Nearsightedness and Intelligence: A Mystery Science Has Begun to Solve

Children in gifted classes are, on average, considerably more nearsighted than their classmates. For decades, some psychologists saw that fact as a clue about the genes behind intelligence. Research over the past twenty years has found a simpler, and more useful, explanation: what links myopia and intelligence runs mainly through the classroom.

The story is worth telling in full, because it’s a good example of how a well-measured correlation can lead to the wrong hypothesis.

A modest, well-documented correlation

Psychologist Arthur Jensen devoted a section of The g Factor (1998) to this link, which he described as “long known” and backed by many studies with very large samples. According to the work he reviews, the correlation between degree of myopia and IQ is around +0.20 to +0.25.

That’s a small correlation: it leaves plenty of room for huge numbers of nearsighted people with average or below-average IQs, and for many high-IQ people who see perfectly well at a distance. But it turns up again and again, and it called for an explanation.

Jensen’s hypothesis: one gene for the eye and the brain

Jensen bet on a pleiotropic explanation: that a single gene, or a set of genes, influenced both the development of the eye and some aspect of brain development related to the g factor, without one causing the other.

His main evidence was a 1988 study he co-authored with S. J. Cohn and C. M. G. Cohn. They selected sixty high-IQ adolescents (measured with Raven’s Matrices) and their full siblings, whose IQ was on average about fourteen points lower. Everyone’s myopia was measured objectively, by refraction. The higher-IQ sibling turned out to be, on average, clearly more nearsighted.

Comparing siblings controls for socioeconomic status and the shared family environment. What it doesn’t control for is what each sibling does with their time: if the more gifted one reads more, studies for more years and spends more hours indoors, they’ll also have more myopia. The authors acknowledged as much in the paper’s abstract: environmental pathways “cannot yet be ruled out”. The result was compatible with pleiotropy, not proof of it.

The Down syndrome argument, and why it doesn’t work

Jensen added a second argument to rule out the studying-and-reading explanation: according to him, people with Down syndrome, who spend much less time on close-up tasks, have the same incidence of myopia as the general population.

That reasoning doesn’t hold up well. Refractive errors are very common in Down syndrome for reasons specific to how the eye develops, and in children the most common one is farsightedness, not myopia. In a review of the medical records of 96 children with Down syndrome in England (Creavin and Brown, 2010), 96% of those assessed had at least one ophthalmic condition. A group whose eyes develop so differently can’t be used to test what role reading plays in myopia in the rest of the population.

The missing piece: the myopia epidemic

While psychologists argued about genes, ophthalmology was confronting a phenomenon no gene could explain. In the cities of East Asia, myopia soared within a few decades to affect 80-90% of young people finishing secondary school, according to the review by Ian Morgan, Kyoko Ohno-Matsui and Seang-Mei Saw in The Lancet (2012).

Myopia has a substantial hereditary component, but a population’s gene pool doesn’t change in two or three generations. A rise that fast points to a change in environment: more years of intensive schooling and fewer hours outdoors.

Mountjoy (2018): education causes myopia, not the other way round

The study that changed the debate was published in the BMJ in 2018. Edward Mountjoy and colleagues used a technique called Mendelian randomization. The idea is to take advantage of the fact that genes are shuffled at random at birth: if certain genetic variants nudge people toward more years of education, they act as a “natural experiment” for seeing what happens to the eyesight of those who inherit them, without the other differences between people getting in the way.

They analyzed 67,798 adults from the UK Biobank and looked in both directions:

• From education to myopia. Each additional year of schooling was linked to 0.18 more diopters of myopia in the observational data, and to 0.27 diopters in the genetic analysis, which estimates the causal effect. By their calculations, a British university graduate, with 17 years of education, would on average have at least one more diopter of myopia than someone who left school at 16, with 12 years. That’s enough to need glasses for driving.
• From myopia to education. Genetic predisposition to nearsightedness barely affected years of schooling.

The main arrow, then, runs from school to the eye. And IQ enters the story by a familiar route: people with greater cognitive ability tend to spend more years in education. No shared gene is needed to explain the correlation that intrigued Jensen.

It’s not just close-up reading: outdoor light

One important nuance remains. If the explanation were only “near work”, reading less would be enough. Research points to another factor that travels with studying: time outdoors.

In the Sydney Myopia Study, Kathryn Rose and her team (2008) examined more than 4,000 schoolchildren and found that 12-year-olds who spent more time outdoors had less myopia, even when they read a lot. Indoor sport showed no such effect, which points to exposure to outdoor light rather than to exercise.

The experimental test came in 2015. In a trial with first-grade schoolchildren in Guangzhou (China), Mingguang He and colleagues added a daily 40-minute class of outdoor activities in six schools and compared them with six others. After three years, the incidence of myopia was 30.4% in the schools with the extra class, versus 39.5% in the rest.

The most studied hypothesis to explain this is that bright outdoor light stimulates the release of dopamine in the retina, which slows the excessive lengthening of the eye that produces myopia. It’s a mechanism with good support in animals and still under study in humans.

What remains open

The educational explanation doesn’t close every question. Myopia still has a genetic component, and it can’t be ruled out that some variant affects both the eye and cognition. But the weight of the evidence has shifted: the clearest, best-demonstrated pathway is that studying for more years, with many hours indoors, increases myopia.

Seen this way, the “mystery” has turned into something more practical. Myopia says nothing useful about any one person’s intelligence, but the story of its link with IQ teaches two things: that a carefully measured correlation can have a very different explanation from the one people look for first, and that the playground counts for eye health too.

If you’re interested in another debate about what we inherit and what the environment does, why two strangers raised in the same house end up nothing alike explains what twin and adoption studies show.

Sources

• Cohn, S. J., Cohn, C. M. G. and Jensen, A. R. (1988). Myopia and intelligence: A pleiotropic relationship? Human Genetics, 80(1), 53-58.
• Creavin, A. L. and Brown, R. D. (2010). Ophthalmic assessment of children with Down syndrome: is England doing its bit? Strabismus, 18(4), 142-145.
• He, M., Xiang, F., Zeng, Y., Mai, J., Chen, Q., Zhang, J., Smith, W., Rose, K. and Morgan, I. G. (2015). Effect of time spent outdoors at school on the development of myopia among children in China: a randomized clinical trial. JAMA, 314(11), 1142-1148. DOI
• Jensen, A. R. (1998). The g Factor: The Science of Mental Ability. Praeger.
• Morgan, I. G., Ohno-Matsui, K. and Saw, S. M. (2012). Myopia. The Lancet, 379(9827), 1739-1748.
• Mountjoy, E., Davies, N. M., Plotnikov, D., Smith, G. D., Rodriguez, S., Williams, C. E., Guggenheim, J. A. and Atan, D. (2018). Education and myopia: assessing the direction of causality by mendelian randomisation. BMJ, 361, k2022. DOI
• Rose, K. A., Morgan, I. G., Ip, J., Kifley, A., Huynh, S., Smith, W. and Mitchell, P. (2008). Outdoor activity reduces the prevalence of myopia in children. Ophthalmology, 115(8), 1279-1285.