title: Characteristics and concordance of autism spectrum disorders among 277 twin pairs.
abstract: OBJECTIVE
  To examine patterns of autism spectrum disorder (ASD) inheritance and other features in twin pairs by zygosity, sex, and specific ASD diagnosis.
  
  
  METHODS
  Cross-sectional study.
  
  
  METHODS
  Internet-based autism registry for US residents.
  
  
  METHODS
  Survey results from 277 twin pairs (210 dizygotic [DZ] and 67 monozygotic [MZ]) aged 18 years or younger with at least 1 affected twin.
  
  
  METHODS
  Zygosity and sex.
  
  
  METHODS
  Concordance within twin pairs of diagnosis, natural history, and results from standardized autism screening.
  
  
  RESULTS
  Pairwise ASD concordance was 31% for DZ and 88% for MZ twins. Female and male MZ twins were 100% and 86% concordant, respectively, and DZ twin pairs with at least 1 female were less likely to be concordant (20%) than were male-male DZ twin pairs (40%). The hazard ratio for ASD diagnosis of the second twin after a first-twin diagnosis was 7.48 for MZ vs DZ twins (95% confidence interval, 3.8-14.7). Affected DZ individual twins had an earlier age at first parental concern and more frequent diagnoses of intellectual disability than did MZ twins; MZ twins had a higher prevalence of bipolar disorder and Asperger syndrome and higher concordance of the latter. Results of autism screening correlated with parent-reported ASD status in more than 90% of cases.
  
  
  CONCLUSIONS
  Our data support greater ASD concordance in MZ vs DZ twins. Overall higher functioning, psychiatric comorbidity, and Asperger syndrome concordance among affected MZ vs DZ twins may also suggest differential heritability for different ASDs. For families in which one MZ twin is diagnosed with ASD, the second twin is unlikely to receive an ASD diagnosis after 12 months. In addition, Internet parent report of ASD status is valid.
authors.affiliation: Department of Medical Informatics, Kennedy Krieger Institute, Baltimore, MD 21211, USA.
authors.forename: Rebecca E
authors.initials: RE
authors.lastname: Rosenberg
authors.forename: J Kiely
authors.initials: JK
authors.lastname: Law
authors.forename: Gayane
authors.initials: G
authors.lastname: Yenokyan
authors.forename: John
authors.initials: J
authors.lastname: McGready
authors.forename: Walter E
authors.initials: WE
authors.lastname: Kaufmann
authors.forename: Paul A
authors.initials: PA
authors.lastname: Law
mesh_terms: D020817:Asperger Syndrome / Q000453:epidemiology* / Q000235:genetics*; D001321:Autistic Disorder / Q000453:epidemiology* / Q000235:genetics*; D002648:Child; D002659:Child Development Disorders, Pervasive / Q000453:epidemiology* / Q000235:genetics*; D015897:Comorbidity; D003430:Cross-Sectional Studies; D004200:Diseases in Twins / Q000453:epidemiology* / Q000235:genetics*; D005260:Female; D006801:Humans; D008297:Male; D017678:Sex Distribution; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic; D014481:United States / Q000453:epidemiology