An important consideration in twin and family studies of addiction is the treatment of scores from participants that have never been exposed to a drug, because their scores may bias genetic and environmental estimates. This is because the assessment of DSM-IV (American Psychiatric Association (APA; 1994) symptoms of dependence via structured interviews usually requires a minimum level of exposure to a drug (e.g., using marijuana at least six times). Thus, how we score participants who have not met these criteria can affect estimates of genetic and environmental influences on these behaviors. Scoring individuals (e.g., abstainers) as having zero symptoms makes it difficult to characterize the etiology of dependence symptoms amongst drug users, because the population would also include non-users. Many factors, such as drug availability which determine whether an individual is able to use a substance, may be largely environmental and twin studies report higher estimates of environmental influences for measures related to substance initiation and use compared to measures of drug problems (Hopfer et al., 2003). One approach to overcoming this problem is the implementation of conditional/multi-stage models that
