What does heritability (h²) measure, and what does it NOT tell us?
A: h² measures the proportion of an individual's trait determined by genes; high h² means genes are more important than environment for that person
B: h² measures the proportion of variance in a trait across a population that is attributable to genetic differences; it does NOT tell us what causes a single individual's trait value or whether a trait can be changed by environment
C: h² measures the probability that a genetic disorder will be passed to offspring; values above 0.5 indicate a dominant trait
D: h² measures how much a trait has been influenced by natural selection; h² of 1.0 means the trait is a perfect adaptation
Correct: h² measures the proportion of variance in a trait across a population that is attributable to genetic differences; it does NOT tell us what causes a single individual's trait value or whether a trait can be changed by environment
Heritability (h²) is a population statistic: the proportion of observed variance in a trait in a specific population that is attributable to genetic differences between individuals in that population at that time. Critical misunderstandings: (1) h² does not apply to individuals — you cannot say "70% of Jim's IQ is genetic." (2) High h² does not mean the environment cannot change the trait — height has h² ~0.80 yet improved nutrition dramatically increased population height across the 20th century. (3) h² is population- and environment-specific: the same trait will have different h² in different environments. (4) h² says nothing about differences between groups — the fact that within each group a trait is highly heritable does not imply that between-group differences are genetic (a common logical error in race-IQ debates).
Why are monozygotic (MZ) twins raised apart particularly valuable for estimating heritability?
A: MZ twins raised apart share the same environment but different genes, allowing gene-environment interactions to be estimated
B: MZ twins raised apart share nearly 100% of their DNA but share no rearing environment — similarities between them estimate heritability directly, while differences estimate non-shared environment
C: MZ twins raised apart allow researchers to compare gene expression in different environments and detect epigenetic differences
D: MZ twins raised apart are identical in personality and intelligence; their data prove genes determine all traits
Correct: MZ twins raised apart share nearly 100% of their DNA but share no rearing environment — similarities between them estimate heritability directly, while differences estimate non-shared environment
Thomas Bouchard's Minnesota Study of Twins Reared Apart (MISTRA) is the most famous example. MZ twins are genetically identical (sharing ~100% of DNA, unlike DZ twins who share ~50%). When raised apart in different families from infancy, MZ twin pairs no longer share the same rearing environment. Their phenotypic similarity — measured by intraclass correlations — provides a direct estimate of heritability: if MZ twins raised apart are highly similar for a trait (e.g., IQ correlation ~.72), this similarity must reflect genetic influence. The difference between MZ-reared-apart similarity and perfect correlation (1.0) estimates non-shared environmental influence. MISTRA reported surprisingly high heritability for personality traits, IQ, occupational interests, and even religiosity, influencing the field substantially (though it has been criticised for selective placement issues).
What is a concordance rate and how is it used to infer genetic versus environmental influences?
A: The probability that both twins will develop a trait if one has it; higher MZ concordance than DZ concordance suggests genetic influence
B: The correlation coefficient between twins' quantitative trait scores; 1.0 = perfect genetic determination
C: The percentage of twins in a study who share the same diagnosis; it measures diagnostic reliability between raters
D: The ratio of MZ to DZ twin pairs in a genetic study; higher ratios improve heritability estimates
Correct: The probability that both twins will develop a trait if one has it; higher MZ concordance than DZ concordance suggests genetic influence
Concordance rate is used for categorical (present/absent) traits, particularly psychiatric diagnoses. It is the percentage of co-twins who share a disorder given that their proband twin has it. The comparison between MZ and DZ concordance rates provides evidence for (or against) genetic influence: if MZ concordance (e.g., 48% for schizophrenia) substantially exceeds DZ concordance (e.g., 17%), this suggests genetic influence because MZ pairs share twice as much DNA as DZ pairs. Perfect MZ concordance (100%) would indicate complete genetic determination; MZ concordance above 0% but below 100% indicates that environment plays a role even for identical genes. Schizophrenia: MZ ~48%, DZ ~17%; Major depression: MZ ~37%, DZ ~22%; Autism: MZ ~77%, DZ ~31% — all showing substantial but incomplete genetic influence.
The "end of the shared environment" (Rowe, 1994; Harris, 1998) refers to which surprising finding in behavioural genetics?
A: That shared rearing environment (same family) accounts for most of the variance in personality and intelligence in adults
B: That shared family environment accounts for very little variance in most adult personality traits — non-shared environment (unique experiences) and genetics explain most variance, making siblings in the same family nearly as different as strangers
C: That shared DNA between siblings explains all similarity; the environment is irrelevant to personality development
D: That parenting style has no measurable effect on any trait because parents have no shared environment with their children
Correct: That shared family environment accounts for very little variance in most adult personality traits — non-shared environment (unique experiences) and genetics explain most variance, making siblings in the same family nearly as different as strangers
One of the most counterintuitive findings in behavioural genetics: for most adult personality traits (Big Five), shared family environment (c²) accounts for nearly 0% of variance. Siblings reared together in the same home by the same parents are often nearly as different in personality as unrelated individuals reared in different families. The variance is partitioned approximately: genetics (~40–60%), non-shared environment (~40–60%), shared environment (~0–10%). This has been called one of the "great embarrassments" of developmental psychology, given the field's emphasis on parenting. Judith Rich Harris's controversial book The Nurture Assumption (1998) popularised this finding and argued that peers, not parents, shape personality. The finding is robust for personality but does NOT hold equally for all outcomes — shared environment does matter for academic achievement, reading, vocabulary, and some psychopathologies.
Sandra Scarr and Kathleen McCartney described three types of gene-environment correlation (rGE). What are they?
