Does Reading Increase IQ? What the Research on Books Shows

Reading probably does increase IQ a little, and most of the gain shows up as vocabulary and general knowledge. In the strongest study, which followed 1,890 pairs of identical twins from age 7 to 16, the twin who read better at age 12 tended to score higher on intelligence tests at 16 than the co-twin, with reading differences explaining about 7% of later IQ differences (about 13% after correcting for measurement error).

That is a meaningful finding, and it comes with two caveats. In the same study, how much the twins read for pleasure added nothing once reading skill was accounted for, and other research shows that much of the link between reading and intelligence runs the other way: children who read well choose to read more. This page covers the identical-twin evidence, how reading builds the kind of knowledge IQ tests sample, the reverse-causation problem, and why the claim that books make you smarter is so widely believed. Effect sizes are given as correlations (r) or as a percentage of variance explained, which is r squared.

Comparing readers with non-readers cannot settle the question, because families that read a lot differ from families that do not in many ways, including their genes. Stuart Ritchie, Timothy Bates and Robert Plomin (2015) used a design that removes most of those differences. They studied monozygotic (identical) twins in the UK Twins Early Development Study, who share all their genes and grow up in the same home. Any difference between two identical twins must come from experiences they do not share, which is why twin studies of IQ are so useful for separating causes.

The researchers tested the twins at ages 7, 9, 10, 12 and 16 on reading and on intelligence tests that included both verbal measures, such as vocabulary, and nonverbal ones, such as Raven's Progressive Matrices. They then asked whether a twin who read better than the co-twin at one age went on to have a higher IQ than the co-twin at a later age.

• Main result: Earlier reading advantages predicted later intelligence advantages across several age gaps. Reading differences at 12 accounted for about 7% of intelligence differences at 16, or around 13% after the authors corrected for measurement error.

• Beyond vocabulary: The links were not limited to verbal tests. Reading differences also predicted later differences on nonverbal intelligence measures, though the summed association was larger for verbal intelligence.

• Amount read: A measure of how much the children read (an author-recognition test given at ages 10 and 12) did not predict later intelligence once reading ability was included in the model.

• Both directions: Higher nonverbal intelligence also predicted later gains in reading, so the two abilities appear to feed each other.

The authors were careful about what this does and does not show. The design rules out genetic and shared family explanations, but something else that differs between twins, such as a teacher who helped one twin more, could improve both early reading and later intelligence without reading itself doing the work. It is also a single study, so its exact estimates should be treated with caution.

The most consistent effect of reading is on language. Written text uses a wider range of words than everyday conversation, and readers meet those words in context again and again. Psychologists call the stored knowledge that results "crystallized intelligence", the words and facts a person has accumulated, which is different from fluid reasoning on new problems (our article on fluid vs. crystallized intelligence explains the distinction). Vocabulary and general-information subtests are standard parts of IQ batteries, so reading can raise scores on those sections directly. Our article on what vocabulary tests measure explains why they track general intelligence so closely.

Suzanne Mol and Adriana Bus (2011) pooled 99 studies with 7,669 participants in a Psychological Bulletin meta-analysis of leisure reading from preschool to university. Print exposure, meaning how much a person reads outside school, showed moderate to strong correlations with reading comprehension and with technical reading and spelling, and its link with oral language grew with age. Print exposure explained 12% of the variance in oral language skills in preschool and kindergarten (r of about .35), rising to 30% in high school and 34% in college and university (r of about .58). The authors read this as an upward spiral: better readers read more, and the extra reading improves their skills further each year.

Anne Cunningham and Keith Stanovich (1997) followed a group of first graders to 11th grade. Reading ability in first grade predicted 11th-grade vocabulary and general knowledge, along with how much the students read, even after the researchers statistically removed the effect of cognitive ability, and differences in print exposure predicted growth in reading comprehension through school.

Correlations between reading and ability are only half the story. Elsbeth van Bergen and colleagues (2018) studied 11,559 twins from the Netherlands Twin Register at about age 7.5. Reading ability and print exposure correlated .41, and models that exploit the differing genetic make-up of the two traits supported a causal path from reading ability to how much children chose to read, rather than the reverse. A 2023 study of 3,690 Finnish twins aged 12 reached a similar conclusion about enjoyment: literacy skills shaped how much children liked reading and writing, while the influence of enjoyment on skills was estimated at zero.

