A notable difference between these studies is in the continuity between regular smoking and subsequent ND. In contrast to our results, Maes et al. (2004) found that this parameter approached unity (β = 0.93). This difference is particularly striking among males in the current study (β = 0.37). Lyons et al. (2008) reported an intermediate result (β = 0.54) among men. An analysis of the pathway from smoking initiation to ND in female twins (Kendler et al. 1999) estimated the β path to be similar to that reported here for females (β = 0.77, v. 0.63 in the current study). Swedish males appear to be outliers in that the sources of variance influencing regular cigarette use are only modestly influential on ND. The causes underlying this deviation from previous results are unclear.
