by Patrick Appel
A reader writes:
Wikipedia and the other studies cited by yourself and readers note correlations between IQ and myopia, and general intelligence, and guess at some causes, but there is a known causal link, between excessive eye length (myopia), and IQ and education. The intervening variable is reading, and how it intersects with the mechanisms of how the eye uses its own activity in early development to regulate its optics.
In the first years of life, the focusing power of the lens and cornea, and the focal distance to the retina are only loosely linked (the whole eye is growing into early childhood, and modifying its size in smaller amounts thereafter). Rather than having genetically-set values for all its optical dimensions, the eye "measures" if it is in focus by several methods, and uses that measurement to control its growth. One measurement strategy uses the clear borders in the retinal image that result from good focus. High contrast borders drive the neurons in the retina briskly; uniform gray doesn't. Therefore, if there is a lot of activity in the retina, that indicates it is in focus, and the high activity is read out in several more steps of cell biology to stop further growth.
Unfortunately, reading interferes with this - if you focus on a page of text, the very center of gaze is in focus, but the rest isn't, it's just gray. I attach a jpg of the July 1987 cover of Science magazine (above) showing this effect in which the study I cite below appeared (featuring, in apparent precognition of the Tea Party movement, the US Constitution as the source of myopia...) With this limited activity, the eye doesn't send the signal to check its growth, and the eye continues to elongate, making the image focus fall short, in front of the retina. There's a bit more to the story, this being the childhood part, but the phenomenon itself is well worked out in several animals (not by requiring them to read, just by defocus or blur, Wallman below). For ophthalmologists, this phenomenon is called "form deprivation myopia" (or even "reading myopia"). Amount of education/reading correlates highly with myopia, prospective experiments between amount of reading and the progression of myopia have been done (Hepsen, below) and in those cases (as in China) where large groups have gone from illiterate to literate, myopia rises to western levels.
So, until contacts came on the scene, glasses for distant vision (not reading glasses) would be a good sign of educational level, not an accidental cultural convention! I have done work in this area, though I'm not at all the main authority.