How heritable is IQ?

Identical (monozygotic) twins share essentially all their genes. Non-identical (dizygotic) twins share about half of the genes that vary between people. Both pairs of twins usually share a home. If identical twins resemble each other on IQ more than non-identical twins do, the extra resemblance is attributed to genes.

A simple version is Falconer's formula: heritability = 2 x (the identical-twin correlation minus the non-identical-twin correlation). Illustration, not data from a particular study: if identical twins correlate .85 and non-identical twins .60, heritability is 2 x 0.25 = .50. The rest is split into shared environment (what makes siblings alike, such as home and school) and non-shared environment (what makes them different, including measurement error).

Modern studies fit these three components, usually written A, C and E, to thousands of pairs at once. The finding that C shrinks and A grows with age is the pattern in the table above. It rests on assumptions, for example that identical and non-identical twins are treated alike by families, and the results are estimates with ranges, not fixed values.

It sounds backwards. Years of life experience might be expected to dilute the effect of genes. The usual explanation is that people choose and shape their environments. Haworth and colleagues suggest that as children grow they increasingly select, modify and create experiences that match their genetic propensities: a child with a talent for reading gets more books, and a stronger reader reads more. Young children are mostly given their environment by parents and schools, so the family explains more of the variation early on.

Thomas Bouchard (2013) named the rise the Wilson effect, after the psychologist Ronald Wilson, whose Louisville Twin Study documented it. It does not mean that adults are less affected by their surroundings. It means that, by adulthood, the differences between people are better lined up with their genes.

A heritability of 60% does not mean that 60% of your IQ comes from genes and 40% from environment. It is a statistic about variation in a particular group at a particular time. Change the group or the environment and the number can change.

It is not a fixed ceiling. The Flynn effect raised average scores by around 3 points a decade while heritability stayed substantial. Education has an effect too: a 2018 meta-analysis by Ritchie and Tucker-Drob found an extra year of school raised cognitive scores by about 1 to 5 IQ points.

It does not explain group differences. A high heritability within each of two groups says nothing about why the groups differ on average. The two questions are separate and need different evidence.

It depends on the environment. In a uniform environment, nearly all remaining differences are genetic by default. In a very unequal one, environmental differences matter more. Turkheimer and colleagues reported in 2003 that among 7-year-old American twins the shared environment explained about 60% of IQ variance in poor families and genes close to zero, while the reverse held in affluent families. Later work found the pattern in US studies but not in western Europe or Australia, where Tucker-Drob and Bates (2016) found it was zero or reversed.

Genome-wide studies look at the DNA of hundreds of thousands of people. A 2018 study by Savage and colleagues of 269,867 people found 205 genomic regions linked to intelligence. The scores built from all the markers predicted about 4.8% of the variance in intelligence in independent samples. The same study put the share explained by common genetic variants at 19%, so the DNA-based estimates are still well below the 60 to 80% found in twin studies. The gap is known as missing heritability.

Two conclusions follow. Intelligence differences are highly polygenic, with thousands of variants each having a tiny effect. And you cannot read a person's IQ from their genome with any useful accuracy today. The IQ genetics calculator estimates a child's likely score from both parents' scores using regression to the mean. Treat it as an illustration of the family pattern, not a prediction.

Heritable does not mean unchangeable, and environmental does not mean easy to change. Schooling, nutrition, health and stimulation can all shift scores, and they matter most where children lack them. At the same time, no quick fix has been shown to raise general ability by a large amount. The boost IQ guides cover what is supported.

For the concept of general ability that these estimates apply to, see the g factor. For how test scores are built, see IQ meaning.