title: Exploring the relationship between genetic and environmental influences on initiation and progression of substance use.
abstract: OBJECTIVE
  To examine the genetic and environmental contributions to the initiation of use and progression to more serious use of alcohol, cigarettes and marijuana during adolescence, and to examine the relationship between initiation and progression of substance use.
  
  
  METHODS
  The study used a twin-based design and a new theoretical model, the causal-common-contingent (CCC) model. This allows modelling of the relationship between initiation of use and progression to heavier use as a two-stage model and the examination of genetic and environmental influences on both stages, while taking into account their relationship.
  
  
  METHODS
  The participants consisted of 1214 twin pairs (69% response rate) aged 11-19 years sampled from the UK population-based Cardiff Study of All Wales and North-west of England Twins (CaStANET).
  
  
  METHODS
  Data on adolescent initiation and progression to more serious use of alcohol, cigarettes and marijuana were obtained using self-report questionnaires.
  
  
  RESULTS
  Initiation of alcohol and progression to heavier alcohol use had separate but related underlying aetiologies. For cigarette and marijuana use the relation between initiation and progression to heavier use was stronger, suggesting greater overlap in aetiologies. For all three substances, environmental influences that make twins more similar (common environment) tended to be greater for initiation, while genetic influences were stronger for heavier use.
  
  
  CONCLUSIONS
  These findings have implications for policy decisions aimed at an adolescent and early adult age group. Specifically, these findings suggest that it may be more efficacious to focus alcohol interventions on risk factors for the development of heavier use rather than initiation of use. In contrast, interventions aimed at reducing the initiation of cigarettes and marijuana use may be more appropriate.
authors.affiliation: Department of Psychological Medicine, Cardiff University, Cardiff, UK. fowlerta@cardiff.ac.uk
authors.forename: Tom
authors.initials: T
authors.lastname: Fowler
authors.forename: Kate
authors.initials: K
authors.lastname: Lifford
authors.forename: Katherine
authors.initials: K
authors.lastname: Shelton
authors.forename: Frances
authors.initials: F
authors.lastname: Rice
authors.forename: Anita
authors.initials: A
authors.lastname: Thapar
authors.forename: Michael C
authors.initials: MC
authors.lastname: Neale
authors.forename: Andrew
authors.initials: A
authors.lastname: McBride
authors.forename: Marianne B M
authors.initials: MB
authors.lastname: van den Bree
mesh_terms: D000293:Adolescent; D018450:Disease Progression; D005260:Female; D020022:Genetic Predisposition to Disease / Q000453:epidemiology / Q000235:genetics; D006801:Humans; D008297:Male; D008962:Models, Theoretical; D015995:Prevalence; D012959:Socioeconomic Factors; D019966:Substance-Related Disorders / Q000453:epidemiology / Q000209:etiology* / Q000235:genetics; D006113:United Kingdom / Q000453:epidemiology