title: Monozygotic twins discordant for attention-deficit/hyperactivity disorder: ascertainment and clinical characteristics.
abstract: OBJECTIVE
  Nongenetic factors and phenomenology of attention-deficit/hyperactivity disorder (ADHD) were examined in monozygotic (MZ) twin pairs discordant for ADHD.
  
  
  METHODS
  Recruitment included telephone screening (n = 297 pairs), behavioral ratings obtained from parents and teachers (n = 59 pairs), and, finally, in-person assessment (n = 25 pairs; structured classroom observation, diagnostic interview, psychoeducational evaluation, birth record review, establishment of monozygosity, and anatomic brain imaging). Affected twins were further contrasted with previously studied affected singletons.
  
  
  RESULTS
  Of the 25 MZ twin pairs qualifying for in-person evaluation, only 10 proved discordant for ADHD. Affected twins were mostly comparable with affected singletons on clinical measures, although fathers' self-ratings of childhood ADHD status were significantly lower in twins than in singletons.
  
  
  CONCLUSIONS
  Discordance for ADHD in MZ twins appears to be ascribable to greater environmental discordance and decreased familiality. Despite these differences, affected twins were phenotypically comparable with affected singletons. Thus MZ twins discordant for ADHD, while rare, can inform research on the etiology and pathophysiology of this disorder.
authors.affiliation: Child Psychology Branch of the NIMH, USA.
authors.forename: Wendy S
authors.initials: WS
authors.lastname: Sharp
authors.forename: Rebecca F
authors.initials: RF
authors.lastname: Gottesman
authors.forename: Deanna K
authors.initials: DK
authors.lastname: Greenstein
authors.forename: Christen L
authors.initials: CL
authors.lastname: Ebens
authors.forename: Judith L
authors.initials: JL
authors.lastname: Rapoport
authors.forename: F Xavier
authors.initials: FX
authors.lastname: Castellanos
mesh_terms: D000293:Adolescent; D001289:Attention Deficit Disorder with Hyperactivity / Q000453:epidemiology / Q000209:etiology*; D002170:Canada / Q000453:epidemiology; D016022:Case-Control Studies; D002648:Child; D004200:Diseases in Twins / Q000453:epidemiology / Q000209:etiology*; D004777:Environment; D005260:Female; D006801:Humans; D008297:Male; D014430:Twins, Monozygotic; D014481:United States / Q000453:epidemiology