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How much of intelligence is hereditary?

Intelligence has a substantial genetic component, but how large that component appears depends on what exactly you measure, when you measure it, and in which population. The scientific shorthand for this proportion is “heritability,” the fraction of the observed variation in a trait that is attributable to genetic differences within a particular group at a particular time. Critical points to keep in mind:

  1. Heritability is a population statistic, not an individual one.
  2. A high heritability does not make a trait “fixed” or unaffectable by the environment.
  3. Heritability can change when environments become more or less uniform, when countries industrialize, or across development.

Below is a synthesis of the major findings on intelligence (often operationalized as IQ or general cognitive ability, g).

  1. Estimates from classical twin and adoption studies
    • Early childhood (ages 2–7): heritability ≈ 0.20–0.40
    • Middle childhood to adolescence: heritability ≈ 0.40–0.60
    • Late adolescence to adulthood: heritability ≈ 0.60–0.80
    • Old age: estimates scatter, typically 0.50–0.70, but data are less plentiful.

    These figures mean, for example, that in well-studied Western populations about 60–80 % of the individual differences in adult IQ scores can be statistically associated with genetic differences.

  2. Developmental trend
    Heritability rises with age. In toddlerhood, the shared family environment (e.g., parental education, home literacy, neighborhood) explains more variance than genes; by late adolescence, the situation reverses. Possible reasons include:
    • Children increasingly select, modify, and create environments that match their genetic predispositions (“gene–environment correlation”).
    • Measurement precision improves with age.
    • Some genetic influences manifest only after certain neural or hormonal maturational events.

  3. Environmental contributions and modulation
    • Shared family environment: 20–30 % of the variance in childhood; almost 0 % by adulthood in many studies.
    • Non-shared environment (experiences that differ between siblings) consistently explains 15–25 % across the lifespan.
    • Socioeconomic status (SES): In higher-income samples, heritability estimates are high because environmental resources are relatively uniform. In very low-SES contexts, genetic influences on IQ are often muted, and environment plays a larger role—a phenomenon called the Scarr–Rowe effect.
    • Prenatal factors (maternal nutrition, toxins, stress), early nutrition, quality of education, infectious disease burden, and social stimulation all measurably affect mean IQ levels and can either mask or amplify genetic potential.

  4. Molecular genetics
    • Intelligence is highly polygenic: thousands of DNA variants, each with minuscule effect sizes, collectively contribute.
    • Current “polygenic scores” (PGS) derived from genome-wide association studies explain about 10–15 % of IQ variance in European-ancestry samples. This is far less than the 50–80 % heritability from twin studies, illustrating that most causal variants are still unidentified.
    • PGS predict better when combined with environmental data, supporting gene–environment interplay rather than a simple nature–versus–nurture dichotomy.

  5. Interpretation cautions
    • Heritability says nothing about causes of differences between populations (e.g., national or ethnic IQ gaps). A trait can be highly heritable within groups yet shaped largely by environmental disparities between groups.
    • High heritability does not imply immutability: height is about 80–90 % heritable, yet average height rose worldwide in the 20th century due to nutrition and health improvements. IQ means have similarly risen (~3 points per decade in many countries), the “Flynn effect,” underscoring environmental leverage.
    • Educational interventions, cognitive training, poverty reduction, and public-health measures can still shift both the average IQ and the realized potential of individuals even when heritability is high.

Bottom line
In contemporary high-income societies, roughly half to three-quarters of the variation in adult intelligence is attributable to genetic differences, with the remainder due to environmental factors and their interaction with genes. The genetic influence strengthens from childhood to adulthood, but environmental opportunities, especially during prenatal development, early childhood, and schooling, remain crucial for allowing genetic potential to be expressed and for raising population-wide cognitive performance.