The main variable used was the discounting gradient (k). k value represents the rate of discounting of the delayed outcome and was calculated for each subject as described by Mitchell (1999) and Wilson et al. (2011). Greater k values indicate greater preference for immediate rewards. As k values are not normally distributed, a natural log-transformation was applied and the transformed values (ln(k)) were used for the analyses. Although some subjects took a different version of the delay-discounting task, as outlined above, k values (and ln(k)) are not influenced by this variation, so all the individuals were included wherever possible.
