Variation in ocular refraction is determined genetically rather than environmentally—that is, according to a Danish study of twins. The question of genes versus environment has been complicated previously by possible natural selection in populations in epidemiological studies and differences in age or environment in family studies. Previous studies of twins have shown a high degree of heritability for ocular refraction—that is, the proportion of biological variation caused by genetic variation—but even these have had major drawbacks.

Lyhne et al set out to put the record straight with a population based study of same sex twins. In all, they studied 114 pairs of identical (53 pairs: 23 male, 30 female) and non-identical (61 pairs: 27 male, 34 female) twins aged 20–45 years, measuring within each pair refraction and measures of the eye that determine refraction. After correcting for any age/sex dependence, they determined best fit for each measure in a model estimating the proportion of biological variation due to additive genetic effects, dominant genetic effects, common environmental effects, and unique environmental effects.

Heritability was high, between 0.89 and 0.94 (95% confidence interval 0.82 to 0.96), for refraction, total refraction, axial length, and radius of corneal curvature, and variation was due to additive genetic effects. Similar values, between 0.88 and 0.94 (0.81 to 0.96), were obtained for anterior chamber depth and lens thickness, and variation was due to dominant genetic effects. Signs of a gene-environment interaction, evident for refraction, suggested that genetically liable individuals might develop myopia under certain conditions—for example, from near work..