The science of intelligence
How environment shapes intelligence
Short answer
Environment clearly matters. A meta-analysis of over 600,000 people found that each extra year of education raises cognitive ability by about 1 to 5 IQ points. Average scores also rose about 3 points per decade through the 20th century, and a study of Norwegian brothers traced both that rise and its later decline to environmental factors.
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Nature and nurture are not rivals
Genes matter for intelligence, and that is covered on the genes and intelligence page. But the same research tradition is clear that high heritability does not mean intelligence is unaffected by environment or fixed at birth (Gottfredson, 1997). One subtle finding: the influence of the shared family environment on IQ falls from about .35 in childhood to quite low by late adolescence (Neisser et al., 1996). Put simply, the home that siblings share accounts for less and less of the differences in their scores as they grow up.
Schooling: the strongest evidence
A 2018 meta-analysis gathered quasi-experimental studies, which use natural differences in how much schooling people received rather than random assignment. Across 142 effect sizes from 42 data sets and over 600,000 participants, it found that an additional year of education raises cognitive ability by approximately 1 to 5 IQ points (Ritchie & Tucker-Drob, 2018). The effects persisted across the life span and appeared on all broad categories of ability studied, and the authors called education the most consistent, robust and durable method yet identified for raising intelligence (Ritchie & Tucker-Drob, 2018).
Reading
Comparing identical twins with each other is a way to hold genes constant and look at experience. In 1,890 identical-twin pairs tested from ages 7 to 16, the twin who read better earlier tended to show higher intelligence later, and the associations were not restricted to verbal abilities (Ritchie, Bates & Plomin, 2015).
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 two solid stars (option 7).
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 triangle) 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 two solid stars.
That was 1 puzzle. The full test has 25.
The Flynn effect: change across generations
Average measured IQ rose through the 20th century. A 2015 meta-analysis of 271 samples (about 4 million people, 31 countries, 1909 to 2013) estimated about 2.8 points per decade and found the gains slowing in recent decades (Pietschnig & Voracek, 2015). Gains were largest for fluid tests (0.41 points a year), then spatial (0.30), full-scale (0.28) and crystallized (0.21) (Pietschnig & Voracek, 2015).
A 2014 meta-analysis of 285 studies (N = 14,031) since 1951 came to a somewhat different view. It estimated 2.31 points per decade overall and 2.93 for modern Stanford-Binet and Wechsler tests, and found the modern-test estimate not consistent with the idea that the effect is diminishing (Trahan et al., 2014). The two analyses used different data, so whether the rise slowed depends partly on which tests and years you look at. The Flynn effect guide covers what this means for reading scores.
The causes are still debated. The 1996 APA task force said they were unknown, considered more complex modern life, better nutrition and Flynn's view that what rose is not intelligence itself, and noted that the effects of nutrition were themselves not firmly established (Neisser et al., 1996). It also judged the gains much too large to come from simple test sophistication (Neisser et al., 1996). Pietschnig and Voracek later concluded that factors associated with life history speed seem mainly responsible (Pietschnig & Voracek, 2015).
Evidence from brothers
If scores changed because of who was having children, comparing brothers in the same family would hide it. Using Norwegian conscription data for male cohorts born 1962 to 1991, Bratsberg and Rogeberg found the rise, its turning point and the decline after about the 1975 cohort all within families, which establishes that the changes reflect environmental factors (Bratsberg & Rogeberg, 2018). See is the average IQ falling?
Early life and early programs
In the PROBIT trial (17,046 infants enrolled in Belarus, 13,889 followed up at 6.5 years), the breastfeeding-promotion group scored +7.5 verbal IQ (95% CI +0.8 to +14.3), +2.9 performance IQ (CI -3.3 to +9.1) and +5.9 full-scale IQ (CI -1.0 to +12.8) (Kramer et al., 2008). The authors concluded that prolonged and exclusive breastfeeding improves cognitive development, though two of the three intervals include zero.
Early education programs show a different pattern: scores in Head Start and similar programs often rise during the program but the gains fade with time (Neisser et al., 1996). What this means for adults who want to improve is covered in can you raise your intelligence?
Sources
- 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
- 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
- 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
- Ritchie, S. J., Bates, T. C., & Plomin, R. (2015). Does learning to read improve intelligence? Child Development, 86(1), 23-36. doi.org/10.1111/cdev.12272
- Pietschnig, J., & Voracek, M. (2015). One century of global IQ gains. Perspectives on Psychological Science, 10(3), 282-306. doi.org/10.1177/1745691615577701
- Trahan, L. H., Stuebing, K. K., Fletcher, J. M., & Hiscock, M. (2014). The Flynn effect: A meta-analysis. Psychological Bulletin, 140(5), 1332-1360. doi.org/10.1037/a0037173
- Bratsberg, B., & Rogeberg, O. (2018). Proceedings of the National Academy of Sciences, 115(26), 6674-6678. doi.org/10.1073/pnas.1718793115
- Kramer, M. S., et al. (2008). Breastfeeding and child cognitive development: New evidence from a large randomized trial. Archives of General Psychiatry, 65(5), 578-584. doi.org/10.1001/archpsyc.65.5.578