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The heritability of intelligence is a complex topic that has been extensively studied through twin studies, adoption studies, and more recently, genome-wide association studies (GWAS). Current research indicates that intelligence has substantial genetic influences, but the degree varies significantly with age and environmental factors.
Twin and adoption studies consistently show that the heritability of intelligence increases with age. In early childhood, heritability estimates are typically around 20-40%, but this increases dramatically through development, reaching 60-80% in adulthood [1][4]. This counterintuitive finding - that genetic influences become stronger over time rather than weaker - is one of the most robust findings in behavioral genetics.
The meta-analysis by Haworth et al. examining longitudinal twin and adoption studies found that heritability increases from approximately 41% in childhood to 55% in adolescence and 66% in young adulthood [4]. This pattern suggests that as individuals age, they increasingly select and create environments that match their genetic predispositions, a process known as gene-environment correlation.
Recent advances in molecular genetics have provided new insights into the genetic architecture of intelligence. Large-scale GWAS studies have identified hundreds of genetic variants associated with cognitive ability [3]. However, these identified variants explain only a small fraction of the total heritability estimated from twin studies - a phenomenon known as the “missing heritability” problem.
The Nature Reviews Genetics analysis indicates that polygenic scores derived from GWAS can predict approximately 4-7% of the variance in intelligence in independent samples [2][7]. While this may seem modest, it represents substantial progress in understanding the molecular basis of cognitive differences and demonstrates that intelligence is highly polygenic, influenced by many genetic variants of small effect.
Despite substantial heritability, environmental factors remain crucial for intelligence. The remaining 20-40% of variance in adult intelligence is attributable to environmental influences, and this proportion is even higher in childhood [1]. Environmental factors such as education quality, socioeconomic status, nutrition, and cultural opportunities all play important roles.
Moreover, gene-environment interactions are significant - the same genetic variants may have different effects depending on environmental context [5]. This means that heritability estimates can vary across different populations and environmental conditions.
Several important caveats must be considered when interpreting heritability estimates:
Population-specific: Heritability estimates apply to specific populations under specific environmental conditions and cannot be generalized universally [1][2].
Not fixed: High heritability does not mean that traits cannot be changed through environmental interventions [1].
Measurement challenges: Intelligence itself is measured through various cognitive tests, and different aspects of cognitive ability may show different patterns of heritability [6].
Between vs. within family effects: Recent sibling studies suggest that genetic effects may operate differently within families compared to between families, potentially affecting interpretation of heritability estimates [8].
The current scientific consensus suggests that intelligence in adulthood is approximately 60-80% heritable in populations of European ancestry living in developed countries, with substantial environmental influences accounting for the remainder [1][2][4]. However, this represents the upper bound observed in relatively privileged environments where basic needs are met.
The molecular genetic revolution has confirmed that intelligence is highly polygenic and has begun to identify specific biological pathways involved in cognitive differences [3][5]. As sample sizes increase and methods improve, our understanding of both the genetic and environmental contributions to intelligence continues to evolve.
[1] Is Intelligence Hereditary? - Scientific American - Provides an accessible overview of heritability research, emphasizing that high heritability doesn’t preclude environmental influence and that estimates vary by population and context.
[2] The new genetics of intelligence - Nature Reviews Genetics - Comprehensive review of molecular genetic findings, discussing the polygenic nature of intelligence and the gap between twin study heritability and molecular genetic predictions.
[3] Genome-wide association meta-analysis in 269,867 individuals identifies new genetic and functional links to intelligence - Nature Genetics - Large-scale GWAS study identifying genetic variants associated with intelligence and demonstrating the highly polygenic architecture of cognitive ability.
[4] Explaining the Increasing Heritability of Cognitive Ability Across Development: A Meta-analysis of Longitudinal Twin and Adoption Studies - Psychological Science - Meta-analysis documenting the increase in heritability from childhood to adulthood and proposing explanations for this developmental pattern.
[5] Genetic Variation, Brain, and Intelligence Differences - Molecular Psychiatry - Examines the relationship between genetic variants, brain structure/function, and cognitive differences, highlighting gene-environment interactions.
[6] DNA and IQ: Big Deal or Much Ado About Nothing? – A Meta-analysis - Intelligence - Meta-analytic examination of the practical significance of genetic findings for intelligence research and applications.
[7] The new genetics of intelligence - Nature Reviews Genetics (PMC Open Access) - Open access version providing detailed discussion of molecular genetic approaches to studying intelligence and their implications.
[8] Polygenic score prediction within and between sibling pairs for intelligence, cognitive abilities and educational traits from childhood to early adult - ICA Journal - Examines how genetic predictions work within families versus between families, providing insights into family-specific effects on cognitive development.