title: Heritability of tic disorders: a twin-family study.
abstract: BACKGROUND
  Genetic-epidemiological studies that estimate the contributions of genetic factors to variation in tic symptoms are scarce. We estimated the extent to which genetic and environmental influences contribute to tics, employing various phenotypic definitions ranging between mild and severe symptomatology, in a large population-based adult twin-family sample.
  
  
  METHODS
  In an extended twin-family design, we analysed lifetime tic data reported by adult mono- and dizygotic twins (n = 8323) and their family members (n = 7164; parents and siblings) from 7311 families in the Netherlands Twin Register. We measured tics by the abbreviated version of the Schedule for Tourette and Other Behavioral Syndromes. Heritability was estimated by genetic structural equation modeling for four tic disorder definitions: three dichotomous and one trichotomous phenotype, characterized by increasingly strictly defined criteria.
  
  
  RESULTS
  Prevalence rates of the different tic disorders in our sample varied between 0.3 and 4.5% depending on tic disorder definition. Tic frequencies decreased with increasing age. Heritability estimates varied between 0.25 and 0.37, depending on phenotypic definitions. None of the phenotypes showed evidence of assortative mating, effects of shared environment or non-additive genetic effects.
  
  
  CONCLUSIONS
  Heritabilities of mild and severe tic phenotypes were estimated to be moderate. Overlapping confidence intervals of the heritability estimates suggest overlapping genetic liabilities between the various tic phenotypes. The most lenient phenotype (defined only by tic characteristics, excluding criteria B, C and D of DSM-IV) rendered sufficiently reliable heritability estimates. These findings have implications in phenotypic definitions for future genetic studies.
authors.affiliation: Department of Biological Psychology,Vrije Universiteit,Amsterdam,The Netherlands.
authors.forename: N R
authors.initials: NR
authors.lastname: Zilhão
authors.affiliation: Department of Psychology,University of Amsterdam,The Netherlands.
authors.forename: M C
authors.initials: MC
authors.lastname: Olthof
authors.affiliation: Department of Biological Psychology,Vrije Universiteit,Amsterdam,The Netherlands.
authors.forename: D J A
authors.initials: DJ
authors.lastname: Smit
authors.affiliation: Department of Clinical Psychology,Utrecht University,The Netherlands.
authors.forename: D C
authors.initials: DC
authors.lastname: Cath
authors.affiliation: Department of Biological Psychology,Vrije Universiteit,Amsterdam,The Netherlands.
authors.forename: L
authors.initials: L
authors.lastname: Ligthart
authors.affiliation: Department of Psychiatry,University of Florida,Gainesville, FL,USA.
authors.forename: C A
authors.initials: CA
authors.lastname: Mathews
authors.affiliation: Department of Psychiatry,University of California,San Francisco, CA,USA.
authors.forename: K
authors.initials: K
authors.lastname: Delucchi
authors.affiliation: Department of Biological Psychology,Vrije Universiteit,Amsterdam,The Netherlands.
authors.forename: D I
authors.initials: DI
authors.lastname: Boomsma
authors.affiliation: Department of Biological Psychology,Vrije Universiteit,Amsterdam,The Netherlands.
authors.forename: C V
authors.initials: CV
authors.lastname: Dolan
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D008297:Male; D008875:Middle Aged; D009426:Netherlands / Q000453:epidemiology; D009679:Nuclear Family; D010375:Pedigree; D012042:Registries; D013981:Tic Disorders / Q000453:epidemiology / Q000235:genetics*; D055815:Young Adult
keywords: DSM; Tourette's syndrome; heritability; structural equation modeling; tic symptoms