title: Genetic influences on mechanically-assessed activity level in children.
abstract: BACKGROUND
  Activity level is an important component of children's temperament, as well as being part of the core symptom domain of hyperactivity-impulsivity within attention deficit hyperactivity disorder (ADHD). Yet it is poorly understood, due partly to limitations on parent and teacher ratings, which are typically used as measurements of these symptoms.
  
  
  METHODS
  We aimed to study the etiology of objectively-measured activity level across two situations, using actigraphs. A population-based sample of 463 7-9-year-old twin pairs were assessed individually both when apart undergoing laboratory-based cognitive testing and when together during a break in testing.
  
  
  RESULTS
  Heritability of activity level was estimated as 24% during the test session and at 30% during the break in testing. Shared environmental influences accounted for 27% of the variance in activity level during the test session and 42% of activity level measured during the break. A genetic correlation of 1.0 indicated that the same genes influenced activity level across the two situations, justifying the use of a composite measure of the two situations. This produced a heritability estimate of 36%.
  
  
  CONCLUSIONS
  Objectively-measured activity level shows a moderate degree of genetic influence, with a common set of genes influencing activity level across situations. This supports the use of actigraphs as an additional source of information in studies that aim to improve phenotype definition for molecular genetic studies of activity level and ADHD.
authors.affiliation: MRC Social, Genetic and Developmental Psychiatry Centre, Institute of Psychiatry, Kings College London, UK. alexis.wood@iop.kcl/ac/uk
authors.forename: Alexis C
authors.initials: AC
authors.lastname: Wood
authors.forename: Kimberly J
authors.initials: KJ
authors.lastname: Saudino
authors.forename: Hannah
authors.initials: H
authors.lastname: Rogers
authors.forename: Philip
authors.initials: P
authors.lastname: Asherson
authors.forename: Jonna
authors.initials: J
authors.lastname: Kuntsi
mesh_terms: D001289:Attention Deficit Disorder with Hyperactivity / Q000175:diagnosis* / Q000453:epidemiology / Q000235:genetics*; D002648:Child; D004594:Electrophysiology / Q000295:instrumentation; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D008297:Male; D015588:Observer Variation; D011159:Population Surveillance / Q000379:methods; D011597:Psychomotor Performance / Q000502:physiology*; D012931:Social Environment; D013694:Temperament; D014427:Twins / Q000235:genetics