title: Relative immaturity and ADHD: findings from nationwide registers, parent- and self-reports.
abstract: BACKGROUND
  We addressed if immaturity relative to peers reflected in birth month increases the likelihood of ADHD diagnosis and treatment.
  
  
  METHODS
  We linked nationwide Patient and Prescribed Drug Registers and used prospective cohort and nested case-control designs to study 6-69 year-old individuals in Sweden from July 2005 to December 2009 (Cohort 1). Cohort 1 included 56,263 individuals diagnosed with ADHD or ever used prescribed ADHD-specific medication. Complementary population-representative cohorts provided DSM-IV ADHD symptom ratings; parent-reported for 10,760 9-year-old twins born 1995-2000 from the CATSS study (Cohort 2) and self-reported for 6,970 adult twins age 20-47 years born 1959-1970 from the STAGE study (Cohort 3). We calculated odds ratios (OR:s) for ADHD across age for individuals born in November/December compared to January/February (Cohort 1). ADHD symptoms in Cohorts 2 and 3 were studied as a function of calendar birth month.
  
  
  RESULTS
  ADHD diagnoses and medication treatment were both significantly more common in individuals born in November/December versus January/February; peaking at ages 6 (OR: 1.8; 95% CI: 1.5-2.2) and 7 years (OR: 1.6; 95% CI: 1.3-1.8) in the Patient and Prescribed Drug Registers, respectively. We found no corresponding differences in parent- or self-reported ADHD symptoms by calendar birth month.
  
  
  CONCLUSIONS
  Relative immaturity compared to class mates might contribute to ADHD diagnosis and pharmacotherapy despite absence of parallel findings in reported ADHD symptom loads by relative immaturity. Increased clinical awareness of this phenomenon may be warranted to decrease risk for imprecise diagnostics and treatment. We speculate that flexibility regarding age at school start according to individual maturity could reduce developmentally inappropriate demands on children and improve the precision of ADHD diagnostic practice and pharmacological treatment.
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden; Karolinska Institutet Center of Neurodevelopmental Disorders (KIND), Karolinska Institutet, Stockholm, Sweden.
authors.forename: Linda
authors.initials: L
authors.lastname: Halldner
authors.forename: Annika
authors.initials: A
authors.lastname: Tillander
authors.forename: Cecilia
authors.initials: C
authors.lastname: Lundholm
authors.forename: Marcus
authors.initials: M
authors.lastname: Boman
authors.forename: Niklas
authors.initials: N
authors.lastname: Långström
authors.forename: Henrik
authors.initials: H
authors.lastname: Larsson
authors.forename: Paul
authors.initials: P
authors.lastname: Lichtenstein
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D001289:Attention Deficit Disorder with Hyperactivity / Q000175:diagnosis* / Q000188:drug therapy; D016022:Case-Control Studies; D000697:Central Nervous System Stimulants / Q000627:therapeutic use; D002648:Child; D004200:Diseases in Twins / Q000175:diagnosis / Q000628:therapy; D006801:Humans; D008875:Middle Aged; D010290:Parents / Q000523:psychology; D010379:Peer Group; D011446:Prospective Studies; D012042:Registries; D057566:Self Report; D013548:Sweden / Q000453:epidemiology; D055815:Young Adult
chemical_list: D000697:Central Nervous System Stimulants
keywords: ADHD; child development; epidemiological studies; pharmacotherapy