title: Are children's activity levels determined by their genes or environment? A systematic review of twin studies.
abstract: BACKGROUND
  The importance of physical activity to paediatric health warrants investigation into its determinants. Objective measurement allows a robust examination of genetic and environmental influences on physical activity.
  
  
  OBJECTIVE
  To systematically review the evidence on the extent of genetic and environmental influence on children's objectively-measured activity levels from twin studies.
  Medline, EMBASE, PsycINFO, Health and Psychosocial Instruments and all Ovid Databases. Search terms: "accelerometer" OR "actometer" OR "motion sensor" OR "heart rate monitor" OR "physical activity energy expenditure" AND "twin". Limited to Human, English language and children (0-18 years).
  
  
  RESULTS
  Seven sets of analyses were included in the review. Six analyses examined children's daily-life activity and found that the shared environment had a strong influence on activity levels (weighted mean 60%), with a smaller contribution from genetic factors (weighted mean 21%). Two analyses examined short-term, self-directed activity in a standard environment and found a smaller shared environment effect (weighted mean 25%) and a larger genetic estimate (weighted mean 45%).
  
  
  CONCLUSIONS
  Although genetic influences may be expressed when children have brief opportunities for autonomous activity, activity levels in daily-life are predominantly explained by environmental factors. Future research should aim to identify key environmental drivers of childhood activity.
authors: Cancer Research UK Health Behaviour Research Centre, Department of Epidemiology and Public Health, University College London, United Kingdom.
authors: Abigail
authors: A
authors: Fisher
authors: Cancer Research UK Health Behaviour Research Centre, Department of Epidemiology and Public Health, University College London, United Kingdom.
authors: Lee
authors: L
authors: Smith
authors: Cancer Research UK Health Behaviour Research Centre, Department of Epidemiology and Public Health, University College London, United Kingdom.
authors: Cornelia H M
authors: CH
authors: van Jaarsveld
authors: Cancer Research UK Health Behaviour Research Centre, Department of Epidemiology and Public Health, University College London, United Kingdom.
authors: Alexia
authors: A
authors: Sawyer
authors: Cancer Research UK Health Behaviour Research Centre, Department of Epidemiology and Public Health, University College London, United Kingdom.
authors: Jane
authors: J
authors: Wardle
keywords: Activity; Child; Environment; Genetic; Twin