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The Finnish Twin Cohort Study was used to compile twin pairs in whom one or both members had astigmatism. Seventy-two pairs of twins (42 monozygotic and 30 dizygotic) were found. Refractive error and astigmatism information was obtained by asking the twins to send their last prescription for glasses to the authors. The correlations between monozygotic twins for astigmatism were not higher than the correlations between dizygotic twins. The differences in the amounts of astigmatism in monozygotic twins was not statistically significantly different than that in dizygotic twins. This suggests that genetic factors do not contribute to astigmatism, leaving environmental causes as major contributors.
Teikari et al. (1989) studied this question.