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July 08, 2004

Lifestyle of myopia

Lifestyle causes myopia, not genes. Money shot:

...There, 80 per cent of 18-year-old male army recruits are myopic, up from 25 per cent just 30 years ago....

For instance, 70 per cent of 18-year-old men of Indian origin living in Singapore have myopia, while in India itself the rate is roughly 10 per cent.

Another study found myopia rates of 80 per cent in 14 to 18-year-old boys studying in schools in Israel that emphasise reading religious texts. The rate for boys in state schools was just 30 per cent.

This does not deny that genetics has some influence of course. Read the full article to get beyond the headline. Differences between groups (that is, religious students in Israel might be of a different ethnic mix) or selection bias (that is, immigrants to Singapore are not a proper sample of Indians) can explain some of the results, but the ones that look at the same population over a few generations are pretty hard to explain genetically (note that of course some people might be genetically predisposed to develop myopia in a given environment, while others might develop it no matter the environment, while some will never develop myopia and are insensitive to environmental inputs).

Godless comments:

NCBI summary article on genetics of myopia:

High-grade myopia is a refractive error greater than or equal to 6 diopters. Several loci for high-grade myopia have been mapped: MYP1 (310460) on Xq28, MYP2 on chromosome 18p, MYP3 (603221) on chromosome 12q, MYP4 (608367) on chromosome 7q, and MYP5 (608474) on chromosome 17q.

Myopia of severe degree was transmitted through 4 generations in the family reported by Francois (1961). Franceschetti (1953) observed a family with 10 cases in 4 generations. Four suffered detachment of the retina. Myopia is, in a sense, a metric character. Variation in many components of the eye contributes to its refractive capacity (Sorsby et al., 1962). Some myopia, perhaps most, is multifactorial in causation. Although autosomal recessive inheritance has been suggested (Macklin, 1927; Karlsson, 1975; Edwards and Lewis, 1991), autosomal dominant myopia has been reported by Flach (1942), Franceschetti (1953), and Francois (1961). DelBono et al. (1995) described 52 2- and 3-generation families with 2 or more individuals affected by juvenile-onset myopia, defined as refractive error of more than -0.75 diopters by age 15 years.

Karlsson (1975) concluded that the 'myopia gene' may influence brain development. Myopic high school students aged 17 or 18 years performed better on IQ tests than their nonmyopic classmates. Comparison with test results obtained 10 years earlier before development of myopia suggested that the influence of the gene on the brain was of fundamental importance. Cohn et al. (1988) investigated the association between myopia and superior intelligence in the general population in a group of intellectually gifted children and their less gifted full sibs. A highly significant gifted-nongifted sib difference in myopia was found consistent with the hypothesis that intelligence and myopia are related pleiotropically.

On the basis of studies in the Finnish Twin Cohort, Teikari et al. (1991) estimated that the heritability of myopia is 0.58 (0.74 for males and 0.61 for females) when myopia is considered a dichotomous variable.

Kolata (1985) summarized the work of Raviola and Wiesel (1985), which is relevant to the nature/nurture controversy in the area of myopia. Their work with an animal model suggested that myopia is caused by abnormal influences of the nervous system on the developing eye. In studying the effects of visual deprivation on the development of the visual system, they sutured shut the eyes of young monkeys. In the course of this they found that the eyeball grew abnormally long as in myopia. Monkeys with sutured eyes reared in the light became myopic whereas those reared in the dark did not. Distortion of the visual image by injecting small polystyrene beads into the corneal stroma likewise led to myopia. In humans it has been found that children with ptosis become myopic and children with unilateral hemangioma of the eyelid develop myopia in the closed eye. Children with corneal opacities tend to be myopic as do those with mild retrolental fibroplasia which distorts vision. In the rhesus macaque monkey, atropine did not prevent development of myopia; in the stumptailed macaque it did. Section of the optic nerve did not prevent development of myopia in the macaque but did in the stumptail. This was interpreted as indicating that growth factors produced by the retina are important in the former and brain impulses in the latter species; both factors may be operative in man.

We're going to be posting on the Karlsson psychosis-IQ connection in a few days. Also see demographics of myopia:

"Race exercises a considerable influence over myopia. High degrees with degenerative changes are very common in certain races, such as Chinese, Japanese, Arab, and Jewish persons. Myopia is uncommon in black, Nubian, and Sudanese persons. The variation probably is due more to heredity than habit. "

Remember, a high heritabilty figure means roughly that rank order is preserved though the mean can translate. In other words, it's a centered correlation. So the rise of myopia prevalence to 70% from 10% in one generation does not invalidate the role of genetics, especially when we have mapped loci already. Instead, what it (usually) means is that if A is more myopic than B, A's child will tend to be more myopic than B's child. In this case that interpretation is complicated because the heritability figure was estimated with a logit value for myopia (1 or 0) rather than a continuous value (i.e. the aforementioned refractive error, in diopters). But heritability alone does not allow you to compare A and A's child in *absolute* value. As an analogy, you could take population A to be Asians, B to be Europeans, and the variable to be height.

Also see Marginal Revolution.

Posted by razib at 09:18 PM