‘No man is an island entire of itself.’ The hidden effect of peers on physical activity: John Donne, Meditation XVII

International public policy emphasises the need to increase current low levels of physical activity (WHO 2010). A large literature examines the reasons for the low levels of physical activity but tends to focus on the correlates of behaviour. This has prompted a call for more causal research to better support policy recommendations to change behaviour (Bauman et al. 2012). Using a large sample of individuals from the British Household Panel Survey (BHPS) between 1996/7 and 2006/7, a dynamic panel data analysis is employed to reveal a causal contemporaneous effect of a household peer’s participation in physical activity on an individual’s behaviour. The effect of a peer’s physical activity on an individual’s physical activity is found to be of a magnitude commensurate with the habits of the individual. An individual’s participation in physical activity is also positively associated with their other leisure activity. The research suggests that an individual’s physical activity takes place as part of a portfolio of household leisure, which health promotion needs to take account of.