When trying to assess the importance of genetic influences in the development of a disorder, the first step usually is to determine whether the disorder aggregates in families. Likewise, the co-aggregation of two disorders within families can provide evidence that common genetic factors influence both conditions. Using data from more than 1,000 families participating in COGA, Bierut and colleagues (1998, 2000) examined the transmission of both alcohol dependence and habitual smoking within families and determined risk factors for both conditions. This analysis found that the greatest risk factor for developing dependence on a drug is being already dependent on another drug.
