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Researchers do not speak of a single fixed percentage of “genes versus environment.” Instead they estimate the heritability of intelligence (the proportion of the observed differences among people that can be traced to genetic differences in a given population at a given time). The most robust findings can be summarized in four points.
Average heritability in modern Western populations is around 50 %.
Large twin- and adoption-based meta-analyses put the overall heritability of general cognitive ability (g) close to one-half (h² ≈ 0.50) across the life span [1].
Heritability rises with age.
The longitudinal meta-analysis of 11 000 twin and adoption pairs found that genetic influence on IQ is modest in infancy (h² ≈ 0.22 at 2 years), about equal to environmental influence in middle childhood (h² ≈ 0.40–0.50), but becomes dominant in late adolescence and adulthood (h² ≈ 0.60–0.80) [3]. Gene–environment correlation (children increasingly selecting or creating environments that fit their genetic propensities) is thought to drive this upward trend [3], [2].
Molecular (DNA-based) methods corroborate but give smaller numbers.
• Genome-wide complex-trait analysis that considers only common single-nucleotide polymorphisms (SNPs) captures roughly 20–25 % of IQ variance (so-called SNP heritability) [5].
• The largest genome-wide association study to date (≈ 270 000 individuals) identified 205 loci; the resulting polygenic scores currently predict 5–10 % of intelligence variation in independent samples, a figure that has been inching upward with sample size and improved methods [4], [6]. These molecular estimates are lower because they do not yet capture rare variants, non-additive effects, or gene–environment interplay.
Heritability is not immutability.
High heritability does not mean intelligence cannot be changed. Environmental factors (schooling quality, nutrition, toxins, childhood adversity, etc.) can still shift the mean level of cognitive performance for whole populations even when genetic differences explain most of the variation within that population. Heritability also differs across socioeconomic strata and across countries; in poorer or more unequal settings environmental constraints usually depress heritability estimates [1], [2].
Putting the pieces together
• In infancy and the preschool years, genes account for roughly one-quarter to one-third of the variability in measured intelligence.
• In middle childhood heritability settles near 40–50 %.
• By the late teens and throughout adulthood, somewhere between 60 and 80 % of the differences in IQ scores can be traced to genetic variation, with the remainder due to non-shared environmental influences and measurement error.
Thus, “about half” is a good rule of thumb, but the proportion ranges from about 20 % to 80 % depending on age, population, and method of estimation.
Sources
[1] Scientific American – “Is Intelligence Hereditary?”: Reviews twin and adoption evidence; concludes that roughly 50 % of IQ variance is genetic on average. (https://www.scientificamerican.com/article/is-intelligence-hereditary/)
[2] Plomin & von Stumm, “The new genetics of intelligence,” Nature Reviews Genetics (2018): Summarizes twin, SNP, and polygenic findings; notes heritability rises with age and can reach 80 % in adulthood. (https://www.nature.com/articles/nrg.2017.104)
[3] Briley & Tucker-Drob, “Explaining the Increasing Heritability of Cognitive Ability Across Development,” Psychological Science (2013): Meta-analysis of longitudinal twin/adoption studies; h² grows from 0.22 in infancy to 0.81 in late adulthood. (https://pmc.ncbi.nlm.nih.gov/articles/PMC3954471/)
[4] Savage et al., “Genome-wide association meta-analysis in 269 867 individuals…” Nature Genetics (2018): Identifies 205 loci; polygenic scores explain up to 5–7 % of IQ variance. (https://pubmed.ncbi.nlm.nih.gov/29942086/)
[5] Posthuma & van Bergen, “Genetic Variation, Brain, and Intelligence Differences,” Molecular Psychiatry (2021): Reviews twin and SNP heritability (≈ 50–80 % twin; ≈ 20–25 % SNP). (https://www.nature.com/articles/s41380-021-01027-y)
[6] Genç et al., “DNA and IQ: Big Deal or Much Ado About Nothing? – A Meta-analysis,” Intelligence (2024): Finds current polygenic scores predict ~10 % of IQ variance; acknowledges rapid gains as GWAS sample sizes grow. (https://www.sciencedirect.com/science/article/abs/pii/S0160289624000655)