## Definition **Nature vs nurture** is the longstanding debate about the relative contributions of genetic inheritance (nature) and environmental experience (nurture) to individual differences in behaviour, personality, and cognition. **Behavioural genetics** is the scientific field that quantifies these contributions using twin, adoption, and family studies. Steven Pinker (*The Blank Slate*, 2002) argues that decades of behavioural genetics research have effectively settled the debate, though the results are counterintuitive and politically contested. ## The Three Laws of Behavioural Genetics Pinker presents what he calls — echoing the field's consensus — the three empirical laws of behavioural genetics: 1. **All human behavioural traits are heritable.** Every reliably measured psychological trait shows a genetic component. There are no known exceptions. 2. **The effect of being raised in the same family is smaller than the effect of genes.** Shared family environment — having the same parents, home, income level, and neighbourhood — accounts for surprisingly little of the variance in adult personality, intelligence, and life outcomes. 3. **A substantial portion of the variance in complex behavioural traits is not accounted for by genes or shared family environment.** The residual is attributed to *non-shared environment* — the experiences unique to each individual: peer groups, classrooms, chance encounters, and idiosyncratic developmental events. ## Heritability Heritability ($h^2$) is the proportion of phenotypic variance in a population attributable to genetic variance. It is not a fixed property of a gene or a trait but a population-level statistic that can vary with environment. For most personality dimensions and cognitive abilities studied in Western samples: $h^2 \approx 0.40\text{–}0.60$ This means roughly 40–60 % of individual differences in, say, extraversion, conscientiousness, or IQ are explained by genetic differences among individuals. Heritability rises across development: it is lower in childhood (shared environment matters more when children are dependent on parents) and approaches 0.6–0.8 for intelligence by adulthood. ## Shared vs Non-Shared Environment The striking finding is that shared environment — what parents can actually control about the home — accounts for approximately 0–10 % of adult personality variance. Children raised in the same household by the same parents end up no more similar in personality than expected from their genetic relatedness alone. Non-shared environment is the complement: all the influences unique to each sibling — different friend groups, different teachers, birth-order effects, chance events, and measurement error. This component typically accounts for 40–50 % of variance. Its specific sources remain poorly understood, which makes it one of the most important open problems in developmental psychology. ## Twin and Adoption Studies The key methodological tools: - **Monozygotic (identical) twins reared apart**: share 100 % of segregating genes but different households. Their correlations for personality, IQ, and attitudes are substantially higher than those of dizygotic twins reared together (who share ~50 % of genes and the same household). This design cleanly separates genetic from shared-environment contributions. - **Adoption studies**: adopted children's adult traits correlate with biological parents (whom they never lived with) more strongly than with adoptive parents (with whom they shared a home). Adopted siblings raised together show near-zero adult personality correlations. Pinker recounts the classic Minnesota Study of Twins Reared Apart: identical twins separated at birth and meeting for the first time as adults showed uncanny convergences — similar posture, identical occupational choices (both fire captains), identical nervous habits, identical car-maintenance observations on meeting the researcher — far beyond what chance could explain. ## Implications for Parenting Pinker draws a pointed conclusion: parents have very little influence on most of the outcomes they most care about — their children's personality, intelligence, mental health, political attitudes, and life success — beyond the genetic endowment they transmit at conception. Children are not blank slates to be written by parenting style. They develop primarily within their peer culture, using their genetic endowments. The relevant parenting task is to keep them safe and provide a stable, stimulating environment — not to micromanage the traits they will emerge with. ## What Nature vs Nurture is Not The debate is not about whether environment matters (it clearly does, via non-shared environment and gene-environment interactions) nor whether genes determine destiny (they do not; heritability is not fate). It is about the relative and specific contributions of different causal pathways, and about which environmental influences actually make a lasting difference — a question that remains largely open. ## Related - [[Human Nature and Innateness]] - [[The Blank Slate Doctrine]] ## Sources - [[The Blank Slate (Pinker 2002)]]