Two twin studies have used this new method to examine the relationship between initiation and progression in cigarette use [24] and illicit drug use, including marijuana [25]. Both studies found a strong relationship between the initiation and progression stages of substance use. Of the underlying liability that was specific to the progression of substance use (after initiation had been taken into account), genetic factors were most important in cigarette use and unique environmental factors were most important in marijuana use. These papers used adult samples and, to our knowledge, no study has examined the relationship between initiation and quantity of use of cigarettes, alcohol or marijuana in adolescents. Although rates of alcohol and marijuana are among the highest in Europe [9], we are not aware of any adult or adolescent twin studies of either substance use in a UK sample.
