Behavioural Processes 73 (2006) 315–324 Maintenance of delay of gratification by four chimpanzees (Pan troglodytes): The effects of delayed reward visibility, experimenter presence, and extended delay intervals Michael J. Beran ∗ , Theodore A. Evans Language Research Center, Georgia State University, P.O. Box 5010, Atlanta, GA 30303, United States Received 7 April 2006; received in revised form 17 July 2006; accepted 26 July 2006 Abstract Previous research in our laboratory has demonstrated that chimpanzees can delay gratification by inhibiting consumption of available food items for as long as 3 min as an experimenter transfers additional food items from a transparent container to a bowl placed in front of the subject. In this study, we examined the influence of the visibility of the food source, as well as the presence of the experimenter, on four chimpanzees’ self-control in this paradigm. In Experiment 1 an experimenter transferred 15 preferred food items between a distant opaque container and a bowl placed in front of the subject. In Experiment 2 we tested the chimpanzees with an automated system that (in the absence of the experimenter) transferred up to 36 highly preferred food items from a universal food dispenser to a container located either inside or outside of the subject’s enclosure. There were no differences in self-directed behaviors or attentiveness to the food items between the self-imposed and externally imposed delay conditions. A final experiment with the automated paradigm indicated that individuals could delay gratification for up to 11 min in order to obtain all 36 food items. © 2006 Elsevier B.V. All rights reserved. Keywords: Chimpanzees; Delay of gratification; Inhibition; Pan troglodytes; Self-control 1. Introduction In certain situations, waiting longer to receive an outcome leads to that outcome increasing in value. The longer one leaves a deposit in an interest-bearing account rather than removing the money and spending it, the larger the amount of money avail- able on withdrawal. This ability to forgo a more immediate, less preferred outcome so as to attain a more preferred outcome at a future time is called delay of gratification, and it involves a two- component process. The first component is the initial choice to leave the more immediate gain alone so as to get the better outcome. This choice can be re-evaluated throughout the delay interval, and an organism can decide at any point that it would rather take the immediate, less preferred outcome rather than continuing to wait for the better outcome. This second com- ponent, called delay maintenance, consists of both behavioral responses and mental processes employed by the organism to ∗ Corresponding author. E-mail address: mjberan@yahoo.com (M.J. Beran). “bridge the delay interval” between choosing to wait for the more preferred outcome and receiving that outcome (Mischel, 1974, 1981). Humans are not the only organisms that attempt to maxi- mize long-term gains by giving up immediate gratification. In some experimental investigations, non-human animals choose between two schedules of reinforcement. One schedule provides a smaller reward or a less preferred reward, but after a shorter period of time compared to the other schedule, which provides a bigger or better reward after a longer period of time. Sometimes, organisms (including humans) choose to wait for better rewards (Chelonis et al., 1994; Logue, 1988; Logue et al., 1996; Tobin et al., 1996; van Haaren et al., 1988) whereas other times they choose the smaller reward or the less preferred reward more immediately (Logue and Chavarro, 1992; Logue et al., 1996; Tobin et al., 1993). In a second test situation readily used with human children, there are two rewards, and children are offered the more pre- ferred reward only if they wait for a period of time whereas the less preferred reward can be received at any time. Unlike the tests outlined above, in these tests children have to maintain 0376-6357/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.beproc.2006.07.005