Amount-dependent temporal discounting?

Amount-dependent temporal discounting has been demonstrated for human choice between outcomes differing in amount and delay. In the only study to date with non-humans, Grace reported no evidence for amount-dependent temporal discounting with pigeons in a concurrent-chains procedure. The present experiments repeated Grace's procedure but with modifications to enhance the discrimination between small and large magnitude outcomes. In Experiment 1, sensitivity of pigeons' initial-link choice to the terminal link delay ratio was greater with large reinforcer durations in the terminal links than with small reinforcer durations. This result is consistent with a greater rate of temporal discounting for larger reinforcers (the reverse of the result for humans), but can also be explained as enhanced discrimination of delay ratios with larger reinforcer durations. The results of a second experiment supported Grace's conclusion that amount-dependent temporal discounting does not characterize pigeons' choice in concurrent chains. Because reinforcer amount was held constant between choice alternatives in the present experiments and that of Grace, but varied in the human studies, our results question whether prior demonstrations of amount-dependent discounting reflect the effects of reinforcer delay or of reinforcer amount. Differences in the procedures used to study discounting in humans (titration procedures) and non-humans (concurrent chains) may contribute to the divergent results across species.