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). Although the DeFries-Fulker model is intended for continuously distributed outcomes, the prevalence of smoking in this study is roughly 50 percent, which makes the linear probability model an appropriate extension of this model. In addition to the survival, bivariate correlation, and DeFries-Fulker models, we also calculate cohort-specific heritability estimates using the maximum likelihood variance components technique described above (Purcell 2008). These heritability estimates are presented in Table 3 and summarized in Figure 1.
