title: Environmental influences on children's physical activity: quantitative estimates using a twin design.
abstract: BACKGROUND
  Twin studies offer a 'natural experiment' that can estimate the magnitude of environmental and genetic effects on a target phenotype. We hypothesised that fidgetiness and enjoyment of activity would be heritable but that objectively-measured daily activity would show a strong shared environmental effect.
  
  
  RESULTS
  In a sample of 9-12 year-old same-sex twin pairs (234 individuals; 57 MZ, 60 DZ pairs) we assessed three dimensions of physical activity: i) objectively-measured physical activity using accelerometry, ii) 'fidgetiness' using a standard psychometric scale, and iii) enjoyment of physical activity from both parent ratings and children's self-reports. Shared environment effects explained the majority (73%) of the variance in objectively-measured total physical activity (95% confidence intervals (CI): 0.63-0.81) with a smaller unshared environmental effect (27%; CI: 0.19-0.37) and no significant genetic effect. In contrast, fidgetiness was primarily under genetic control, with additive genetic effects explaining 75% (CI: 62-84%) of the variance, as was parent's report of children's enjoyment of low 74% (CI: 61-82%), medium 80% (CI: 71-86%), and high impact activity (85%; CI: 78-90%), and children's expressed activity preferences (60%, CI: 42-72%).
  
  
  CONCLUSIONS
  Consistent with our hypothesis, the shared environment was the dominant influence on children's day-to-day activity levels. This finding gives a strong impetus to research into the specific environmental characteristics influencing children's activity, and supports the value of interventions focused on home or school environments.
authors.affiliation: Department of Epidemiology and Public Health, Cancer Research UK Health Behaviour Research Centre, University College London, London, United Kingdom.
authors.forename: Abigail
authors.initials: A
authors.lastname: Fisher
authors.forename: Cornelia H M
authors.initials: CH
authors.lastname: van Jaarsveld
authors.forename: Clare H
authors.initials: CH
authors.lastname: Llewellyn
authors.forename: Jane
authors.initials: J
authors.lastname: Wardle
mesh_terms: D002648:Child; D004777:Environment; D006240:Happiness; D006801:Humans; D009043:Motor Activity; D011795:Surveys and Questionnaires; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic