Introduction

Experimental economics has illustrated that uncertainty is ubiquitous in decision-making, influences learning, and contributes crucially to the valuation of options in diverse situations. One situation where uncertainty is endemic is deciding to trust another. In fact, a significant feature of any economic transaction (Arrow, 1974) is our ability to trust and cooperate with non-related others. Learning who to trust and deciding to trust requires the evaluation of numerous factors, including various risk and ambiguity considerations (Gambetta, 1988). For instance, an individual typically must evaluate how trustworthy another person appears to be, while also weighing whether past experiences to trust others has led to reciprocal exchanges. In these unknown environments, choosing to trust is tantamount to making a decision of uncertainty. While stress is known to affect choices involving uncertainty in non-social contexts (Starcke & Brand, 2012), little is known about how stress affects choices of uncertainty within social contexts.

Current models of uncertainty preferences assume that people assess the desirability and likelihood of possible outcomes through some type of expectation-based calculus. In social contexts, uncertainty considerations become especially important when one must decide whether to trust another—a dynamic captured in the Trust game (Berg, Dickhaut, & Mccabe, 1995). Since an individual can make more money by trusting another, but runs the risk of losing all the money if another player decides not to reciprocate, deciding to trust is highly uncertain (Ben-Ner & Putterman, 2001) and akin to playing a gamble where the probabilities of winning are unknown (Knight, 1921).

One critical question that has received little attention is how uncertainty considerations are differentially valued in social and non-social contexts. Extensive research within non-social contexts illustrates that individuals can be averse to decisions of uncertainty, choosing the safe option that yields small but reliable payouts over the uncertain option that yields large but unreliable payouts (Holt & Laury, 2002). Notably, individuals react more aversively to decisions of uncertainty that have unknown probabilities (i.e. ambiguity) compared to known probabilities, such as a 50% chance of winning (i.e. risk). In the social domain where an individual must engage with others, relatively few decisions of uncertainty have known outcomes, and thus are best characterized by ambiguous uncertainty than risky uncertainty. Since little is known about how ambiguity considerations are valued in social situations, our first goal was to juxtapose how uncertainty considerations in non-social contexts are valued compared to uncertainty considerations in social contexts. To explore these putative differences in ambiguity preferences, we use two tasks—the Trust game and a matched Lottery task—where all components of the two tasks are held constant except the source of uncertainty (i.e. trusting others versus unknown probabilistic gambles). By directly measuring ambiguity under social and non-social contexts, we can observe whether individuals differentially compute ambiguous uncertainty for choices made during social interactions compared with choices devoid of any social component.

A second question examines how acute stress might differentially affect the valuation of these uncertainty preferences within social and non-social contexts. Stress has a profound—albeit inconsistent (Pabst, Schoofs, Pawlikowski, Brand, & Wolf, 2013)—effect on risky decision-making in non-social contexts. In some cases individuals become less risk averse under stress (Lighthall, Mather, & Gorlick, 2009; Preston, Tansfield, Buchanan, & Bechara, 2007; Starcke, Wolf, Markowitsch, & Brand, 2008; van den Bos, Harteveld, & Stoop, 2009), while in other cases, stress makes individuals more risk averse (Lighthall et al., 2009; Porcelli & Delgado, 2009). Far less is known about how stress affects ambiguity considerations within either non-social or social contexts. One recent study suggests that socially stressed individuals engage in greater trusting behavior than individuals who are not socially stressed (von Dawans, Fischbacher, Kirschbaum, Fehr, & Heinrichs, 2012). Evidence from this study of such trusting behavior is thought to demonstrate a need to ‘tend and befriend’ in order to strengthen potentially fragile social ties under stress (Taylor et al., 2000). However, it is unclear whether the social nature of the stressor used in this study (i.e. being evaluated while publicly speaking), rather than the neurobiological stress response itself, is responsible for the increase in trusting behavior. By directly manipulating the effects of acute stress—indexed by increased levels of cortisol (Axelrod & Reisine, 1984)—using a non-social stressor in both social and non-social contexts, we can probe whether there are domain specific effects of acute stress on ambiguity considerations.

