Prisoner's dilemma and the pigeon: Control by immediate consequences.

In three experiments pigeons played (i.e., chose between two colored keys) iterated prisoner's dilemma and other 2 x 2 games (2 participants and 2 options) against response strategies programmed on a computer. Under the prisoner's dilemma pay-off matrix, the birds generally defected (i.e., pecked the color associated with not cooperating) against both a random response (.5 probability of either alternative) and a tit-for-tat strategy (on trial n the computer "chooses" the alternative that is the same as the one chosen by the subject on trial n - 1) played by the computer. They consistently defected in the tit-for-tat condition despite the fact that as a consequence they earned about one third of the food that they could have if they had cooperated (i.e., pecked the "cooperate" color) on all the trials. Manipulation of the values of the food pay-offs demonstrated that the defection and consequent loss of food under the tit-for-tat condition were not due to a lack of sensitivity to differences in pay-off values, nor to strict avoidance of a null pay-off (no food on a trial), nor to insensitivity to the local (current trial) reward contingencies. Rather, the birds markedly discounted future outcomes and thus made their response choices based on immediate outcomes available on the present trial rather than on long-term delayed outcomes over many trials. That is, the birds were impulsive, choosing smaller but more immediate rewards, and did not demonstrate self-control. Implications for the study of cooperation and competition in both humans and nonhumans are discussed.