title: Estimating heritability and shared environmental effects for refractive error in twin and family studies.
abstract: OBJECTIVE
  Twin studies have demonstrated a high heritability for refractive error of up to 90%, but some family studies have suggested up to one-third of population variance is attributable to the effects of shared family environment. This large twin study aimed to explore the role of shared environment in refractive error.
  
  
  METHODS
  Refractive error was measured using autorefraction in 4602 subjects (1152 monozygotic and 1149 dizygotic twin pairs), aged between 16 and 82 years, recruited from the TwinsUK Adult Twin Registry. Maximum-likelihood methods were used to estimate the variance of genetic, environmental, and age variance components.
  
  
  RESULTS
  Maximum likelihood model fitting estimate of the heritability from the best-fit model was 77% (95% confidence interval [CI], 68%-84%). Shared environmental effects explained 7% (95% CI, 0%-15%) and individual environmental effects explained 16% (95% CI, 15%-18%) of the spherical equivalent variance, respectively. Inclusion of age effects into the modeling reduced shared environmental effects to an estimated 2% of variation.
  
  
  CONCLUSIONS
  Analysis of 2301 twin pairs confirms that the twin study design results in a very low estimate of shared family environmental effects in refractive error. Several factors may explain these differences; we believe the most likely is that twins are perfectly age matched and do not include cross-generation or cohort effects. This means twin study designs have more power to detect heritable effects in variance component models of myopia, whereas family studies have more power to detect shared environment effects.
authors.affiliation: Department of Twin Research and Genetic Epidemiology, King's College London School of Medicine, London, United Kingdom. margarida.lopes@kcl.ac.uk
authors.forename: Margarida C
authors.initials: MC
authors.lastname: Lopes
authors.forename: Toby
authors.initials: T
authors.lastname: Andrew
authors.forename: Francis
authors.initials: F
authors.lastname: Carbonaro
authors.forename: Tim D
authors.initials: TD
authors.lastname: Spector
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Hammond