Twin studies can make an important contribution to understanding the relationship between the developmental stages of substance involvement [16]. In a seminal study by Heath and colleagues [17], the relationship between whether people have ever drunk alcohol and the quantity of alcohol consumed as well as the underlying genetic aetiology was explored with three separate conceptual models; the single liability dimension (SLD) model, the independent liability dimension (ILD) model and the combined model. The SLD model assumes a single underlying liability dimension, with abstinence at one end and heavy drinking at the other. The ILD model assumes two independent liability dimensions, the first determining abstinence/alcohol use and the second determining quantity of alcohol use. The combined model assumes that there are also two liability dimensions, positioned on the first determining whether a person is abstinent or influenced by the second liability dimension, and positioned on the second ranging from abstinence to heavy alcohol use. Research with adults has found support for this third ‘combined model’ for alcohol and tobacco use, with findings suggesting that common environment plays a larger role
