DF-Analyses of heritability with double-entry twin data: Asymptotic standard errors and efficient estimation

In this paper we establish the asymptotic distribution of DeFries Fulker (1 985) regression estimates for heritability and shared environmental influen ces with double-entry twin data. A simple formula to estimate the covarianc e matrix of the coefficients in DF-regressions is provided, and application s to simulated data and Danish twin data show that this method can substant ially increase the statistical power of the analyses. We also provide an 'e fficient DF-analysis' that yields more precise estimates when additional co variates are included among the explanatory variables.