trait is heritable.

Question 29

What is the logic of family studies?

Answer 29

Closer genetic relatives (e.g., siblings) should be more similar on traits than distant relatives if traits are heritable.

Question 30

What is a limitation of family studies?

Answer 30

Families share both genes and environments, so it's hard to separate the two.

Question 31

What is the difference between monozygotic and dizygotic twins?

Answer 31

MZ (identical) twins share 100% of genes DZ (fraternal) twins share ~50% of genes.

Question 32

What does the twin study method assume?

Answer 32

The equal environments assumption: MZ and DZ twins experience equally similar environments.

Question 33

How is heritability estimated using twin studies?

Answer 33

Formula: heritability = 2(rMZ – rDZ) Example: If rMZ = .93 and rDZ = .48 → heritability = 2(.93 – .48) = .90

Question 34

What evidence supports the equal environments assumption?

Answer 34

Mislabeling studies show twins' similarity is due to genetics, not how they’re treated ## Footnote (Plomin et al., 2008)

Question 35

How do adoption studies separate genes and environment?

Answer 35

By comparing adopted children to biological and adoptive parents.

Question 36

What does a positive correlation between adopted children and adoptive parents suggest?

Answer 36

Environmental influences.

Question 37

What does a positive correlation between adopted children and biological parents suggest?

Answer 37

Genetic (heritable) influences.

Question 38

What are two potential issues in adoption studies?

Answer 38

* Representativeness: Are adoptive families typical of the general population? * Selective placement: Do adoption agencies place children into similar environments as birth parents?

Question 39

What is the strongest behavioral genetic design?

Answer 39

Studying identical twins reared apart—it isolates genetic influence by removing shared environment.

Question 40

What are the heritability estimates for Extraversion and Neuroticism based on twin studies?

Answer 40

Extraversion: ~60% heritability Neuroticism: ~54% heritability ## Footnote Studies: Floderus-Myrhed et al. (1980), Birley et al. (2006), Boomsma et al. (2018)

Question 41

What do adoption studies suggest about the heritability of Extraversion and Neuroticism?

Answer 41

Extraversion: ~40% Neuroticism: ~30% Adoptive parent–child correlations near zero indicate limited direct environmental influence.

Question 42

Which personality traits have shown moderate heritability in children and adults?

Answer 42

Activity level: ~40–50% Temperaments (e.g., emotionality, sociability): ~50% Aggressiveness: 51–72% ## Footnote Studies: Spinath et al. (2002); Oniszczenko et al. (2003); Hudziak et al. (2003)

Question 43

What is the heritability of shyness in children?

Answer 43

~44% at age 6 ## Footnote (Morneau-Vaillancourt et al., 2019)

Question 44

Are psychopathic personality traits heritable?

Answer 44

Yes—traits like Coldheartedness and Fearlessness show moderate to high heritability. ## Footnote Examples: Coldheartedness: rMZ = +.34, rDZ = −.16 Fearlessness: rMZ = +.54, rDZ = +.03 (Blonigen et al., 2003)

Question 45

What were the average correlations for personality traits in the Minnesota Twin Study (MZA)?

Answer 45

Neuroticism: .70 Social potency: .57 Traditionalism: .59 Absorption: .74 Average correlation: .54 ## Footnote (Tellegen et al., 1988)

Question 46

What do large studies suggest about the heritability of the Big Five traits?

Answer 46

Generally ~50% heritability HEXACO traits: 34–58% Meta-analyses: 40–49% ## Footnote (De Vries et al., 2021; Vukasovic & Bratko, 2015; van den Berg et al., 2014)

Question 47

Is there heritability for political attitudes like traditionalism or conservatism?

Answer 47

Yes. Traditionalism: ~.59 heritability (Minnesota Twin Study) Political ideologies: 30–60% ## Footnote (Abrahamson et al., 2002; Hatemi et al., 2014)

Question 48

Are occupational preferences heritable?

Answer 48

Yes. Genetic children’s preferences correlated significantly with parental status, while adopted children's did not. Suggests genetic influence on job preferences and social status. ## Footnote (Ellis & Bonin, 2003)

Question 49

What is the heritability of religiousness?

Answer 49

Adolescents: ~12% Adults: ~44% ## Footnote (Abrahamson et al., 2002; Koenig et al., 2005; Button et al., 2011)

Question 50

What’s the heritability of smoking?

Answer 50

MZ twins: 16x more likely to both smoke DZ twins: 7x more likely ## Footnote (Hooper et al., 1992)

Question 51

What’s the heritability of alcohol use and alcoholism?

Answer 51

Drinking: .36–.56 Alcoholism: .50–.71 Genetic link found between alcoholism and antisocial behavior. ## Footnote (Heath et al., 1994; Kendler et al., 1992)

Question 52

What is the estimated heritability of sexual orientation?