The picture is not one-directional at every age. Minna Torppa and colleagues (2020) followed 2,525 Finnish students from age 7 to 16 and separated within-person changes from stable differences between children. In grades 1 to 3, weaker comprehension and fluency led to less leisure reading. In later grades, reading more books predicted better comprehension, while digital reading was negatively associated with reading skills.

The idea that good readers pull further ahead every year was popularized by Stanovich (1986) as the "Matthew effect" in reading, after the biblical line about the rich getting richer. A 2014 review of 25 years of studies by Pfost and colleagues found no strong general support for steadily widening gaps. Widening gaps were more likely on precisely measured decoding and vocabulary tests, and a pooled correlation of r = -.21 between starting level and later growth meant that, on average, weaker starters gained slightly more.

Part of the appeal is that schooling, which is built around reading, does raise measured intelligence. Stuart Ritchie and Elliot Tucker-Drob (2018) meta-analyzed 142 effect sizes from 42 data sets with more than 600,000 participants and estimated that each additional year of education adds approximately 1 to 5 IQ points, with effects on every broad ability studied. Reading is one of the main channels through which school delivers knowledge, so it is plausible that some of that gain runs through literacy. The meta-analysis did not isolate reading, though, and the education effect cannot be credited to books alone.

Another source of the idea is a single well-publicized experiment. Kidd and Castano (2013) reported in Science that reading short passages of literary fiction immediately improved performance on a test of reading emotions from photographs of eyes. Three independent research groups repeated the study with 792 participants and found no advantage for literary fiction over the other reading conditions or over no reading at all. They did find that people with more lifetime exposure to fiction scored higher, a correlation that could reflect either a long-term effect of reading or the kind of people who are drawn to novels.

Probably a little, mostly through vocabulary and general knowledge. A study of identical twins found that the better reader of each pair tended to score higher on later IQ tests, but it found no added effect of simply reading more.

There is limited evidence. The twin study found links from reading to later nonverbal reasoning scores, but no experiment has shown that assigning people to read more raises fluid reasoning.

Yes, on average, and much of the link runs in that direction. Twin studies suggest that children who read well choose to read more and enjoy it more.

A 2013 study reported that reading literary fiction boosted social understanding, but a replication with 792 participants found no such effect. Lifelong fiction readers do score higher on those tests, which may reflect who chooses to read fiction.

Leisure reading correlates with vocabulary at every age studied, including university students. Whether reading in adulthood raises IQ itself has not been tested in controlled trials.

Reading is one of the few everyday activities with credible evidence that it adds to measured intelligence, and the gains are concentrated in vocabulary and knowledge, with smaller and less certain effects on reasoning. Skill at reading seems to matter more than hours spent with a book, and a large share of the reading-IQ correlation exists because capable readers seek out more reading. To see where your verbal and nonverbal reasoning stand, try a professionally developed IQ test.

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2. Mol, S. E., & Bus, A. G. (2011). To read or not to read: A meta-analysis of print exposure from infancy to early adulthood. Psychological Bulletin, 137(2), 267-296. [doi.org/10.1037/a0021890](. doi.org

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6. Torppa, M., Niemi, P., Vasalampi, K., Lerkkanen, M.-K., Tolvanen, A., & Poikkeus, A.-M. (2020). Leisure reading (but not any kind) and reading comprehension support each other: A longitudinal study across grades 1 and 9. Child Development, 91(3), 876-900. [doi.org/10.1111/cdev.13241](. doi.org

7. Stanovich, K. E. (1986). Matthew effects in reading: Some consequences of individual differences in the acquisition of literacy. Reading Research Quarterly, 21(4), 360-407. [doi.org/10.1598/RRQ.21.4.1](. doi.org

8. Pfost, M., Hattie, J., Dörfler, T., & Artelt, C. (2014). Individual differences in reading development: A review of 25 years of empirical research on Matthew effects in reading. Review of Educational Research, 84(2), 203-244. [doi.org/10.3102/0034654313509492](. doi.org

9. 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](. doi.org

10. Kidd, D. C., & Castano, E. (2013). Reading literary fiction improves theory of mind. Science, 342(6156), 377-380. [doi.org/10.1126/science.1239918](. doi.org

11. Panero, M. E., Weisberg, D. S., Black, J., Goldstein, T. R., Barnes, J. L., Brownell, H., & Winner, E. (2016). Does reading a single passage of literary fiction really improve theory of mind? An attempt at replication. Journal of Personality and Social Psychology, 111(5), e46-e54. [doi.org/10.1037/pspa0000064](. doi.org

Hero image: Open book with apples placed on a wooden table, by Shixart1985, CC BY 2.0. Via Wikimedia Commons.