Towards validation of delay discounting in the pigeon

A series of procedures were conducted in an attempt to assess various forms of validity related to the use of pigeons in research on delay discounting. In separate experimental arrangements, pigeons pressed a treadle, pecked a lit key, pecked a darkened key, or pecked a lit key with a hold as the required response. First, the obtained results were consistent with what would be necessary if the construct of delay discounting were being measured, which provides evidence of face validity. Second, criterion validity was assessed by comparing individual differences in rates of discounting across procedures. Third, to assess internal validity, each pigeon repeated the Treadle, Key Peck, and Dark Key procedures. Again, at both the aggregate and individual levels, the obtained indifference points did not differ systematically between replications. Finally, to assess external validity, discounting was observed regardless of the procedure, where the patterns of data at the aggregate level, and generally at the individual level, were orderly and well described by a hyperbolic function. In addition, rates of discounting were similar when pigeons pecked a lit key, a dark key, or a key with a hold; and each of those rates of discounting tended to be steeper than when treadle pressing. Generally speaking, pigeons that discounted relatively steeply on one procedure also tended to discount relatively steeply on the other procedures. The procedures evidence some of the necessary elements involved with the use of pigeons in research on delay discounting.