reply: Myopia and ambient night-time lighting

Quinn et al. reply. In not being able to find the strong association reported by us1 of childhood myopia with night-time ambient lighting before age 2 years, Zadnik et al. and Gwiazda et al. ascribe our results to a tendency of myopic parents to illuminate their children's rooms at night. Family studies of myopia typically have difficulty separating environmental from genetic factors, however, as sib–sib correlations for myopia decrease with increasing age difference2 and within-family refractive similarities decrease with adjustment for the ‘classic’ environmental factors of education and close work3. Thus, shared inter-generational behaviour (such as use of night lighting) cannot be excluded a priori as contributing to any familial association for myopia.