The rate of affection with multiple forms of substance dependence was high among the family members recruited for our genetic studies of CD and OD. The rate of comorbidity (i.e., at least two SD disorders) was 80.1% and 88.5% in AAs and EAs, respectively. The break-down by the number of SD disorders is shown at the bottom of Table I. In our sample, the rate of OD was much higher in EAs than AAs (65.3% vs. 27.4%); the difference was not as great for AD (62.6% vs. 55.9%). CD was less common in EAs (81.4% vs. 85.7%). There were no other significant differences between the AA and EA samples on the characteristics listed in Table I. The complex comorbidity of SD disorders is depicted in a 3-D plot for dependence on the three illegal drugs, cannabis, cocaine, and opioids, regardless of alcohol and nicotine (Fig. 1). The complete dissection of all five studied SD disorders is shown in 3-D plots as well (Supplementary Fig. S1).
