Elective abortion is examined in subsidiary analyses. Results from a handful of studies suggest alcohol and other substance use, including smoking, are more common among women who abort versus carry pregnancies to term (Coleman, 2005), thus abortion could explain at least in part observed associated between reproductive timing and AD. Furthermore, evidence of heritable variation in AD is now considerable (e.g., Heath, Bucholz, Madden, Dinwiddie, Slutske et al., 1997; Kendler, Neale, Heath, Kessler, & Eaves, 1994; McGue, 1994; Prescott & Kendler, 1999), with heritable variation in age at first reproduction also reported (Kirk, Blomberg, Duffy, Heath, Owen, & Martin, 2001; Neiss, Rowe, & Rodgers, 2002). As part of subsidiary analyses, genetic sources of covariation are also examined.
