T. Kasser and R. M. Ryan (2001), ‘Be careful what you wish for: optimal functioning and the relative attainment of intrinsic and extrinsic goals’, in P. Schmuck and K. M. Sheldon (eds.) Life Goals and Well-Being: Towards a Positive Psychology of Human Striving, Hogrefe and Huber: Goettingen, Germany, pp. 116–131.