A potentially serious flaw in this research strategy, however, stems from studies showing that most individuals identified with one internalizing disorder also meet diagnostic criteria for additional internalizing disorders; i.e., internalizing disorder are correlated (Andrews et al., 2002; Boyd et al., 1984; Brown et al., 2001; Kushner et al., 2005; Magee et al., 1996). Consider that there are 2n unique combinations that are possible for n binary (i.e., present vs. absent) disorders; e.g., there are 16, 32 and 64 possible combinations of four, five and six internalizing disorders, respectively. While uncorrelated disorders would co-occur by chance according to the disorders’ base-rates, correlated disorders co-occur at rates that significantly exceed chance (Boyd et al., 1984). This creates various configurations of shared and unique variances that are theoretically discernable in correlated disorders. However, it is presently unknown how the shared versus unique variances of correlated internalizing disorders relate to alcohol dependence.