In addition, we can explore whether social context and acute stress biases the integration of past experiences and interactions when processing ambiguity considerations. For instance, given the nature of repeated one-shot games where each gamble and interaction is independent, there should be no influence of past experiences when deciding to take a new gamble or engage in trusting behavior with a new person. In other words, each new decision to gamble or trust should be treated independently. However, countering this rationalist perspective, it is well documented within the non-social domain that one's willingness to engage in decisions of uncertainty is highly influenced by recent outcomes, even when those past choices are completely independent. This is illustrated by the fact that individuals routinely exhibit the gamblers’ fallacy (Oskarsson, Van Boven, McClelland, & Hastie, 2009; Rabin, 2002; Tversky & Kahneman, 1974), believing that a streak of losses indicates a greater likelihood of wins for future gambles. Given the robustness of this finding in the non-social domain, it is possible that similar behavior also occurs in social contexts, such that a decision to trust a new individual (i.e. in one-shot games) is influenced by whether previous, unrelated partners were trustworthy. By comparing wins and losses in a non-social lottery task to receiving either a ‘defect’ or ‘reciprocate’ after an initial decision to trust a stranger in the Trust game, we can assess whether social context and acute stress differentially influences how feedback is incorporated into decisions of uncertainty.

Materials and Methods

Participants

58 participants were recruited and randomly assigned to be in either the stress condition or in the control condition (sample size based on extant research (Otto, Raio, Chiang, Phelps, & Daw, 2013)). One participant was subsequently excluded from the analysis for exhibiting decreased cortisol levels from baseline in the stress condition, and another subject was excluded from the analysis for exhibiting significantly increased cortisol levels from baseline in the control condition. The final sample included 56 participants, 28 of whom underwent the cold pressor manipulation (N=28; 13 males, mean age 22.33±3.15) and 28 who underwent the control manipulation (N=28, 15 males, mean age 21.0±2.49). Groups were matched on age (t(54)=1.76, p=.09) and gender (t(54)=−.814, p=.42), and all participants provided written consent in accordance with the standards of the New York University Committee on Activities Involving Human Subjects. Participants were paid $15 and received additional compensation based on the result of one randomly selected trial from the Trust Game and one randomly selected trial from the Lottery Game.

Task Procedures

Before starting the experiment, participants were asked to read instructions about each game. They were given additional verbal and visual instructions to ensure full comprehension (see supplement). Participants were endowed with $20, which was placed on the desk before them, $10 to be used for the Trust Game and $10 to be used for the Lottery Game. Participants also completed three practice trials before beginning each task. Both the Trust and Lottery games each had 36 trials and were matched on visual, temporal, and monetary dimensions. In order to counterbalance the order of the games, half of the participants played the Trust game first, while the other half played the Lottery game first.

The Trust Game (social task)

A typical Trust game involves a one-shot social interaction between two players, an Investor and a Trustee (Figure 1A). The first player (Investor) is initially faced with a decision to keep a sum of money (e.g., $10) or share part of it with a Trustee. If shared, the investment is quadrupled ($40) and the Trustee now faces the decision to repay the trust by sending back half of the increased sum (e.g., $20 for each player), or to defect and violate trust by keeping the money (e.g. $40 for the Trustee), leaving the Investor with nothing. The social dilemma for the Investor is clear as it is more profitable to trust, if trust is reciprocated, but doing so leaves the Investor susceptible to the risk of a breach in trust, and ultimately, the loss of money. Notably, these socially uncertain decisions combine risk (known probabilities, such as a 50% chance of winning) and ambiguity (unknown probabilities) (Knight, 1921)—parameters that behavioral economists have successfully deconstructed within the non-social domain. FIG 1

FIG 1 TASKS AND EXPERIMENTAL PROTOCOL A. Trust and Lottery games and their payoff structures. An Investor (participants) is endowed with money and can decide whether to send money to their partner (trust, no trust), in which case the money is multiplied four times. The Trustee can then decide to reciprocate by splitting the money between the two players, or can defect by keeping all the money for him or herself. The Lottery task (gambles) is identical in all respects except for the social component. B. Task structure. Participants were shown a picture of their partners, before being asked how much money they would like to trust to their partner (between $0 and $10, in increments of $2). After making a decision, participants were informed of whether their partners decided to defect or reciprocate (feedback). C. Experimental parameters. Cortisol measurements (Cort 1-4) were taken before the stress or control manipulation (baseline), ten minutes after the manipulation, between the two games (which were counterbalanced), and following the final game.

