To our knowledge, no single previous report has examined the same three questions investigated here. However, we are able to make comparisons to previous studies. First, our report confirms a general vulnerability to substance use (Swan et al. 1996; Tsuang et al. 1998; Hettema et al. 1999; Kendler et al. 2003, 2007). A previous examination of the covariation between tobacco, alcohol and other substance use at ages 17–18 also showed that a common factor was responsible for covariance (Han et al. 1999). However, Han et al. (1999) reported the heritability of the substance use factor at 23% whereas shared environment contributed 62%. Our estimates at age 16–17 were 54% and 34% respectively. The reasons for this discrepancy are difficult to determine but may be due to sample characteristics, length of assessment period or differing definitions of substance use.
