However, genetic AD liability is not necessarily discrete; clinically, AD frequently co-occurs with other psychiatric disorders (especially other forms of substance dependence and antisocial personality disorder), and it has been demonstrated that at least some of this comorbidity is attributable to shared genetic liability. For example, AD shares genetic vulnerability with other externalizing disorders (conduct disorder, antisocial personality disorder, and drug dependence), and this common liability showed heritability of 80% (Hicks et al. 2004). Consistent with the notion of a general liability, AD has also been shown, specifically, to share genetic risk with nicotine dependence (ND) (Swan et al. 1997; True et al. 1999; Madden et al. 2000), and these traits also share genetic variance with caffeine use (Hettema et al. 1999). An obvious implication of these findings, which are based on twin samples, is that genes that influence risk for ND may also increase risk for AD and vice versa; this has been observed and several examples are discussed below.
