Maternal contribution to the familial risk for smoking was modeled using the Heaviness of Smoking Index (HSI) defined by time to 1st cigarette upon waking and number of cigarettes smoked per day when smoking the most (Heatherton et al. 1989). Based on evidence that HSI scores of 4 or greater indicate high ND, (Diaz et al. 2005; Heatherton et al. 1989), and consideration of the distribution of HSI scores, we created a dichotomized HSI score so that values of 1–2 defined low and values ≥3 defined medium to severe ND mothers. Lifetime never smokers were the reference group.
