The effects of real versus hypothetical reward on delay and probability discounting.

Delay discounting and probability discounting tasks providing hypothetical rewards have commonly been used to measure levels of impulsivity and risk taking. Results have been inconsistent as to whether real, compared with hypothetical, rewards influence choice behaviour in delay discounting tasks. This experiment examined choice behaviour in 38 healthy individuals using delay and probability discounting tasks involving both real and hypothetical rewards (pound sterling 0.10/pound sterling 0.20), real delays, and probabilistic outcomes. It was hypothesized that choice behaviour when receiving real rewards would be less impulsive and less risky than when receiving hypothetical rewards. The tasks were designed so that a mathematical model (the multiplicative hyperbolic model of choice) could be applied to assess effects on discounting parameters. The provision of real rewards was associated with significantly decreased impulsive, but not risky, choice although the size of effect was small and was influenced by the order of presentation. Repeated presentation of both tasks increased quantity discounting but not delay or probability discounting parameters. These results indicate that the nature of the reward provided, and repetition effects, need to be taken into account when designing discounting studies.