Genetic and family studies in psychiatric illness and alcohol and drug dependence

The high prevalence of comorbid psychiatric disorders among patients with substance dependence both confuses diagnosis and complicates treatment. One way of separating substance dependence and comorbid conditions is to use the family, rather than the individual, as the unit of study. Results from family, twin, and adoption studies indicate the existence of a heritable and specific liability to alcohol dependence independent of other psychiatric disease, and perhaps to other substance dependence, as well. However, substance dependence, like many other psychiatric disorders, is a clinically and etiologically heterogeneous entity, which both complicates patterns of familial transmission and underlines the need for more precise description of subforms of illness.