Science Brief

Delay of gratification in nonhuman animals

Chimps and monkeys can display patterns of self-control similar to humans.

By Michael J. Beran

Michael J. BeranMichael J. Beran is a senior research scientist at the Language Research Center of Georgia State University and the associate director of the Language Research Center at Georgia State. He received his BA in Psychology from Oglethorpe University in 1997, his MA in 1997, and his PhD in 2002, both from Georgia State University. Beran is a fellow of Division 6 and Division 3 of APA. He was the inaugural Duane M. Rumbaugh Fellow at Georgia State University. He is the current president of the Southern Society for Philosophy and Psychology. His research interests include numerical cognition, metacognition, planning and prospective memory, self-control, decision making and language acquisition, and his research is supported by the National Institute of Child Health and Human Development and the National Science Foundation. Author website.

“Should I watch this movie, or study for my exam coming up next week?” “The doctor says I need to cut back on my alcohol intake, but I sure would enjoy another drink.” “I’ll have just one more cupcake, but tomorrow I am going to start losing that weight.”

Frequently, we find ourselves in exactly these kinds of situations. Sometimes, willpower wins out, but often, we act impulsively, going against our long-term interests. And, unfortunately, this can be at great consequence. Some of the most devastating health and social problems occur because of choices we make that do not reflect self-control (e.g., Baumeister, Heatherton, & Tice, 1994; Baumeister & Vohs, 2004). Even simple choices can have big impacts. Take a test as simple as being told that one can have a smaller, sooner reward right now, or one can choose to wait a while to get a bigger or better reward. This test, commonly called the marshmallow test (Mischel, 1974; Mischel, Shoda, & Rodriguez, 1989), gives a simple measure of delay of gratification, and many factors impact how long children will wait to try to get the better reward (Mischel & Baker, 1975; Mischel, Ebbeson, & Zeiss, 1972). And, we now know that performance on this test correlates with many outcomes later in life, including performance in school, standard test scores, salary earned, trouble with the law and other measures of success (e.g., Mischel, Shoda, & Peake, 1988). So, it is a critical health mission to understand the nature of self-control in humans, and how to improve it. My interest has been in the comparative approach to this issue.

Why test nonhuman animals on self-control? There are a number of reasons, besides the general need for us always to put our own behavior in perspective by comparing and contrasting it with the behavior of other species. It is critical to dissociate certain things such as human morality, social norms and even the role of language that might contribute to choice behavior (whether self-controlled or not) so that we can see what other cognitive and emotional processes contribute to decision-making in the face of tempting, but shortsighted, outcomes. And, we can even develop techniques to help nonhuman animals improve their own self-control in ways that will offer insights for interventions with humans.

My interest in this topic came from an observation by psychologist Sally Boysen of Ohio State University that chimpanzees had the hardest time doing what seemed to be a relatively easy task — pointing to which of two piles of candy they did not want (e.g., Boysen & Berntson, 1995). No matter how many times they faced the test, they kept pointing to more candy even though that meant they got less, and this suggested that they had trouble inhibiting themselves from reaching toward immediately available food. This is a great finding, replicated with many species now (see Shifferman, 2009), and it would suggest that impulsiveness is tough for animals to overcome. I set out to see if chimpanzees could pass different tests of self-control. I created a version of the marshmallow test (Beran, Savage-Rumbaugh, Pate, & Rumbaugh, 1999) where chimpanzees either could wait for a better reward (delivered some minutes later) or press a button to take the reward right next to the button. When the immediate reward was the better one (such as banana instead of carrot), they pressed immediately, of course. But, when they had to wait to get the better reward, they often did. So, at least in that test, chimpanzees delayed gratification.

The chimpanzees Panzee (left) and Sherman (center) will distract themselves by looking at magazines or doing other things when they need to delay gratification and let more candies accumulate rather than eat what they already have accumulated. Capuchin monkeys such as Lily (right) also will wait for better food items to move themselves into reach rather than just take the first thing that comes close enough to reach. This also requires some degree of self-control and the ability to wait for more delayed rewards.

