1. Centre for Behaviour Change, University College London, 1-19 Torrington Place, London WC1E 7HB, UK
2. Department of Kinesiology, University of Rhode Island, 25 W Independence Way, Kingston, RI 02881, USA
3. Aberdeen Health Psychology Group, Institute of Applied Health Sciences, University of Aberdeen, Aberdeen, UK
4. Department of Psychology, University of Minnesota, Minneapolis, MN, USA
5. Primary Care Unit, Institute of Public Health, University of Cambridge, Cambridge, UK