How much of IQ is genetic?

Twin and adoption studies estimate the heritability of adult IQ at approximately 0.5 to 0.8 in the populations studied. This means 50 to 80% of the variance in adult IQ within a population is statistically attributable to genetic differences. Importantly, heritability is a population-level statistic, not an individual one — it tells you about variance within a population, not about what proportion of any individual's IQ is 'due to genes'. Heritability also depends on the environmental range of the studied population: in highly equal environments, more residual variance is genetic; in highly unequal environments, more residual variance is environmental. Genome-wide association studies (GWAS) have identified thousands of variants that contribute to IQ, each with very small individual effects. Polygenic scores currently account for 10 to 20% of variance in IQ in the populations studied — useful for research, currently of very limited individual predictive power.

This question comes up frequently from users of free online IQ tests and from people considering whether to pursue a clinical evaluation. The full answer depends on context — what the score will be used for, how recently the test was administered, and what other information is available. The brief answer above captures the broad consensus from the published research literature; the linked deep-dive articles cover the underlying evidence in more detail.

Related considerations include the standard error of measurement on the relevant test, the population the test was normed against, and the specific cognitive abilities the test samples. A score is much more informative when interpreted alongside these contextual variables than when reported as a bare number.

If this answer raises further questions, see the related FAQ entries listed in the sidebar and the longer-form articles on the same topic in the article library. The site is designed to provide layered depth: the FAQ entries offer concise answers, the deep-dive articles offer the underlying research, and the score-interpretation pages tie the abstract concepts to specific result bands.