Answer 52

Previous estimates: 30–70% More recent, controlled studies: ~20% concordance for MZ twins Childhood gender nonconformity: 50% heritability in males, 37% in females. ## Footnote (Bailey et al., 2000)

Question 53

What did LeVay (1991) find in brain studies of gay vs. straight men?

Answer 53

Gay men had a smaller medial preoptic area of the hypothalamus—suggesting biological differences.

Question 54

Is gender identity heritable?

Answer 54

Yes, modest heritability has been found. ## Footnote (Polderman et al., 2018)

Question 55

Is the propensity to marry heritable?

Answer 55

Yes—estimated at 68% (Johnson et al., 2004), partly through traits like social potency and achievement.

Question 56

What personality traits in wives predict marital satisfaction?

Answer 56

Optimism, warmth, and low aggressiveness. These traits also contribute to husbands’ satisfaction. ## Footnote (Spotts et al., 2005)

Question 57

Is life satisfaction heritable?

Answer 57

Moderately—traits like sense of purpose, personal growth, and positive relationships predict well-being. ## Footnote (Bartels, 2015; Archontaki et al., 2013)

Question 58

What is the difference between shared and nonshared environments?

Answer 58

Shared: Aspects of the environment siblings share (e.g., home, parenting, school) Nonshared: Unique experiences (e.g., different friends, treatment, rooms, life events)

Question 59

Do shared environments have a large impact on personality traits like extraversion or neuroticism?

Answer 59

No—studies show little or no effect from shared environments. Most environmental influence is nonshared.

Question 60

Which domains do show shared environmental influences?

Answer 60

Smoking & drinking (Willerman, 1979) Religious beliefs, political attitudes, and adjustment traits like depression and autonomy

Question 61

Why might psychologists have overlooked nonshared environmental effects?

Answer 61

Historically, most theories focused on shared environments (e.g., parenting), and nonshared influences are more difficult to identify.

Question 62

What is genotype–environment interaction?

Answer 62

When people with different genotypes respond differently to the same environment. ## Footnote Example: Introverts vs. extraverts in noisy settings.

Question 63

What is an example of genotype–environment interaction with the MAOA gene?

Answer 63

Abused children with low MAOA levels were more likely to develop antisocial behavior (Caspi et al., 2003).

Question 64

What gene is linked to persistent depression in response to maltreatment?

Answer 64

The short 5-HTT gene variant (Uher et al., 2011).

Question 65

What is genotype–environment correlation?

Answer 65

When a person’s genotype influences the environment they experience.

Question 66

What are the three types of genotype–environment correlation?

Answer 66

Passive: Parents provide genes and matching environment (e.g., verbal ability & books). Reactive (Evocative): Environment responds to individual traits (e.g., cuddly babies get more affection). Active: Individuals seek environments that match their traits (e.g., extraverts seek parties).

Question 67

Can genotype–environment correlations be negative or positive?

Answer 67

Yes— Positive: Environment enhances trait expression. Negative: Environment works against trait expression (e.g., parents calm hyperactive kids).

Question 68

What did Krueger et al. (2003) find about personality and family environment?

Answer 68

Family cohesion was linked to traits like Constraint and low Negative Emotionality, showing a genetic mediation of perceived environments.

Question 69

What is the DRD4 gene associated with?

Answer 69

Novelty seeking and financial risk-taking (Benjamin et al., 1996; Dreber et al., 2009). It affects dopamine receptors.

Question 70

Why is the DRD4 gene not a definitive predictor of personality?

Answer 70

Findings are mixed and effects are small. Many genes (possibly 500+) likely influence personality.

Question 71

What is a GWAS (Genome-Wide Association Study)?

Answer 71

A method that scans the entire genome to identify genes linked to traits like extraversion and well-being.

Question 72

What are polygenic risk scores?

Answer 72

Sums of small effects from many genes used to predict personality traits (e.g., depression, low well-being).

Question 73

What gene interacts with stress to predict depression?

Answer 73

The short version of the 5-HTT gene—linked to depression only when combined with stressful life events.

Question 74

Why was behavioral genetics controversial historically?

Answer 74

It challenged environmentalist views and raised fears of misuse (e.g., eugenics, labeling people as inferior).

Question 75

How are science and values different?

Answer 75

Science = discovering what exists Values = what people want to exist Science is self-correcting; findings must be interpreted responsibly.

Question 76

What percentage of personality is heritable and environmental?

Answer 76

Heritability: ~30–50% Environmental: ~50–70% (mostly nonshared)

Question 77

What are the four traditional behavioral genetic methods?

Answer 77

Selective breeding Family studies Twin studies Adoption studies

Question 78

What is the key insight from behavioral genetics about 'nurture'?

Answer 78

The most important environmental influences are nonshared, not shared—contrary to traditional socialization theories.