Rhesus monkeys lack a consistent peak-end effect

In humans, the order of receiving sequential rewards can significantly influence the overall subjective utility of an outcome. For example, people subjectively rate receiving a large reward by itself significantly higher than receiving the same large reward followed by a smaller one (Do, Rupert, & Wolford, 2008). This result is called the peak-end effect. A comparative analysis of order effects can help determine the generality of such effects across primates, and we therefore examined the influence of reward-quality order on decision making in three rhesus macaque monkeys (Macaca mulatta). When given the choice between a high-low reward sequence and a low-high sequence, all three monkeys preferred receiving the high-value reward first. Follow-up experiments showed that for two of the three monkeys their choices depended specifically on reward-quality order and could not be accounted for by delay discounting. These results provide evidence for the influence of outcome order on decision making in rhesus monkeys. Unlike humans, who usually discount choices when a low-value reward comes last, rhesus monkeys show no such peak-end effect.     

Gambling for Gatorade: risk-sensitive decision making for fluid rewards in humans

Determining how both humans and animals make decisions in risky situations is a central problem in economics, experimental psychology, behavioral economics, and neurobiology. Typically, humans are risk seeking for gains and risk averse for losses, while animals may display a variety of preferences under risk depending on, amongst other factors, internal state. Such differences in behavior may reflect major cognitive and cultural differences or they may reflect differences in the way risk sensitivity is probed in humans and animals. Notably, in most studies humans make one or a few choices amongst hypothetical or real monetary options, while animals make dozens of repeated choices amongst options offering primary rewards like food or drink. To address this issue, we probed risk-sensitive decision making in human participants using a paradigm modeled on animal studies, in which rewards were either small squirts of Gatorade or small amounts of real money. Possible outcomes and their probabilities were not made explicit in either case. We found that individual patterns of decision making were strikingly similar for both juice and for money, both in overall risk preferences and in trial-to-trial effects of reward outcome on choice. Comparison with decisions made by monkeys for juice in a similar task revealed highly similar gambling styles. These results unite known patterns of risk-sensitive decision making in human and nonhuman primates and suggest that factors such as the way a decision is framed or internal state may underlie observed variation in risk preferences between and within species.     

Analogical reasoning and the differential outcome effect: transitory bridging of the conceptual gap for rhesus monkeys (Macaca mulatta)

Monkeys, unlike chimpanzees and humans, have a marked difficulty acquiring relational matching-to-sample (RMTS) tasks that likely reflect the cognitive foundation upon which analogical reasoning rests. In the present study, rhesus monkeys (Macaca mulatta) completed a categorical (identity and nonidentity) RMTS task with differential reward (pellet ratio) and/or punishment (timeout ratio) outcomes for correct and incorrect choices. Monkeys in either differential reward-only or punishment-only conditions performed at chance levels. However, the RMTS performance of monkeys experiencing both differential reward and punishment conditions was significantly better than chance. Subsequently when all animals experienced nondifferential outcomes tests, their RMTS performance levels were at chance. These results indicate that combining differential reward and punishment contingencies provide an effective, albeit transitory, scaffolding for monkeys to judge analogical relations-between-relations.     

Copying without rewards: socially influenced foraging decisions among brown capuchin monkeys

An individual’s foraging activity can be influenced by the choices made by nearby conspecifics. The interest shown in the location and characteristics of a feeding patch may depend on the feeding success of a conspecific there, a process that needs to be distinguished from choices guided by rewards to the observer itself. We investigated how rewards for both self and others influence the foraging choices of captive capuchin monkeys (Cebus apella). Thirteen adult capuchins observed familiar female conspecific models explore one of three opaque boxes under three conditions. In the first, there were no rewards available to either monkey; in the second, rewards were available to the model only; and in the third, both monkeys could retrieve a reward. Under all conditions, subjects more often explored the same box as the model than was expected by chance. Thus, without ever receiving a reward themselves or without seeing another receive rewards, subjects’ searches were directed at the box explored by another monkey. The tendency to match the model’s choice increased if the subject was rewarded. We compared these results to control conditions in which the model was either absent, or present but not allowed to demonstrate. Subjects’ located the reward less often in control conditions, than in the experimental conditions. We conclude that extrinsic rewards, while helpful, are not required for partners to influence the foraging choices of capuchins, and that the unrewarded copying of foraging choices demonstrated here may provide the basis for additional social influences on learning. This contribution is part of the Special Issue “A Socioecological Perspective on Primate Cognition” (Cunningham and Janson 2007)     

