Classical twin studies, which compare the similarity of monozygotic and dizygotic twins, are a powerful method for assessing the magnitude of the impact of aggregate genetic risk factors in complex diseases such as SUD. Furthermore, they have been crucial in understanding causes of comorbidity and clarifying the etiologic inter-relationship of genetic and environmental factors. Although they are not without methodological limitations, a careful review of the impact of these limitations suggests that, in most cases, biases are modest2.
