Nearsightedness and Intelligence: The Unsolved Mystery

Children in gifted classes have three to five times more nearsightedness than their classmates in regular classes. Nobody, not even the researchers who’ve measured it most carefully, has a complete explanation for why.

It’s one of those facts that sound like a typo the first time you read them.

It isn’t.

And the closer you look, the less it looks like a simple demographic coincidence.

A finding repeated across “many studies and huge samples”

Psychologist Arthur Jensen devotes a section of The g Factor (1998) to this relationship, which he describes as “long known” and backed by many studies and very large samples.

• The correlation between degree of nearsightedness and IQ runs, according to those studies, between +0.20 and +0.25.
• Nearsightedness is highly heritable, and a specific gene regulating the shape of the eyeball has been identified as the main culprit.
• As early as infancy, from one year of age, an eye exam can predict with some reliability whether that person will develop nearsightedness in adolescence or adulthood.

On the surface, it’s one of those curious, low-stakes correlations. The surprise comes when you try to explain why it exists.

The obvious explanation, and why it doesn’t hold up

The first hypothesis anyone comes up with is the most intuitive one: people who read more, study more, and therefore use their eyes up close more, end up developing more nearsightedness. And people who read more also tend to have better academic results.

Jensen explains why this idea, known as the “near-work” hypothesis, is largely ruled out:

• People with trisomy 21 (Down syndrome), a condition that markedly reduces intellectual capacity and, with it, time spent reading and doing other close-up tasks, show the same rate of nearsightedness as the general population.
• If nearsightedness depended only on reading habits, you’d expect much less of it in this group. That isn’t what’s observed.
• Nearsightedness also has a strong hereditary component, something a simple reading habit wouldn’t explain.

Jensen’s conclusion is that “near-work” probably interacts with an already-existing genetic predisposition, but doesn’t create it from scratch.

The experiment that isolates the effect: comparing siblings to each other

The most convincing piece of evidence the book documents comes from a 1988 study by Cohn, Cohn, and Jensen, designed specifically to answer an uncomfortable question: is this correlation real within each family, or is it just an effect of different population groups differing in both nearsightedness and IQ?

• Sixty adolescents with high IQ (measured with the Raven’s Matrices) were selected, along with their full siblings, whose IQ averaged fourteen points lower.
• Everyone’s degree of nearsightedness was measured objectively, through optical refraction, not simply whether or not they wore glasses.
• The sibling with the higher IQ showed, on average, significantly more nearsightedness than their own lower-IQ sibling, within the same family, with the same environment and largely the same shared genetic background.

Because they were siblings, nearly every variable that could “inflate” a correlation like this (socioeconomic level, educational environment, family culture) is controlled for. What’s left is an intrinsic relationship: something within each individual connects both traits, not just the fact of belonging to a particular family or social group.

Genes that do two things at once, without anyone quite knowing how

This is where the mystery gets, honestly, more interesting than if it had a clear answer.

Jensen doesn’t offer a direct causal explanation. What he proposes is the hypothesis of a pleiotropic relationship: the possibility that a single gene, or a set of related genes, influences both eye development and some aspect of neurological development related to g, without one directly causing the other.

It’s a bit like discovering that two light switches, in different rooms of a house, are wired to the same hidden cable in the wall: you can confirm they turn on together, but that tells you nothing about why someone wired them that way.

Why this correlation isn’t practically useful (and why that’s fine)

Before jumping to conclusions, it’s worth being clear about the limits of this finding:

• A correlation of +0.20 to +0.25 is modest: it leaves plenty of room for lots of nearsighted people with no above-average IQ, and vice versa.
• There’s no practical mechanism that turns this into useful information about a specific person based on whether they wear glasses.
• What’s valuable about this finding isn’t its usefulness, it’s what it reveals about how intertwined traits can be that, on the surface, seem to have nothing to do with each other.

Sometimes science doesn’t close a mystery, it just describes it more precisely. This is one of those cases: a real link, carefully measured, still without a complete explanation for why the same body that sharpens vision for what’s close by also seems to sharpen something else, at times.

If another link this unexpected between biology and the g factor interests you, why two strangers raised in the same house end up nothing alike explains what twin and adoption studies show.

Sources

• Jensen, A. R. (1998). The g Factor: The Science of Mental Ability. Praeger.
• Cohn, S. J., Cohn, C. M. G., & Jensen, A. R. (1988). Myopia and intelligence: A pleiotropic relationship? Human Genetics, 80, 53-58.