The hypothesis of a multiplicative interaction between the diplotypes and psychiatric disorder was tested1. For example, both comorbid alcohol dependence and diplotype risks remained significantly associated with the risk for nicotine dependence in an additive model. However, there is no evidence for a multiplicative interaction between the diplotypes and alcohol dependence (Interaction wald=2.52, d.f.=5, p=0.77). Similarly, we found that psychiatric disorders and diplotypes independently contribute to the risk of nicotine dependence and saw no evidence of genetic risk moderation by any comorbid disorders.
