Partly. Twin, family and adoption studies put the heritability of IQ at roughly 0.4 to 0.8, and reviews usually summarise it at about 0.5. That means around half the variation in IQ within a population is linked to genetic differences between people. It does not mean half of your own intelligence comes from your genes, and environment still matters a great deal.
What does it mean to say IQ is genetic?
Researchers measure this with a statistic called heritability. It estimates how much of the variation in a trait, within a particular population, is explained by genetic variation between individuals. It runs from 0 (none of the variation is genetic) to 1 (all of it is).
Three caveats matter more than the number itself:
- It describes differences between people, not how much of one person's trait is caused by genes.
- It is not fixed. If environments become more varied, heritability falls; if everyone had the same environment, it would reach 100%. Populations in developing nations often face more diverse environments, so heritability there would be expected to be lower.
- It applies to the population studied, not to humanity in general.
The psychologist Thomas Bouchard suggests the sharper question is not whether intelligence is hereditary but whether variation in intelligence is influenced by genes. His answer is yes, to a degree that depends on age.
What do twin studies show?
Twin studies are the most common way to estimate heritability. Identical (monozygotic) twins come from one fertilised egg and are largely genetically identical, while fraternal (dizygotic) twins share about 50% of their DNA. If identical twins are more alike in IQ than fraternal twins, the extra similarity is attributed to genes.
That is what the research finds, and it is why most behavioural geneticists place IQ heritability between 0.4 and 0.8. A 2006 text by Alan S. Kaufman and Elizabeth O. Lichtenberger reports IQ correlations of 0.86 for identical twins raised together, 0.76 for identical twins raised apart and 0.47 for ordinary siblings.
The method has critics. The main target is the equal environment assumption: the idea that identical and fraternal twins have equally similar environments. Modelling shows that if identical twins share more similar environments, heritability estimates can come out large even with no genetic effect at all. Twins raised apart may also have had similar schooling, which would inflate their resemblance. Bouchard, for his part, has called some critical reanalyses of twins-apart data "pseudoanalysis".
What do adoption studies add?
Adoption separates genes from family environment: adoptive parents share a home with the child but not DNA. Broadly, adoptees' IQs turn out more similar to their biological parents than to their adoptive parents.
Timing matters. The psychologist Nicholas Grahame notes that children adopted very young resemble their adoptive parents in IQ at age 5 and younger, but increasingly resemble their biological parents as they grow. By around 16, he puts the share of IQ differences linked to biological parents' genes at 70 to 80%.
Does heritability change with age?
Most researchers think it rises over the lifespan, though the exact pattern is still uncertain. One estimate has heritability climbing from about 20% in infancy to as much as 80% in later adulthood. As children become adults, they increasingly choose their own surroundings, books, friends and work, and those choices tend to amplify their genetic leanings.
Is there a gene for intelligence?
No. Intelligence in the normal range is polygenic, influenced by at least 500 genes, each with a tiny effect. Everyone carries these genes; people differ in which versions (alleles) they have.
Single-gene disorders can affect intelligence outside the normal range. Phenylketonuria once caused intellectual disability in everyone who had it. A modified diet now prevents this, which lowered its heritability.
DNA-based studies have produced a puzzle. Genome-wide methods estimate IQ heritability at only about 0.10 to 0.20, far below the 40 to 80% from twin and family studies. This is the missing heritability problem. Robert Plomin and Sophie von Stumm expect the gap to close with more genetic data; Eric Turkheimer and Lucas J. Matthews argue it exposes deeper problems with the concept of heritability itself.
How much does environment matter?
A lot, and in ways that sit alongside genetic influence rather than against it. IQ scores are associated with nutrition, parental socioeconomic status and the perinatal environment. Research on gene-by-socioeconomic-status interaction suggests genetic influences on intelligence are more fully realised in advantaged conditions and suppressed under deprivation.
Height is a useful comparison. It is highly heritable, yet average heights keep rising because the shared environment has improved. IQ shows something similar: raw scores in many populations have risen by about three points per decade since the early 20th century, known as the Flynn effect. Genes cannot change that quickly.
Vocabulary makes the same point another way. It is substantially heritable, yet every word is learned. Where words are plentiful, differences in how many people pick up reflect their predispositions.
Will clever parents have clever children?
More likely than average, but not guaranteed. Polygenic traits show regression towards the mean: the more extreme the parents, the more likely their child lands closer to average. Two very tall parents usually have a child taller than most people, but rarely taller than themselves. IQ works the same way, so two parents with very high scores are more likely to have a child nearer the middle than at their level.
Do genes explain differences between groups?
No. Heritability within a group says nothing about the causes of differences between groups, and the scientific consensus is that genetics does not explain average differences in IQ test performance between racial groups.
This matters because IQ has a dark history. The eugenicist Henry H. Goddard argued low test scores were hereditary, and his test was used to push forced sterilisation laws. Upheld by the US Supreme Court in Buck v. Bell in 1927, those laws led to over 60,000 sterilisations. Mainstream science firmly rejects such ideas.
What does this mean for your own score?
Very little directly. Modern tests set the average IQ at 100 with a standard deviation of 15, so about two-thirds of people score between 85 and 115. Where you fall in the IQ score ranges cannot tell you how much reflects genes and how much reflects upbringing, schooling or health. Any score, including one from online IQ tests, is an estimate, not a fixed measure.
If you are worried about a child's development or a change in your own thinking, speak to a doctor or qualified psychologist rather than relying on a test.
Frequently asked questions
Is intelligence genetic or learned?
Both. Studies put heritability at roughly 0.4 to 0.8, so genes account for a large share of differences between people, but environment, learning and health also shape scores.
What percentage of IQ is genetic?
Reviews usually summarise it at about 50%, with twin and family studies ranging from 40 to 80%. DNA-based studies give lower figures, around 10 to 20%.
Do you inherit intelligence from your mother or father?
From both. Intelligence is influenced by hundreds of genes inherited from both parents, not passed down through a single line.
Can you raise an IQ that is partly genetic?
Heritability does not mean fixed. Height is highly heritable yet averages have risen, and IQ scores have climbed about three points per decade in many populations.
Are identical twins' IQs the same?
Very similar but not identical. One summary reports a correlation of 0.86 for identical twins raised together and 0.76 for those raised apart.
Sources
- Heritability of IQ (Wikipedia)
- Intelligence quotient (Wikipedia)
- Is Intelligence Hereditary? - Consensus
- genetics - Its there a gene or genes for intelligence? - Biology Stack Exchange
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