Purpose: To evaluate the role of environment variance in the refraction discordance in monozygotic (MZ) twin.

Methods:  Monozygotic twins aged 7 to 20 years old from Guangzhou Twin Eye Study in 2009 were enrolled. Eye behavior information was obtained by a standardized questionnaire, which define eye behavior as near work, middle-distance work, indoor leisure time and outdoor activity, the refraction phenotype was measured by auto-refraction under cycloplegia. 

Results: A total of 1042 questionnaires were eligible in this study. In singleton analysis, age (β=-0.24,P<0.001) and near work (β=-0.21, P<0.001) are risk factor while outdoor activity (β=0.09, P<0.001) is a protective factor for spherical equivalent. Among 521 MZ pairs, the logistic regression analyses, factors associated with having greater discordance of SE among MZ was age (OR=1.16, 95% confidence interval 1.09,1.25, P<0.001), but near work and outdoor activity were no longer associated (all P>0.05). 

Conclusions: Near work and age are risk factors while outdoor activity is protective factor for myopia development. Among MZ discordance, age but near work or outdoor activity is risk factor.