Myopia: is the nature-nurture debate finally over?

In the nineteenth century, the prevalence of myopia began to rise, and Cohn stressed the role of education. Later, based on twin studies, Sorsby argued that refraction was almost totally genetically determined. This became the dominant view. However, rapid increases in the prevalence of myopia were then reported, especially in East and Southeast Asia, where the prevalence of myopia in children completing secondary school is now 80-90 per cent, with around 20 per cent highly myopic, and at risk of ocular pathology. It is not possible to explain these rapid changes genetically, since gene pools cannot change that fast. Nevertheless, there are at least 200 genetic forms of myopia, but these account for myopia in only a low percentage of the population. Genome-wide association studies have identified over 150 single-nucleotide polymorphisms (SNPs) associated with myopia, but they account for < 10 per cent of the variation in refraction. In contrast, twin studies have given consistently high heritability estimates for myopia. The high twin study heritability may be explained by the limited environmental variation within twin pairs, combined with basic assumptions made in twin studies. The SNP-heritability approach suggests that 25-35 per cent of the variation may be accounted for by hundreds or even thousands of SNPs. The apparent conflicts in the literature can be explained by the aetiological heterogeneity of myopia, and the fact that estimates of heritability are population-specific. It has been proposed that environmental variation is more relevant to variations between populations; however, the current differences between populations are due to changes within populations, driven by educational pressures, and limited time spent outdoors. Ethnic differences in myopia and the effects of parental myopia now seem more likely to be explained by environmental influences. Genetic studies have not yet defined molecular pathways and preventive interventions, and the predictive power of current genetic data is limited.