In behavioural genetics, what does a heritability coefficient (h²) of 0.50 mean?

A: 50% of a person's personality is determined by their genes

B: 50% of the observed variance in a personality trait within a particular population can be attributed to genetic differences among individuals

C: Genes and environment contribute equally to each individual's personality score

D: 50% of personality traits have been identified in the human genome

Correct: 50% of the observed variance in a personality trait within a particular population can be attributed to genetic differences among individuals

Heritability is a population-level statistic, not an individual-level claim. A h² of 0.50 means that 50% of the variance in a trait — the differences between people in that population, at that time — is associated with genetic differences. It does not mean that 50% of any one person's personality comes from their genes. Heritability estimates can change across populations, cultures, and historical periods because they depend on the range of genetic and environmental variation present in the sample.

What heritability estimates do twin studies generally find for Big Five personality traits?

A: Very low: approximately 10–20%

B: Moderate: approximately 40–60%

C: High: approximately 70–90%

D: Near-zero: genes play a negligible role in personality

Correct: Moderate: approximately 40–60%

The consistent finding from large twin studies is that Big Five traits are approximately 40–60% heritable. Jang et al. (1996), using a sample of over 1,000 Canadian twin pairs, found heritabilities ranging from 41% (Agreeableness) to 61% (Openness). Vukasović and Bratko's (2015) meta-analysis of 134 twin studies confirmed a mean heritability of approximately 40% across personality traits. This leaves substantial room for environmental influences, but these are predominantly nonshared rather than shared.

The "equal environments assumption" is a critical methodological assumption of twin studies. What does it state, and why is it important?

A: It assumes MZ and DZ twins are raised in equally loving environments; violations would inflate heritability estimates

B: It assumes the environments of MZ (identical) and DZ (fraternal) twin pairs are equally similar, so that any greater similarity in MZ twins reflects genetic rather than environmental sharing

C: It assumes all twins are raised in middle-class environments to control for socioeconomic confounds

D: It assumes that genes and environment do not interact, only add together

Correct: It assumes the environments of MZ (identical) and DZ (fraternal) twin pairs are equally similar, so that any greater similarity in MZ twins reflects genetic rather than environmental sharing

The logic of twin studies depends on MZ twins (who share 100% of their DNA) being more similar than DZ twins (who share ~50%) because of genetics, not because their environments are more similar. The equal environments assumption holds that MZ and DZ pairs are exposed to equally similar environments — that parents, peers, and teachers do not treat MZ twins more similarly than DZ twins. If this assumption is violated (if MZ twins are treated more similarly), heritability would be overestimated. Research generally supports the assumption, though it remains a topic of methodological debate.

Bouchard's Minnesota Study of Twins Reared Apart (MISTRA) provided especially compelling evidence for heritability because it:

A: Used the largest sample of MZ twins ever assembled, exceeding 10,000 pairs

B: Studied MZ twins who had been raised in different families and environments since birth, so their similarity cannot be attributed to shared upbringing

C: Tracked twins from birth to age 70, allowing analysis of lifespan personality change

D: Included cross-cultural samples from 50 countries to test whether heritability varies with culture

Correct: Studied MZ twins who had been raised in different families and environments since birth, so their similarity cannot be attributed to shared upbringing

Bouchard's MISTRA study is among the most powerful designs in behavioural genetics. MZ twins reared apart share 100% of their DNA but have had different family environments, friends, schools, and experiences. If they are still similar in personality, this similarity can be attributed to their shared genes rather than shared environment. Bouchard found that MZ twins reared apart were nearly as similar in personality as MZ twins reared together (r ≈ .50 vs r ≈ .52), suggesting that shared family environment adds little to personality similarity.

True or False: In behavioural genetics research on adult personality, shared environmental factors (being raised in the same family) account for very little variance — typically less than 10%.

Answer: True

One of the most counterintuitive and replicated findings in behavioural genetics is that growing up in the same family — sharing parents, home, neighbourhood, socioeconomic status — accounts for surprisingly little variance in adult personality. Plomin and Daniels (1987) highlighted this finding in their influential paper "Why are children in the same family so different from each other?" The variance left after heritability and shared environment is termed nonshared environment — unique experiences (different peer groups, different treatment by the same parents, chance events) that make siblings different despite their common upbringing.

What is "nonshared environment" in behavioural genetic models, and why is it important for personality research?

