Reported myopia in opposite sex twins: a hormonal hypothesis.

Two studies have reported poor vision in opposite sex twins (evidenced by wearing glasses or low visual acuity, both of which are interpreted here as evidence of myopia), whereas none have reported an absence of such effects. If these reports are replicable, it would suggest a hormonal effect. There is one report of higher testosterone levels in those suffering from high myopia. A possible mechanism would be if sex hormones in opposite sex pairs transfer from one fetus to the other. There is evidence that sex hormones can cross the placenta, and reports of sex differences in the development of opposite sex twins are consistent with such transfers. If different parts of the eye respond differentially to sex hormones, eyes developing in the unusual hormonal environment of opposite sex twins would be expected to have high myopia rates.