The chimpanzees Panzee (left) and Sherman (center) will distract themselves by looking at magazines or doing other things when they need to delay gratification and let more candies accumulate rather than eat what they already have accumulated. Capuchin monkeys such as Lily (right) also will wait for better food items to move themselves into reach rather than just take the first thing that comes close enough to reach. This also requires some degree of self-control and the ability to wait for more delayed rewards.

But, often in the real world, self-control is needed in a different context — one in which it becomes harder to leave a delayed reward alone because that reward keeps getting better or bigger, and thus more appealing (Toner & Smith, 1977). Think about saving money over time, where each month the amount gets bigger, and more tempting to take and spend on something now rather than on whatever you are saving for. Could chimpanzees resist this kind of urge? What about other primates? This is one issue we have been addressing now for more than a decade. My colleague Ted Evans and I use a test we call the accumulation task (e.g., Beran, 2002; Beran & Evans, 2006). Food items accumulate within reach of the animal, and the rule of the game is simple — the longer you leave the accumulation alone, the bigger it gets. With chimpanzees, for example, they might have a tube that runs right into their enclosure, and an automated dispenser might drop a candy every 10 seconds, and as long as they do not detach the tube and eat the candy, the dispenser will keep running until it is out of candy. Chimpanzees play the game well, sometimes waiting for over 20 minutes to get all of the candy that is available. Other laboratories now use this test as well, with other apes, some monkey species and even some non-primates (e.g., Anderson, Kuroshima, & Fujita, 2010; Evans & Beran, 2007a; Pelé et al., 2011; Vick, Bovet, & Anderson, 2010). Performance varies, but the idea is that the test lets one assess how long an animal is willing to wait and not take a tempting reward, so that something better can be obtained.

And, the test allows for presenting a number of manipulations that might help or hinder delay of gratification. For example, Evans and I were interested in whether chimpanzees might show one of the best behavioral strategies for improving self-control and sustaining delay of gratification — self-distraction. In nearly any instance in which one finds oneself trying to resist temptation, it is a good idea to find something to distract you (e.g., Peake, Hebl, & Mischel, 2002). Rather than thinking about the jar of cookies on kitchen counter, instead read a book, or grab your guitar to keep you occupied. Those of us who recognize this relation will resist longer. Do chimpanzees use a similar strategy to resist temptation? We asked the question by presenting three conditions in our accumulation task (Evans & Beran, 2007b). In the baseline condition, they had the tube within reach, and candies accumulated like always. In the second condition, they also had toys in their enclosure, things like magazines, crayons, paper and string. We found that when they had those things, they waited longer than when they had nothing to play with. But, this did not necessarily mean they were intentionally distracting themselves, because it might only have been that having toys to play with led to more play behavior, which then led to less attention to the accumulating treats. This relation would be beneficial, no doubt, but not necessarily indicative that the chimps knew that playing with toys was useful. This is where the third condition comes into play — in that condition, the chimpanzees again had the same kinds of toys, but now the tube that collected the candies was never in reach during the trial. It sat right outside the enclosure, and so there was no temptation to take it because there was no opportunity to take it. Waiting was required, whether the chimps wanted to wait or not. We matched the delay exactly to those trials with toys inside and the tube inside the enclosure, and what we found was that toy use was much higher when the tube was available. The chimpanzees played with the toys more when they needed to play with the toys as a distraction. To us, this suggests the chimpanzees knew that toys were useful in that context, and that by playing with them, they could dampen their urge to take the accumulating candy.

Nonhuman primates, including capuchin monkeys (Cebus apella; left and center) and chimpanzees (Pan troglodytes; right) make excellent models for studying self-control and behavioral inhibition. Comparing these species to each other, and to other primates including humans, provides an excellent way to better understand the evolution of cognition.

Nonhuman primates, including capuchin monkeys (Cebus apella; left and center) and chimpanzees (Pan troglodytes; right) make excellent models for studying self-control and behavioral inhibition. Comparing these species to each other, and to other primates including humans, provides an excellent way to better understand the evolution of cognition.

