The science of intelligence
Genes and intelligence
Short answer
Partly. Twin and adoption studies estimate that the heritability of intelligence rises from about 20% in infancy to perhaps 80% in later adulthood. Heritability describes how much of the variation in a group goes with genetic differences. It does not say how much of one person’s IQ came from their genes, and it does not mean IQ is fixed.
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How researchers estimate genetic influence
Intelligence was the first behavioral trait studied with twin and adoption designs, nearly a century ago, and those studies have consistently found substantial genetic influence (Plomin & Deary, 2015). Twin studies estimate genetic influence by comparing identical twins with fraternal twins (Plomin & Deary, 2015). When identical twins turn out more alike in IQ than fraternal twins, that gap is the signal researchers use to estimate how much genes contribute.
Family studies point the same way. A 1981 review of 111 family studies found average correlations in a pattern consistent with polygenic inheritance, with marked variation between studies (Bouchard & McGue, 1981). Polygenic means many genes, each with a small effect, not a single "intelligence gene." For a short version of this topic, see is IQ genetic?
Heritability rises as children grow up
One of the most replicated findings is that genetic influence on intelligence gets stronger with age, not weaker. In a sample of 11,000 twin pairs from four countries, heritability rose steadily through childhood (Haworth et al., 2010):
| Age | Heritability of general cognitive ability |
|---|---|
| 9 | 41% |
| 12 | 55% |
| 17 | 66% |
Across the whole lifespan, Plomin and Deary put heritability at about 20% in infancy and perhaps 80% in later adulthood (Plomin & Deary, 2015). The 1996 APA task force gave a similar picture: about .45 in childhood and about .75 by late adolescence, while the influence of the shared family environment fell from about .35 to quite low (Neisser et al., 1996). The 1994 mainstream statement gave a range of 0.4 to 0.8 (Gottfredson, 1997).
Try one · Abstract Reasoning
Which tile completes the grid?
Tap the option you think is right, or show the answer.
Show the answer
The missing tile has one solid diamond (option 6).
How to solve it
- Start with the first column and name what changes from tile to tile.
- Every row and every column contains each shape (star, diamond and cross) exactly once.
- Every row and every column contains each number of shapes (one, two and three) exactly once.
- The fill (solid) stays the same in every tile.
- Check the rules on the second column, then apply them to the third.
- So the missing tile has one solid diamond.
That was 1 puzzle. The full test has 25.
Why the genetic share grows
Genes do not switch on in the teenage years. The leading explanation is gene-environment correlation: as children grow up they increasingly select, modify and even create their own experiences, partly based on genetic propensities (Haworth et al., 2010). A child who enjoys reading seeks out books, and the books build the skill further. Plomin and Deary call this the most likely explanation, with small genetic differences magnified over time (Plomin & Deary, 2015).
A meta-analysis of longitudinal twin and adoption studies (11,500 twin and sibling pairs, ages 6 months to 18 years) adds detail: after age 8, rising heritability is driven mainly by amplification of genetic influences already present, rather than by new ones (Briley & Tucker-Drob, 2013).
Genes act broadly
Different cognitive and learning abilities correlate about 0.30 on average at the observed level, but about 0.60 or higher genetically (Plomin & Deary, 2015). In other words, many of the same genes influence reading, math and reasoning, which fits the idea of a general factor described in the g factor guide. Spouses also tend to be similar: they correlate about 0.40 for intelligence, more than for personality (about 0.10) or for height and weight (about 0.20) (Plomin & Deary, 2015).
What heritability is not
- Not fixed. High heritability does not mean intelligence is unaffected by environment or fixed at birth (Gottfredson, 1997). An extra year of education raises cognitive ability by about 1 to 5 IQ points (Ritchie & Tucker-Drob, 2018). The environment page covers that side.
- Not about one person. Heritability is a population statistic. A figure of 80% does not mean 80% of your score came from your genes. See the heritability glossary entry.
- Not destiny within a family. Siblings differ by an average of about 12 IQ points (Gottfredson, 1997).
How genes might act through brain structure is covered in the brain and intelligence, and the wider point that measured ability can move is in the myth that IQ is fixed.
Sources
- Plomin, R., & Deary, I. J. (2015). Molecular Psychiatry, 20, 98-108. doi.org/10.1038/mp.2014.105
- Haworth, C. M. A., et al. (2010). The heritability of general cognitive ability increases linearly from childhood to young adulthood. Molecular Psychiatry, 15(11), 1112-1120. doi.org/10.1038/mp.2009.55
- Briley, D. A., & Tucker-Drob, E. M. (2013). Explaining the increasing heritability of cognitive ability across development: A meta-analysis of longitudinal twin and adoption studies. Psychological Science, 24(9), 1704-1713. doi.org/10.1177/0956797613478618
- Bouchard, T. J., Jr., & McGue, M. (1981). Familial studies of intelligence: A review. Science, 212(4498), 1055-1059. doi.org/10.1126/science.7195071
- Neisser, U., Boodoo, G., Bouchard, T. J., Jr., Boykin, A. W., Brody, N., Ceci, S. J., Halpern, D. F., Loehlin, J. C., Perloff, R., Sternberg, R. J., & Urbina, S. (1996). Intelligence: Knowns and unknowns. American Psychologist, 51(2), 77-101. doi.org/10.1037/0003-066X.51.2.77
- Gottfredson, L. S. (1997). Mainstream science on intelligence: An editorial with 52 signatories, history, and bibliography. Intelligence, 24(1), 13-23. doi.org/10.1016/S0160-2896(97)90011-8
- Ritchie, S. J., & Tucker-Drob, E. M. (2018). How much does education improve intelligence? A meta-analysis. Psychological Science, 29(8), 1358-1369. doi.org/10.1177/0956797618774253