A: Environmental exposures that are shared by all members of a culture, such as historical events or national institutions

B: Unique environmental experiences that differ between siblings raised in the same family — different friendships, treatment by parents, school experiences, and chance events — that make siblings different from each other

C: The portion of personality variance that cannot be explained by either genes or environment

D: The genetic variation not shared between parents and offspring

Correct: Unique environmental experiences that differ between siblings raised in the same family — different friendships, treatment by parents, school experiences, and chance events — that make siblings different from each other

Nonshared environment (NSE) refers to environmental influences that differ between siblings in the same family. These include peer group differences, birth order effects, differential parental treatment, school experiences, and random chance factors. NSE is important because it is typically the largest single environmental contributor to personality variance in adulthood — larger than shared environment. Identifying which specific nonshared experiences matter has proved difficult; measurement error in personality assessments also inflates apparent NSE estimates.

What do adoption studies generally find about personality similarity between adoptive parents and their adopted children?

A: Adoptive parents and their adopted children show strong personality similarity, confirming the power of the shared environment

B: Adopted children show modest similarity to both adoptive parents and biological parents, suggesting both genetic and environmental contributions

C: Adopted children show little or no personality resemblance to their adoptive parents by adulthood, but do resemble their biological parents whom they have never met

D: Adoption studies have found no evidence that genes contribute to personality differences

Correct: Adopted children show little or no personality resemblance to their adoptive parents by adulthood, but do resemble their biological parents whom they have never met

Adoption studies provide a natural experiment: if shared environment matters, adoptive parents and children should resemble each other. The consistent finding is that they show near-zero personality correlations by adulthood, despite sharing a home for 18+ years. Adopted siblings raised together also converge on zero resemblance in adulthood. In contrast, adopted children placed at birth do show personality similarity to their biological parents, supporting genetic transmission. This pattern confirms high heritability and low shared environmental influence.

What is a gene-environment interaction (G×E) in personality research?

A: The finding that genes and environment contribute equal amounts to personality variance

B: A statistical phenomenon in which the effect of a genetic variant on a trait depends on the environmental context, or the effect of an environment on a trait depends on genotype

C: The tendency for individuals with particular genetic profiles to seek out specific environments (gene-environment correlation)

D: The process by which environmental experiences can modify gene expression across generations

Correct: A statistical phenomenon in which the effect of a genetic variant on a trait depends on the environmental context, or the effect of an environment on a trait depends on genotype

A G×E interaction occurs when the effect of a genetic variant on a phenotype depends on the environment, and vice versa. For example, Caspi et al. (2003) found that a functional polymorphism in the serotonin transporter gene (5-HTTLPR) moderated the relationship between life stress and depression — individuals with the short allele who experienced stressful life events were more likely to develop depression. Whether this specific finding replicates robustly has been debated, but G×E interactions remain an important theoretical framework for understanding how genetic risk is expressed differently across contexts.

What is "gene-environment correlation" (rGE), and which three types has the literature distinguished?

A: A correlation between genetic risk and measured environments; types: additive, multiplicative, and interactive

B: The tendency for genetically influenced traits to lead people into particular environments; types: passive (parents provide both genes and environment), evocative (traits elicit reactions from others), and active (niche-picking based on traits)

C: The degree to which genes predict environments; types: direct, indirect, and mediated correlations

D: The correlation between twin similarity and shared environmental variance; types: between-family, within-family, and cross-generational

Correct: The tendency for genetically influenced traits to lead people into particular environments; types: passive (parents provide both genes and environment), evocative (traits elicit reactions from others), and active (niche-picking based on traits)

Gene-environment correlation (rGE) refers to the non-random association between genetic predispositions and the environments people encounter. Scarr and McCartney (1983) identified three types: passive rGE — parents provide both genes and home environment (e.g., a musically gifted child born to musical parents who also provide instruments and lessons); evocative rGE — genetically influenced traits elicit particular reactions from others (e.g., a sociable child elicits more social engagement from adults); active rGE — people actively select environments that fit their genetically influenced traits (niche-picking). rGE makes it difficult to separate genetic and environmental contributions cleanly.

What does the concept of "reaction range" propose about the relationship between genes and personality?

A: Genes determine an exact fixed personality score; the environment can only reduce, not increase, it

B: Genes establish a range of possible phenotypic outcomes; the actual outcome within that range depends on environmental quality and experience

C: The range of personality variation is determined entirely by environmental inputs, with genes setting only a floor

D: Genetic influences on personality react to each other through epistatic interactions, making prediction of individual outcomes impossible

Correct: Genes establish a range of possible phenotypic outcomes; the actual outcome within that range depends on environmental quality and experience

The reaction range concept (Gottesman, 1963) proposes that a genotype does not fix a single phenotypic outcome but establishes a range of possible outcomes. The environment determines where within that range the individual ends up. In favourable environments, individuals may approach the upper limit of their genetic potential; in impoverished or adverse environments, they may fall at the lower end. The concept captures the interactive relationship between genes and environments without reducing either to a fixed determinant, and is widely used to explain how high heritability is compatible with significant environmental influence.