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 (1985) regression estimates for heritability and shared environmental influences with double-entry twin data. A simple formula to estimate the covariance matrix of the coefficients in DF-regressions is provided, and applications to simulated data and Dabish twin data show that this method can substantially increase the statistical power of the analyses. We also provide an ´efficient DF-analysis´ that yields more precise estimates when additional covariates are included among the explanatory variables. (AUTHORS)