Developmental patterns of change in the influence of safe and risky peer choices on risky decision‐making

Adolescents take more risks when peers monitor their behavior. However, it is largely unknown how different types of peer influence affect adolescent decision‐making. In this study, we investigate how information about previous choices of peers differentially influences decision‐making in adolescence and young adulthood. Participants (N = 99, age range 12–22) completed an economic choice task in which choice options were systematically varied on levels of risk and ambiguity. On each trial, participants selected between a safer choice (low variability in outcome) and a riskier choice (high variability in outcome). Participants made choices in three conditions: a solo condition in which they made choices with no additional information, a social condition in which they saw choices of supposed peers, and a computer condition in which they saw choices of a computer. Results showed that participants’ choices conform to the choices made by the peers, but not a computer. Furthermore, when peers chose the safe option, late adolescents were especially likely to make a safe choice. Conversely, when the peer made a risky choice, late adolescents were least likely to follow choices made by the peer. We did not find evidence for differential influence of social information on decisions depending on their level of risk and ambiguity. These results show that information about previous decisions of peers are a powerful modifier for behavior and that the effect of peers on adolescents’ decisions is less ubiquitous and more specific than previously assumed.     

Do humans produce the speed–accuracy trade-off that maximizes reward rate?

In this paper we investigate trade-offs between speed and accuracy that are produced by humans when confronted with a sequence of choices between two alternatives. We assume that the choice process is described by the drift diffusion model, in which the speed–accuracy trade-off is primarily controlled by the value of the decision threshold. We test the hypothesis that participants choose the decision threshold that maximizes reward rate, defined as an average number of rewards per unit of time. In particular, we test four predictions derived on the basis of this hypothesis in two behavioural experiments. The data from all participants of our experiments provide support only for some of the predictions, and on average the participants are slower and more accurate than predicted by reward rate maximization. However, when we limit our analysis to subgroups of 30–50% of participants who earned the highest overall rewards, all the predictions are satisfied by the data. This suggests that a substantial subset of participants do select decision thresholds that maximize reward rate. We also discuss possible reasons why the remaining participants select thresholds higher than optimal, including the possibility that participants optimize a combination of reward rate and accuracy or that they compensate for the influence of timing uncertainty, or both.     

The evolution of decision-making under risk: Framing effects in monkey risk preferences

When making choices between risky options, human decision-makers exhibit a number of framing effects. One of the most prominent framing effects is the tendency for decision makers to evaluate gambles relative to a reference point, and to act risk-seeking when prospects are framed as losses but risk-averse when identical prospects are framed as gains. This tendency for risk-preferences to reverse between loss and gain frames has been termed the reflection effect, and is one of the primary predictions of Prospect Theory. Here, we explore whether non-human primates exhibit a similar reflection effect. Using a token-trading task, we show that capuchin monkeys (Cebus apella) exhibit an analogous reversal of risk preferences depending on whether outcomes are presented as gains or losses, suggesting that similar framing effects also influence choice in non-human primates. This finding suggests that the mechanisms that drive framing effects in humans may be evolutionarily ancient, extending broadly across the primate order.     

