title: The opposite end of the attention deficit hyperactivity disorder continuum: genetic and environmental aetiologies of extremely low ADHD traits.
abstract: BACKGROUND
  Although attention deficit hyperactivity disorder (ADHD) is thought to reflect a continuously distributed quantitative trait, it is assessed through binary diagnosis or skewed measures biased towards its high, symptomatic extreme. A growing trend is to study the positive tail of normally distributed traits, a promising avenue, for example, to study high intelligence to increase power for gene-hunting for intelligence. However, the emergence of such a 'positive genetics' model has been tempered for ADHD due to poor phenotypic resolution at the low extreme. Overcoming this methodological limitation, we conduct the first study to assess the aetiologies of low extreme ADHD traits.
  
  
  METHODS
  In a population-representative sample of 2,143 twins, the Strength and Weaknesses of ADHD Symptoms and Normal behaviour (SWAN) questionnaire was used to assess ADHD traits on a continuum from low to high. Aetiological influences on extreme ADHD traits were estimated using DeFries-Fulker extremes analysis. ADHD traits were related to behavioural, cognitive and home environmental outcomes using regression.
  
  
  RESULTS
  Low extreme ADHD traits were significantly influenced by shared environmental factors (23-35%) but were not significantly heritable. In contrast, high-extreme ADHD traits showed significant heritability (39-51%) but no shared environmental influences. Compared to individuals with high extreme or with average levels of ADHD traits, individuals with low extreme ADHD traits showed fewer internalizing and externalizing behaviour problems, better cognitive performance and more positive behaviours and positive home environmental outcomes.
  
  
  CONCLUSIONS
  Shared environmental influences on low extreme ADHD traits may reflect passive gene-environment correlation, which arises because parents provide environments as well as passing on genes. Studying the low extreme opens new avenues to study mechanisms underlying previously neglected positive behaviours. This is different from the current deficit-based model of intervention, but congruent with a population-level approach to improving youth wellbeing.
authors.affiliation: Department of Cognitive Neuroscience, Donders Institute for Brain, Cognition and Behaviour, Radboud University Medical Center, Nijmegen, The Netherlands.|Karakter Child and Adolescent Psychiatry University Center, Nijmegen, The Netherlands.|Medical Research Council Social, Genetic & Developmental Psychiatry Centre, Institute of Psychiatry, Psychology & Neuroscience, King's College London, London, UK.
authors.forename: Corina U
authors.initials: CU
authors.lastname: Greven
authors.affiliation: Medical Research Council Social, Genetic & Developmental Psychiatry Centre, Institute of Psychiatry, Psychology & Neuroscience, King's College London, London, UK.|Department of Psychology, University of Bath, Bath, UK.
authors.forename: Andrew
authors.initials: A
authors.lastname: Merwood
authors.affiliation: Department of Cognitive Neuroscience, Donders Institute for Brain, Cognition and Behaviour, Radboud University Medical Center, Nijmegen, The Netherlands.|Karakter Child and Adolescent Psychiatry University Center, Nijmegen, The Netherlands.
authors.forename: Jolanda M J
authors.initials: JM
authors.lastname: van der Meer
authors.affiliation: School of Experimental Psychology and School of Social and Community Medicine, MRC Integrative Epidemiology Unit at the University of Bristol, Coventry, UK.
authors.forename: Claire M A
authors.initials: CM
authors.lastname: Haworth
authors.affiliation: Karakter Child and Adolescent Psychiatry University Center, Nijmegen, The Netherlands.|Department of Psychiatry, Donders Institute for Brain, Cognition and Behaviour, Radboud University Medical Center, Nijmegen, The Netherlands.
authors.forename: Nanda
authors.initials: N
authors.lastname: Rommelse
authors.affiliation: Department of Cognitive Neuroscience, Donders Institute for Brain, Cognition and Behaviour, Radboud University Medical Center, Nijmegen, The Netherlands.|Karakter Child and Adolescent Psychiatry University Center, Nijmegen, The Netherlands.
authors.forename: Jan K
authors.identifier: http://orcid.org/0000-0001-8288-7757
authors.initials: JK
authors.lastname: Buitelaar
mesh_terms: D000293:Adolescent; D001289:Attention Deficit Disorder with Hyperactivity / Q000453:epidemiology / Q000209:etiology* / Q000235:genetics / Q000503:physiopathology*; D004200:Diseases in Twins; D004739:England / Q000453:epidemiology; D005260:Female; D059647:Gene-Environment Interaction; D006801:Humans; D008297:Male; D012720:Severity of Illness Index; D014852:Wales / Q000453:epidemiology
keywords: Attention deficit hyperactivity disorder; extremes; positive genetics; quantitative trait; twin