title: Shared cognitive impairments and aetiology in ADHD symptoms and reading difficulties.
abstract: BACKGROUND
  Twin studies indicate that the frequent co-occurrence of attention deficit hyperactivity disorder (ADHD) symptoms and reading difficulties (RD) is largely due to shared genetic influences. Both disorders are associated with multiple cognitive impairments, but it remains unclear which cognitive impairments share the aetiological pathway, underlying the co-occurrence of the symptoms. We address this question using a sample of twins aged 7-10 and a range of cognitive measures previously associated with ADHD symptoms or RD.
  
  
  METHODS
  We performed multivariate structural equation modelling analyses on parent and teacher ratings on the ADHD symptom domains of inattention and hyperactivity, parent ratings on RD, and cognitive data on response inhibition (commission errors, CE), reaction time variability (RTV), verbal short-term memory (STM), working memory (WM) and choice impulsivity, from a population sample of 1312 twins aged 7-10 years.
  
  
  RESULTS
  Three cognitive processes showed significant phenotypic and genetic associations with both inattention symptoms and RD: RTV, verbal WM and STM. While STM captured only 11% of the shared genetic risk between inattention and RD, the estimates increased somewhat for WM (21%) and RTV (28%); yet most of the genetic sharing between inattention and RD remained unaccounted for in each case.
  
  
  CONCLUSIONS
  While response inhibition and choice impulsivity did not emerge as important cognitive processes underlying the co-occurrence between ADHD symptoms and RD, RTV and verbal memory processes separately showed significant phenotypic and genetic associations with both inattention symptoms and RD. Future studies employing longitudinal designs will be required to investigate the developmental pathways and direction of causality further.
authors.affiliation: MRC Social, Genetic and Developmental Psychiatry Centre, Institute of Psychiatry, King's College London, London, United Kingdom.
authors.forename: Celeste H M
authors.initials: CH
authors.lastname: Cheung
authors.forename: Alexis C
authors.initials: AC
authors.lastname: Frazier-Wood
authors.affiliation: Human Genetics Center, Division of Epidemiology, Human Genetics and Environmental Science, School of Public Health, University of Texas Health Sciences Center at Houston, Houston, Texas, United States of America.
authors.forename: Alexis C
authors.initials: AC
authors.lastname: Fazier-Wood
authors.affiliation: MRC Social, Genetic and Developmental Psychiatry Centre, Institute of Psychiatry, King's College London, London, United Kingdom.
authors.forename: Philip
authors.initials: P
authors.lastname: Asherson
authors.affiliation: MRC Social, Genetic and Developmental Psychiatry Centre, Institute of Psychiatry, King's College London, London, United Kingdom.
authors.forename: Fruhling
authors.initials: F
authors.lastname: Rijsdijk
authors.affiliation: MRC Social, Genetic and Developmental Psychiatry Centre, Institute of Psychiatry, King's College London, London, United Kingdom.
authors.forename: Jonna
authors.initials: J
authors.lastname: Kuntsi
mesh_terms: D001289:Attention Deficit Disorder with Hyperactivity / Q000503:physiopathology*; D003072:Cognition Disorders / Q000503:physiopathology*; D006801:Humans