Biases in preferences for sequences of outcomes in monkeys

Movies, vacations, and meals are all examples of events composed of a sequence of smaller events. How do we go from our evaluations of each scene in a movie to an evaluation of the sequence as a whole? In theory, we should simply average the values of the individual events. In practice, however, we are biased towards sequences where each element tends to be better than the previous, where the last value is large, and we overweight the best (or worst) part of the sequence. To study how general these biases are we examined monkeys’ preferences for sequences of rewards in a novel reward repeat task. Monkeys were first given a sequence of rewards and then chose between repeating the sequence or receiving a standard comparator sequence. We found that, like humans, monkeys overweight events that happen later in a sequence, so much so that adding a small reward to the end of a sequence can paradoxically reduce its value. Monkeys were also biased towards sequences with large peak values (the highest value in the sequence), but only following a working memory challenge, suggesting that this preference may be driven by memory limitations. These results demonstrate the cross-species nature of biases in preferences for sequences of outcomes. In addition, monkeys’ consistent preference for sequences in which large values occur later challenges the generality of discounting models of intertemporal choice in animals.     

Perceived control in rhesus monkeys (Macaca mulatta): enhanced video-task performance

This investigation was designed to determine whether perceived control effects found in humans extend to rhesus monkeys (Macaca mulatta) tested in a video-task format, using a computer-generated menu program, SELECT. Choosing one of the options in SELECT resulted in presentation of 5 trials of a corresponding task and subsequent return to the menu. In Experiments 1-3, the animals exhibited stable, meaningful response patterns in this task (i.e., they made choices). In Experiment 4, performance on tasks that were selected by the animals significantly exceeded performance on identical tasks when assigned by the experimenter under comparable conditions (e.g., time of day, order, variety). The reliable and significant advantage for performance on selected tasks, typically found in humans, suggests that rhesus monkeys were able to perceive the availability of choices.     

Do monkeys think in metaphors? Representations of space and time in monkeys and humans

Research on the relationship between the representation of space and time has produced two contrasting proposals. ATOM posits that space and time are represented via a common magnitude system, suggesting a symmetrical relationship between space and time. According to metaphor theory, however, representations of time depend on representations of space asymmetrically. Previous findings in humans have supported metaphor theory. Here, we investigate the relationship between time and space in a nonverbal species, by testing whether non-human primates show space–time interactions consistent with metaphor theory or with ATOM. We tested two rhesus monkeys and 16 adult humans in a nonverbal task that assessed the influence of an irrelevant dimension (time or space) on a relevant dimension (space or time). In humans, spatial extent had a large effect on time judgments whereas time had a small effect on spatial judgments. In monkeys, both spatial and temporal manipulations showed large bi-directional effects on judgments. In contrast to humans, spatial manipulations in monkeys did not produce a larger effect on temporal judgments than the reverse. Thus, consistent with previous findings, human adults showed asymmetrical space–time interactions that were predicted by metaphor theory. In contrast, monkeys showed patterns that were more consistent with ATOM.     

The benefit of symbols: monkeys show linear, human-like, accuracy when using symbols to represent scalar value

When humans and animals estimate numbers of items, their error rate is proportional to the number. To date, however, only humans show the capacity to represent large numbers symbolically, which endows them with increased precision, especially for large numbers, and with tools for manipulating numbers. This ability depends critically on our capacity to acquire and represent explicit symbols. Here we show that when rhesus monkeys are trained to use an explicit symbol system, they too show more precise, and linear, scaling than they do using a one-to-one corresponding numerosity representation. We also found that when taught two different types of representations for reward amount, the monkeys systematically undervalued the less precise representation. The results indicate that monkeys, like humans, can learn alternative mechanisms for representing a single value scale and that performance variability and relative value depend on the distinguishability of each representation.     

Extensive practice does not eliminate human switch costs

Numerous human task-switching studies have shown that decision making that follows a task switch is slower and less accurate than that which follows a task repetition. Stoet and Snyder (2003a) tested humans and rhesus monkeys on a task-switching paradigm, but found, surprisingly, no switch costs in the monkeys. We hypothesized that the exceptional monkey behavior may have been due to the more extensive practice the monkeys received in comparison with human subjects. In the present study, we tested the hypothesis that extensive practice can abolish switch costs in humans. Four human subjects each performed 23,000 trials in a task-switching paradigm. We found that this amount of practice does not abolish switch costs.     

