Third, and related to the second, heritability estimates from twin and family studies are possibly inflated by common environment of family members because of deviations from assumptions that underpin the classical twin design such as equal common environmental influences for monozygotic and dizygotic twins, although credible arguments against this special ‘MZ twin environment' have been made.64 In this study, the variance explained by all SNPs is estimated through such distant genetic relationships that confounding with common environment is unlikely. Furthermore, it is common practice in twin and family studies to ‘drop' nonsignificant variance components from the model and subsequently report only the remaining significant variance components. In this way the common environmental term is often dropped but if it truly contributes to variance between individuals and if the reduced most parsimonious model reflects sample size and power, the reported estimates of heritability may be biased upwards with their s.e. biased downwards.
