We then use two traditional behavioral genetics techniques to estimate genetic contributions to regular smoking. The first is a DeFries-Fulker model, which is an efficient and robust method to estimate genetic and environmental components (DeFries and Fulker 1985). The DeFries-Fulker model (see equation 2) predicts the outcome of the second sibling of a pair (y2) as a function of the first sibling’s score on the same outcome (y1), a measure of genetic similarity, i.e., proportion of alleles shared identical by descent by the pair—(g = 1 for MZ pairs and g = .5 for DZ pairs), and an interaction between genetic similarity and the sibling’s score (y1g). Two of the parameter estimates obtained from this model (b1 and b3) describe the relative contribution of shared environment (c2) and heritability (h2), respectively, and the remaining proportion is due to nonshared environmental characteristics (e2). Although the most basic DeFries-Fulker model has undergone considerable modifications (Purcell 2002), it is still widely used to assess the genetic contribution to a trait’s overall variation (Cherny, DeFries, and Fulker 1992; Rende 1993; Rodgers and McGue 1994).
