Does Preference for a Larger Delayed Reward Necessarily Indicate Self-control? An Evaluation of the Validity of the Intertemporal Choice Task in Capuchin Monkeys

The capacity of delaying gratification, or foregoing an immediate reward to obtain a better but future reward, involves two components: (i) delay choice (selecting a delayed reward over an immediate one), and (ii) delay maintenance (keeping the decision of delaying gratification even if the immediate reward is made available during the delay). In both humans and non-human primates, two classes of self-control tasks have explored these two components of delaying gratification. In the intertemporal choice task, where the subject faces choices between a small immediate reward and a large delayed reward, the selection of the large delayed reward is regarded as behavioural evidence of self-control. In the accumulation task, where the amount of reward increases as long as the subject refrains from taking it, the more the subject waits before taking the reward the higher the assumed self-control level. Recent data on capuchins cast doubts on the validity of the intertemporal choice task as a tool to assess self-control. Thus, we employed a traditional method used to assess the validity of psychological tests and compared the performance of 14 capuchins in the intertemporal choice task and in the accumulation task. Overall, performance in these two tasks showed a not significant trend towards a negative correlation. Interestingly, whereas in the intertemporal choice task females showed a significantly higher delay tolerance than males, in the accumulation task the opposite pattern was found. Thus, our findings provide for the first time evidence that choices for the larger delayed outcome in the intertemporal choice task actually reflect a form of impulsivity rather than self-control.