Temporal framing and the hidden‐zero effect: rate‐dependent outcomes on delay discounting

Recent research suggests that presenting time intervals as units (e.g., days) or as specific dates, can modulate the degree to which humans discount delayed outcomes. Another framing effect involves explicitly stating that choosing a smaller–sooner reward is mutually exclusive to receiving a larger–later reward, thus presenting choices as an extended sequence. In Experiment 1, participants (N = 201) recruited from Amazon Mechanical Turk completed the Monetary Choice Questionnaire in a 2 (delay framing) by 2 (zero framing) design. Regression suggested a main effect of delay, but not zero, framing after accounting for other demographic variables and manipulations. We observed a rate‐dependent effect for the date‐framing group, such that those with initially steep discounting exhibited greater sensitivity to the manipulation than those with initially shallow discounting. Subsequent analyses suggest these effects cannot be explained by regression to the mean. Experiment 2 addressed the possibility that the null effect of zero framing was due to within‐subject exposure to the hidden‐ and explicit‐zero conditions. A new Amazon Mechanical Turk sample completed the Monetary Choice Questionnaire in either hidden‐ or explicit‐zero formats. Analyses revealed a main effect of reward magnitude, but not zero framing, suggesting potential limitations to the generality of the hidden‐zero effect.     

Rhesus monkeys (<Emphasis Type="Italic">Macaca mulatta</Emphasis>) discriminate between knowing and not knowing and collect information as needed before acting

Humans use memory awareness to determine whether relevant knowledge is available before acting, as when we determine whether we know a phone number before dialing. Such metacognition, or thinking about thinking, can improve selection of appropriate behavior. We investigated whether rhesus monkeys (Macaca mulatta) are capable of a simple form of metacognitive access to the contents of short-term memory. Monkeys chose among four opaque tubes, one of which concealed food. The tube containing the reward varied randomly from trial to trial. On half the trials the monkeys observed the experimenter baiting the tube, whereas on the remaining trials their view of the baiting was blocked. On each trial, monkeys were allowed a single chance to select the tube containing the reward. During the choice period the monkeys had the opportunity to look down the length of each tube, to determine if it contained food. When they knew the location of the reward, most monkeys chose without looking. In contrast, when ignorant, monkeys often made the effort required to look, thereby learning the location of the reward before choosing. Looking improved accuracy on trials on which monkeys had not observed the baiting. The difference in looking behavior between trials on which the monkeys knew, and trials on which they were ignorant, suggests that rhesus monkeys discriminate between knowing and not knowing. This result extends similar observations made of children and apes to a species of Old World monkey, suggesting that the underlying cognitive capacities may be widely distributed among primates.     

Delay-discounting probabilistic rewards: Rates decrease as amounts increase

The independence of delay-discounting rate and monetary reward size was tested by offering subjects (N = 621) a series of choices between immediate rewards and larger, delayed rewards. In contrast to previous studies, in which hypothetical rewards have typically been employed, subjects in the present study were entered into a lottery in which they had a chance of actually receiving one of their choices. The delayed rewards were grouped into small ($30–$35), medium ($55–$65), and large amounts ($70–$85). Using a novel parameter estimation procedure, we estimated discounting rates for all three reward sizes for each subject on the basis of his/her pattern of choices. The data indicated that the discounting rate is a decreasing function of the size of the delayed reward (p < .0001), whether hyperbolic or exponential discounting functions are assumed. In addition, a reliable gender difference was found (p = .005), with males discounting at higher rates than females, on average.     

