title: Genetic and environmental influences on illicit drug use and tobacco use across birth cohorts.
abstract: BACKGROUND
  The prevalence of use of many psychoactive substances has changed considerably in recent years. While genetic factors impact on overall risk for substance use, we know little about whether the etiological importance of these factors differs across birth cohorts. One theory, which postulates that heritability of deviant traits increases in permissive environments, predicts a positive relationship across cohorts between prevalence and heritability of substance use.
  
  
  METHODS
  The lifetime history of use of tobacco, cannabis, cocaine, sedatives and stimulants were assessed in 4826 twins from male-male and female-female pairs born in Virginia from 1934 to 1974. Using empirical methods based on prevalence by birth year, these twins were divided into three cohorts for each substance (e.g. for cannabis 1934-1953, 1954-1968 and 1969-1974). Structural equation modeling was performed using the Mx software package.
  
  
  RESULTS
  Prevalence rates for psychoactive substance use differed substantially across cohorts, most markedly for cocaine, sedatives and stimulants, which were highest in the 1958-1963 cohort. However, for all substances, the best-fit model constrained estimates of the etiological role of genetic and environmental risk factors to be equal across both sex and cohort.
  
  
  CONCLUSIONS
  We found no evidence in this sample for any systematic relationship between heritability and prevalence of psychoactive substance use--which should be a rough index of drug availability and/or acceptability. This sample had reasonable power to detect large changes in heritability across cohorts and at least moderate power to detect relatively small changes.
authors.affiliation: Virginia Institute for Psychiatry and Behavioral Genetics and Department of Psychiatry, Medical College of Virginia Commonwealth University, Richmond, VA 23298-0126, USA. kendler@hsc.vsu.edu
authors.forename: Kenneth S
authors.initials: KS
authors.lastname: Kendler
authors.forename: Charles
authors.initials: C
authors.lastname: Gardner
authors.forename: Kristen C
authors.initials: KC
authors.lastname: Jacobson
authors.forename: Michael C
authors.initials: MC
authors.lastname: Neale
authors.forename: Carol A
authors.initials: CA
authors.lastname: Prescott
mesh_terms: D000328:Adult; D000368:Aged; D015331:Cohort Studies; D004777:Environment; D005260:Female; D006801:Humans; D040582:Inheritance Patterns; D008297:Male; D008875:Middle Aged; D015995:Prevalence; D019966:Substance-Related Disorders / Q000453:epidemiology* / Q000235:genetics*; D014768:Virginia / Q000453:epidemiology