A population-based twin study in women of smoking initiation and nicotine dependence

Background. The development of drug dependence requires prior initiation. W hat is the relationship between the risk factors for initiation and depende nce? Methods. Using smoking as a model addiction, we assessed smoking initiation (SI) and nicotine dependence (ND) by personal interview in 1898 female twi ns from the population-based Virginia Twin Registry. We developed a twin st ructural equation model that estimates the correlation between the liabilit y to SI and the liability to ND, given SI. Results. The liabilities to SI and ND were substantially correlated but not identical. Heritable factors played an important aetiological role in SI a nd in ND. While the majority of genetic risk factors for ND were shared wit h SI, a distinct set off familial factors, which were probably partly genet ic, solely influenced the risk for ND. SI was associated with low levels of education and religiosity, high levels of neuroticism and extroversion and a history of a wide range of psychiatric disorders. ND was associated with low levels of education, extroversion, mastery, and self-esteem, high leve ls of neuroticism and dependency and a history of mood and alcohol use diso rders. Conclusions. The aetiological factors that influence SH and ND, while overl apping, are not perfectly correlated. One set of genetic factors plays a si gnificant aetiological role in both SI and ND, while another set of familia l factors, probably in part genetic, solely influences ND. Some risk factor s for SI and ND impact similarly on both stages, some act at only one stage and others impact differently and even in opposite directions at the two s tages. The pathway to substance dependence is complex and involves multiple genetic and environmental risk factors.