This study has a number of limitations. Due to lack of information about the childhood household of study participants, the estimated heritability may be inflated by shared environment among siblings. In the present study, substance dependent individuals were oversampled for genetic studies. Thus, the heritability of OD as estimated here is likely not to be representative of that in the general population. Additional sources of information concerning the identified subtypes, such as follow-up studies or molecular genetic correlation, are needed to validate these findings. The heritability estimates shown here are consistent with estimates for other substance dependence disorders. For example, it is estimated that alcohol dependence is 50–60% heritable and dependence on illicit drugs is 45–79% heritable (Dick and Agrawal (2008). The heritability estimates for the subtypes in the present study are in this range, with the two most heritable subtypes at the high end of the range.
