Many studies have shown a relationship between myopia and IQ. According to Arthur Jensen, myopes average 7-8 IQ points higher than non-myopes. The relationship also holds within families, and siblings with a higher degree of refraction error average higher IQs than siblings with less refraction error
Of course, the causality can be reversed as well -- worse vision can lead to less proficiency in things like sports and other fast paced stuff, meaning you're more likely to start reading in the first place.