Our treatment of lifetime abstainers has implications for the interpretation of heritability estimates. Lifetime abstainers are coded as “0” at each wave rather than as missing. Because we did not explicitly model alcohol use initiation, the heritability estimates presented are potentially inflated: some proportion of the heritability of alcohol use is transmitted through a contingency pathway from initiation (as described in Kendler et al., 1999), but that source of genetic variation is not parsed out in the current study. Finally, as noted elsewhere, the processes and paths we refer to as “causal” are theoretically so, but given the nature of the study, we cannot ascribe causality with certainty.
