A SIMPLE ALGEBRAIC DEMONSTRATION OF THE VALIDITY OF DEFRIES-FULKER ANALYSIS IN UNSELECTED SAMPLES WITH MULTIPLE KINSHIP LEVELS

DeFries and Fulker's (Behav. Genet. 15, 467-473, 1985) regression proc edure (DF analysis) to estimate c(2) and h(2) was originally applied t o selected twin data. Since then, DF analysis has been applied more br oadly in unselected data and with multiple (nontwin) kinship levels.:T heoretical work based on the matrix algebra of variance-covariance mat rices has shown that estimates of c(2) and h(2) are unbiased in select ed two-group settings. In this article, a simple proof is presented su pporting the validity of DF analysis in broader settings. We use scala r algebra to show that parameter estimates of h(2) and c(2) are unbias ed in unselected settings with multiple (more than two) kinship levels . Caveats are offered, and other DF analysis problems are identified.