DF-Analyses of Heritability with Double-Entry Twin Data: Asymptotic Standard Errors and Efficient Estimation
Hans-Peter Kohler and Joseph. L. Rodgers
Abstract: 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 Danish 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.