A: Positive, negative, and neutral GE correlation — reflecting whether genes and environment push traits in the same or opposing directions
B: Passive (parents provide both genes and environment), evocative (genotype elicits reactions from others), and active (niche-picking — organisms seek environments matching their genotype)
C: Proximal (immediate environmental effects), distal (background environmental effects), and transactional (bidirectional gene-environment effects)
D: Additive, dominance, and epistatic — reflecting how multiple genes combine with environmental effects
Correct: Passive (parents provide both genes and environment), evocative (genotype elicits reactions from others), and active (niche-picking — organisms seek environments matching their genotype)
Scarr and McCartney (1983) described three types of gene-environment correlation — the tendency for genetic dispositions to correlate with exposure to certain environments: (1) Passive rGE: parents pass on both genes and correlated environments without the child doing anything — musical parents give their child musical genes AND a music-rich home. (2) Evocative rGE: a child's genetically influenced traits elicit particular responses from others — a sociable child elicits more social interactions from others, creating a socially stimulating environment. (3) Active (niche-picking) rGE: as children gain agency, they seek out environments matching their genetic predispositions — a high-intelligence adolescent selects challenging books, courses, and peers. Active rGE increases with age, explaining why genetic influences on intelligence and personality increase across development while shared environment effects diminish.
Research consistently shows that the heritability of general intelligence (g) changes across the lifespan. What is the direction of this change?
A: Heritability decreases with age as accumulated life experience becomes the dominant determinant of intelligence
B: Heritability increases from childhood (~40%) through adolescence to adulthood (~60–80%), while shared environment effects decrease to near zero
C: Heritability is fixed at birth and remains constant at ~50% throughout life
D: Heritability peaks in adolescence (~70%) and then declines as age-related cognitive changes introduce greater individual variation
Correct: Heritability increases from childhood (~40%) through adolescence to adulthood (~60–80%), while shared environment effects decrease to near zero
One of the most replicable and counterintuitive findings in the psychology of intelligence: heritability of IQ increases substantially across development. In childhood, h² ≈ 40–50%; by late adolescence, h² ≈ 55–60%; in adulthood, h² ≈ 60–80%. Simultaneously, shared environment effects (c²) decline from ~30–40% in childhood to near 0% in adulthood. The explanation lies in gene-environment correlation, particularly active niche-picking: as individuals gain independence, they increasingly select environments that match their genetic predispositions — amplifying genetic effects. In childhood, parents control the environment; by adulthood, individuals choose their own educational path, career, social circle, and intellectual environment — all influenced by their genetic propensities. This means the effect of early childhood shared environment on adult intelligence is substantially smaller than its effect during childhood itself.
What do adoption studies contribute to understanding gene-environment effects that twin studies cannot provide?
A: Adoption studies directly measure gene expression; twin studies can only infer it from phenotypic similarity
B: Adoption studies allow direct comparison between genetic relatives reared apart and non-genetic relatives reared together — fully separating genetic from rearing environment contributions
C: Adoption studies always produce larger heritability estimates than twin studies because they eliminate shared environment confounds
D: Adoption studies are uniquely able to measure the contribution of prenatal environment, which twin studies cannot separate from genetic effects
Correct: Adoption studies allow direct comparison between genetic relatives reared apart and non-genetic relatives reared together — fully separating genetic from rearing environment contributions
Adoption creates natural experiments that complement twin research. Key comparisons: (1) Adoptees vs biological parents: if adoptees resemble their biological parents (whom they've never lived with) more than their adoptive parents for a trait, this strongly implicates genetic influence. (2) Adoptees vs adoptive siblings (genetically unrelated, reared together): if they resemble each other, this implicates shared environment — but for most personality traits in adults, adopted siblings show near-zero resemblance, again undermining shared environment's role. (3) Selective placement: a complication — adoption agencies sometimes place children in families similar to their biological families, potentially confounding genetic and environmental effects. The Colorado Adoption Project and Texas Adoption Project have been major longitudinal adoption studies informing these questions.
What does the heritability of psychiatric disorders tell us about causation, and what are the implications for treatment?
A: High heritability proves disorders are incurable because they are genetically determined
B: High heritability tells us that genetic factors contribute to risk, but it does not mean disorders are untreatable or that environmental interventions are futile
C: High heritability means disorders require genetic treatments (pharmacogenomics); psychological therapies can only be effective for low-heritability disorders
D: High heritability means the environment plays no causal role — only genetic mutations can be the target of intervention
Correct: High heritability tells us that genetic factors contribute to risk, but it does not mean disorders are untreatable or that environmental interventions are futile
A common but incorrect inference: high heritability does not imply immutability or poor treatment response. Height has h² ~80%, yet nutrition dramatically changes it. Phenylketonuria (PKU) has h² near 100% but is prevented by a simple dietary change. Heritability tells us about the sources of variation in a population under current environmental conditions — it does not determine how a single individual's outcome can be modified by an intervention. For psychiatric disorders, high heritability (schizophrenia ~80%, bipolar disorder ~60–80%, ADHD ~75%) establishes that genetic risk factors matter, which is important for identifying biomarkers and developing targeted pharmacological treatments. But it does not preclude effective psychological therapies (CBT for psychosis, DBT for BPD), environmental interventions (parenting programmes, trauma treatment), or nutritional/lifestyle approaches. Genetic risk interacts with protective environments to determine outcome.
Behavioural Genetics
What does heritability (h²) measure, and what does it NOT tell us?
About this quiz
Behavioural genetics asks a deceptively simple question: why do people differ from one another? By studying twins, adoptees, and families, researchers can parse how much of the variation in personality, intelligence, mental health, and other traits is attributable to genetic differences versus shared and unique environmental experiences.
This quiz covers the core methods and concepts of behavioural genetics — heritability, twin studies, adoption studies, concordance rates, shared and non-shared environments, and gene-environment correlations.