Although the great apes have succeeded with accumulation tasks, monkeys struggle to perform well in these kinds of tests, perhaps in part because of methodological issues (Addessi et al., in press; Evans & Beran, 2007a; Genty, Karpel, & Silberberg, 2012; Paglieri et al., 2013). But, there is reason to believe that monkeys, as social animals that must often inhibit responses in a variety of social and foraging contexts, should possess some degree of self-control. And so we have tried other tests to assess their competence in waiting for something better. With postdoctoral associate Bonnie Perdue, graduate student Jessica Bramlett, and Evans, we developed a new test (Bramlett, Perdue, Evans, & Beran, 2012) that was inspired by a visit to a sushi restaurant in which individual pieces of food came by on a conveyor, and patrons had to decide whether to take what was in front of them, or let that pass to get a more preferred type of sushi still on its way. We gave this same kind of option to capuchin monkeys by using a rotating tray that successively brought different kinds and different amounts of food within their reach. The monkeys learned that they only got to choose one item from the tray. So, if they took the first, then the second (which they could see) would not ever make it within reach. Although it took a little time, these monkeys learned to let the first reward pass them by, and wait for the second, if the second reward was better. This was true even when the same high preference food (banana) was in both locations, and only differed in size with the larger piece coming later. Here, one could see the monkeys struggle, reaching toward the first (smaller) item, but then pulling their arms back, letting it pass and waiting for the bigger piece to arrive. Looking at these responses and some of those of the chimpanzees when they self-distract, and comparing those reactions to the reactions of children in different versions of the marshmallow test is striking for what it shows by way of commonalities in dealing with these kinds of situations across the primates.

One of the relatively unexplored questions is the role of social context on self-control. Many failures in our own lives such as eating dessert we had not intended to eat (because others ordered dessert themselves), or smoking that cigarette we planned to avoid (because someone invited us to go out for a smoke break), occur because of social contexts that lead to poor short-term choices. At the same time, social contexts can facilitate self-control through support systems (e.g., Alcoholics Anonymous) that put multiple individuals into situations where they help each other when temptations for immediate gratification arise. So, we are starting to ask questions about how well animals can maintain self-control when more than just their own actions are relevant. For example, with graduate student Audrey Parrish (and Evans and Perdue) we came up with a game that involved two chimpanzees passing a token to each other in order to accumulate rewards (Parrish, Perdue, Evans, & Beran, in press). As in our other tests, the game was simple — each time a chimpanzee passed a token to the partner, he or she earned a treat. But, both partners had to inhibit eating the treats they were accumulating and instead keep the game going or else that would end the game and remaining treats would be lost for both individuals. The chimpanzees learned to do this, and they transferred their performance to new partners, and even kept the game going when we were not in the test area and a computer gave them all of their rewards. In fact, sometimes they could choose between free food, or the chance to get the token instead and play the exchange game with a partner. When more food could be won by exchanging, they chose to put themselves into a situation where they had to count on their own self-control and that of their partner to be successful.

The ability to delay gratification, to wait for something better, and even to put behavioral strategies into play to help bridge the delay to that better reward is within the capability of at least some nonhuman animals. Their successes and failures seem to relate to our own, with many of the same factors contributing to self-control or impulsivity. Their lack of language, their lack of cultural norms regarding patience as a virtue, and their lack of the same extensive future-oriented foresight that humans have (e.g., saving for retirement 40 years in the future) does not seem to hinder their ability to make choices that suggest some foresight, some inhibitory control, and perhaps even some conscious access to understanding and dealing with their own fallibility in these situations. These results indicate that a full account of the emergence of self-control in humans is well-served by taking a comparative approach, and I am thankful to have been able to contribute to that effort by studying captive chimpanzees and other primates and documenting the cognitive abilities of these animals.


This research has been supported by the National Institutes of Health (HD-38051 and HD-060563), the National Science Foundation (BCS-0924811), the Duane M. Rumbaugh Fellowship at Georgia State University, and the College of Arts and Sciences at Georgia State University. I am grateful to many wonderful colleagues for their assistance with this research program, including Ted Evans, Bonnie Perdue, Audrey Parrish, Jessica Bramlett, Emilie Menzel, Charles Menzel, Duane Rumbaugh, James Pate, Fabio Paglieri, Elsa Addessi, William Hopkins, John Kelley, Sarah Brosnan, Amanda Bania, Erin Stromberg and David Washburn.


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