Altering the Shape of Punishment Distributions Affects Decision Making in a Modified Iowa Gambling Task

Neuroeconomics research has shown that preference for gambling is altered by the statistical moments (mean, variance, and skew) of reward and punishment distributions. Although it has been shown that altered means can affect feedback‐based decision making tasks, little is known if the variance and skew will have an effect on these tasks. To investigate, we systematically controlled the variance (high, medium, and low) and skew (negative, zero, and positive) of the punishment distributions in a modified version of the Iowa Gambling Task. The Iowa Gambling Task has been used extensively in both academic and clinical domains to understand decision making and diagnose decision making impairments. Our results show that decision making can be altered by an interaction of variance and skew. We found a significant decrease over trials in choices from the decks with high variance and asymmetrically skewed punishments and from the decks with low variance and zero skew punishments. These results indicate that punishment distribution shape alone can change human perception of what is optimal (i.e., mean expected outcome) and may help explain what guides our day‐to‐day decisions. Copyright © 2013 John Wiley & Sons, Ltd.     

Affect regulation and temporal discounting: interactions between primed, state, and trait affect

Previous research has largely focused on the influence of experienced affect on decision making; however, other sources of affective information may also shape decisions. In two studies, we examine the interacting influences of affective information, state affect, and personality on temporal discounting rates (i.e., the tendency to choose small rewards today rather than larger rewards in the future). In Study 1, participants were primed with either positive or negative affect adjectives before making reward choices. In Study 2, participants underwent either a positive or negative affect induction before making reward choices. Results in both studies indicate that neuroticism interacts with state unpleasant affect and condition (i.e., positive or negative primes or induction) to predict discounting rates. Moreover, the nature of the interactions depends on the regulatory cues of the affective information available. These results suggest that irrelevant (i.e., primes) and stable (i.e., personality traits) sources of affective information also shape judgments and decision making. Thus, current affect levels are not the only source of affective information that guides individuals when making decisions.     

State-based versus reward-based motivation in younger and older adults

Recent decision-making work has focused on a distinction between a habitual, model-free neural system that is motivated toward actions that lead directly to reward and a more computationally demanding goal-directed, model-based system that is motivated toward actions that improve one’s future state. In this article, we examine how aging affects motivation toward reward-based versus state-based decision making. Participants performed tasks in which one type of option provided larger immediate rewards but the alternative type of option led to larger rewards on future trials, or improvements in state. We predicted that older adults would show a reduced preference for choices that led to improvements in state and a greater preference for choices that maximized immediate reward. We also predicted that fits from a hybrid reinforcement-learning model would indicate greater model-based strategy use in younger than in older adults. In line with these predictions, older adults selected the options that maximized reward more often than did younger adults in three of the four tasks, and modeling results suggested reduced model-based strategy use. In the task where older adults showed similar behavior to younger adults, our model-fitting results suggested that this was due to the utilization of a win-stay–lose-shift heuristic rather than a more complex model-based strategy. Additionally, within older adults, we found that model-based strategy use was positively correlated with memory measures from our neuropsychological test battery. We suggest that this shift from state-based to reward-based motivation may be due to age related declines in the neural structures needed for more computationally demanding model-based decision making.     

Spontaneous versus Controlled Influences of Stimulus-Based Affect on Choice Behavior

This article identifies two routes through which affect and cognitions arising from a stimulus can influence choices: a “lower order” route, where choices are influenced through automatic affective processes, and a “higher order” route, where choices are influenced through more controlled affective or consequence-related cognitive processes. Across three experiments the extent of deliberation, mental preoccupation, and the nature of exposure to the stimuli were manipulated to identify conditions under which lower order affect, higher order affect, or higher order cognitions impact choices. Respondents chose between two alternatives: one that was associated with more intense positive affect but less favorable cognitions (e.g., chocolate cake), and one that was associated with less intense positive affect but more favorable cognitions (e.g., fruit salad). Findings suggest that when the individual makes the decision quickly and is mentally preoccupied while making the decision, choices are driven by lower order affect. When the individual deliberates on the decision without being mentally preoccupied and the affect-laden option is in full view while the decision is being deliberated upon, choices are driven by higher order affect. In both cases, the affect-laden option (e.g. chocolate cake) is selected. In all other situations, choices are driven by higher order consequence-related cognitions and the alternative that is superior on the cognitive dimension (e.g. fruit salad) is selected. It is suggested that the effects of affective reactions on choice occur through the activation of appetitive (i.e., gratification-seeking) goals.