Heritability is the proportion of variation in a trait among individuals in a population that can be attributed to genetic differences. It is a statistic about populations, not a fixed property of individuals or of the trait itself.

Twin and adoption studies are the classic methods for estimating heritability. Identical twins reared apart provide a powerful natural experiment: similarities between them are more likely to reflect genes, while differences reflect environment. Heritability estimates for height are high; estimates for most psychological traits, including intelligence and personality, are moderate. The remaining variance is attributed to environmental factors, both shared and unique.

Heritability can change with the environment. In a population where everyone has adequate nutrition, genetic differences account for more of the variation in height; in a population with large nutritional disparities, environmental differences loom larger. High heritability does not mean that a trait is unchangeable; it means that, under current conditions, genetic differences explain much of the observed variation.

Heritability answers a specific question: of the differences we see, how much is linked to genetic differences? It does not answer how a trait develops in any single person.