title: Genetic and environmental influences on cannabis use initiation and problematic use: a meta-analysis of twin studies.
abstract: BACKGROUND
  Because cannabis use is associated with social, physical and psychological problems, it is important to know what causes some individuals to initiate cannabis use and a subset of those to become problematic users. Previous twin studies found evidence for both genetic and environmental influences on vulnerability, but due to considerable variation in the results it is difficult to draw clear conclusions regarding the relative magnitude of these influences.
  
  
  METHODS
  A systematic literature search identified 28 twin studies on cannabis use initiation and 24 studies on problematic cannabis use. The proportion of total variance accounted for by genes (A), shared environment (C) and unshared environment (E) in (i) initiation of cannabis use and (ii) problematic cannabis use was calculated by averaging corresponding A, C and E estimates across studies from independent cohorts and weighting by sample size.
  
  
  RESULTS
  For cannabis use initiation, A, C and E estimates were 48%, 25% and 27% in males and 40%, 39% and 21% in females. For problematic cannabis use A, C and E estimates were 51%, 20% and 29% for males and 59%, 15% and 26% for females. Confidence intervals of these estimates are considerably narrower than those in the source studies.
  
  
  CONCLUSIONS
  Our results indicate that vulnerability to both cannabis use initiation and problematic use was influenced significantly by A, C and E. There was a trend for a greater C and lesser A component for cannabis use initiation compared to problematic use for females.
authors.affiliation: Genetic Epidemiology, Queensland Institute of Medical Research, Brisbane, Qld, Australia.
authors.forename: Karin J H
authors.initials: KJ
authors.lastname: Verweij
authors.forename: Brendan P
authors.initials: BP
authors.lastname: Zietsch
authors.forename: Michael T
authors.initials: MT
authors.lastname: Lynskey
authors.forename: Sarah E
authors.initials: SE
authors.lastname: Medland
authors.forename: Michael C
authors.initials: MC
authors.lastname: Neale
authors.forename: Nicholas G
authors.initials: NG
authors.lastname: Martin
authors.forename: Dorret I
authors.initials: DI
authors.lastname: Boomsma
authors.forename: Jacqueline M
authors.initials: JM
authors.lastname: Vink
mesh_terms: D000293:Adolescent; D000328:Adult; D002648:Child; D004200:Diseases in Twins / Q000235:genetics* / Q000523:psychology; D005260:Female; D020022:Genetic Predisposition to Disease / Q000235:genetics* / Q000523:psychology; D006801:Humans; D008297:Male; D002189:Marijuana Abuse / Q000235:genetics* / Q000523:psychology; D008875:Middle Aged; D010641:Phenotype; D012307:Risk Factors; D012737:Sex Factors; D012931:Social Environment; D018433:Twin Studies as Topic; D055815:Young Adult
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