title: A twin study of genetic and environmental influences on tobacco initiation, regular tobacco use and nicotine dependence.
abstract: BACKGROUND
  Numerous twin studies have reported significant genetic contributions to the variability of tobacco initiation (TI), while fewer studies have shown similar results for the persistence of smoking behavior, or nicotine dependence (ND). As the development of ND requires regular tobacco use (RTU) which in turn requires TI, a conditional approach is necessary.
  
  
  METHODS
  We used structural equation modeling of multi-step conditional processes to examine the relationship between genetic and environmental risk factors for TI, RTU and ND. The tobacco variables were assessed by personal interview in female, male and opposite-sex twin pairs from the population-based Virginia Twin Registry.
  
  
  RESULTS
  The results suggested that the liabilities to TI, RTU and ND were correlated. Over 80 % of the variance in liability to TI and RTU were shared, and a smaller proportion was shared between RTU and ND. The heritabilities were estimated at 75 %, 80 % and 60 % respectively for TI, RTU and ND. The variance specific to liability to RTU was entirely accounted for by additive genetic factors. Only a modest part of the heritability in liability of ND was due to genetic factors specific to ND. Shared environmental factors were not significant. No sex differences were found for the sources of variation or causal paths, but prevalences were significantly greater in males versus females.
  
  
  CONCLUSIONS
  This study showed significant overlap in the contribution of genetic factors to individual differences in TI, RTU and ND. Furthermore, there was evidence for significant additional genetic factors specific to RTU and ND.
authors.affiliation: Departments of Human Genetics and Psychiatry, Virginia Institute for Psychiatric and Behavioral Genetics, Virginia Commonwealth University, Richmond, VA 23298-0003, USA. hmaes@hsc.vcu.edu
authors.forename: Hermine H
authors.initials: HH
authors.lastname: Maes
authors.forename: Patrick F
authors.initials: PF
authors.lastname: Sullivan
authors.forename: Cynthia M
authors.initials: CM
authors.lastname: Bulik
authors.forename: Michael C
authors.initials: MC
authors.lastname: Neale
authors.forename: Carol A
authors.initials: CA
authors.lastname: Prescott
authors.forename: Lindon J
authors.initials: LJ
authors.lastname: Eaves
authors.forename: Kenneth S
authors.initials: KS
authors.lastname: Kendler
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D015984:Causality; D005260:Female; D020022:Genetic Predisposition to Disease / Q000235:genetics* / Q000523:psychology; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D012307:Risk Factors; D012907:Smoking / Q000453:epidemiology / Q000235:genetics* / Q000523:psychology*; D012931:Social Environment; D013223:Statistics as Topic; D014029:Tobacco Use Disorder / Q000453:epidemiology / Q000235:genetics* / Q000523:psychology; D014429:Twins, Dizygotic / Q000235:genetics / Q000523:psychology; D014430:Twins, Monozygotic / Q000235:genetics / Q000523:psychology