In our task, participants were assigned to be the Investor and were informed that they would play 36 trials, each with a different Trustee. On each trial participants could choose to invest anywhere between $0 and $10, in increments of $2 (i.e. $0, $2, $4, etc). If the participant decided to invest, they could double their investment if the Trustee shared back with them, or lose the money if the Trustee decided to keep the money. For example, if the participant shares $4 with the Trustee, the money quadruples to $16. The Trustee can then either keep the $16, or split the increased sum with the Investor such that both players each get $8. In other words, if Trustees chose to share the money, participants could double their earnings. If Trustees chose to keep the money, participants could lose whatever money they invested. Although in the eyes of the participant the outcomes of whether a Trustee would keep or share back the money was not known and thus characteristic of ambiguous uncertainty, in reality, the payoff structure was calculated at 50% reciprocation. Participants were further informed that only one trial would be randomly realized to be paid out at the end of the experiment.

Stimuli

Participants were told that on each trial they would play with a different partner (the Trustee), and that they would be able to view a photo of the Trustee before making their decision to invest. All the photos were faces of white males, pre-rated by an independent group (N=50; http://www.pnas.org/content/108/19/7710.long) and selected according to their levels of trustworthiness (within one standard deviation from mean trustworthiness ratings). Participants were further told that these Trustees had been previously brought into the lab and interviewed about their willingness to share or keep money with future partners, and that they would also be paid with a mailed check according to the decisions of the Investor. In reality, the Trustees’ decisions to reciprocate or defect were created by a computer algorithm. To ensure that participants’ believed they were interacting with real offers from real players, we probed participants’ beliefs about their partners during a tunnel debriefing session at the end of the experiment. Participants (N=2) who expressed doubts were not included in the behavioral or neuroendocrine analyses.

Timing structure

On each trial participants were presented with a photo of the Trustee and were given unlimited time to make their decision to invest (Figure 1B). Following their decision, participants were presented with a 2-6 second jitter of a fixation cross before either observing positive feedback “Your partner decided to share the money” or negative feedback “Your partner decided to keep the money” for three seconds. After receiving feedback, there was an inter-trial-interval jittered 2-6 seconds. In reality, the Trustee was a computer algorithm in which half the time the Trustee shared back and half the time the Trustee kept the money. Participants were not given any information about the probability distributions of Trustees reciprocating or defecting. The trials were pseudo-randomized such that a participant would never observe more than two ‘shares’ or two ‘keeps’ in a row.

The Lottery Game (non-social task)

The Lottery game was structured in the same manner as the Trust game, with the exception that there was no cover story about playing with partners and investing money in a Trustee. In essence, all components of the Trust and Lottery tasks were held constant except the social interaction of the Trust game. Accordingly, instead of being presented with photos of partners, participants viewed a stock image of a computer on each trial. Participants were told that on each trial, they could choose to gamble between $0 and $10 of their $10 endowment, in increments of $2. If they won the lottery they would double their gamble (a win). If they lost the gamble, they would loose the money (a loss). Wins and losses followed the same algorithm used in the Trust game, and trials were pseudo-randomized, such that no more than two wins or two losses were presented in a row. Furthermore, like the Trust game, participants were given no information about the probability of winning or losing a gamble. Thus, these lotteries are considered ambiguous probabilities. In reality, however, the lotteries were reinforced at a 50% win rate.

Stress Induction: Cold Pressor Task

Acute stress was induced by asking participants randomly assigned to be in the stress group to submerge their right forearm, hand through elbow, in ice-water (0-4 degree Celsius) for three consecutive minutes. The cold pressor task (CPT) has been shown to reliably increase sympathetic nervous system and HPA axis activity by activation of thermal and nociceptor afferents (Bullinger et al., 1984; Edelson & Robertson, 1986; Kelly & Cooper, 1998; Velasco, Gomez, Blanco, & Rodriguez, 1997) and has been previously used to elicit a stress response (Errico, Parsons, King, & Lovallo, 1993; Pascualy et al., 2000). Critically, the CPT does not have any lasting psychological effects typically associated with other types of laboratory stressors {McRae, 2006 #3138}, and thus provides an ideal technique for isolating an increased neurohormonal stress response exclusive of ancillary effects that could bias behavior. Participants selected to be in the control group were asked to submerge their right forearm in room temperature water (32-35 degree Celsius) for three consecutive minutes.

Physiological Stress Measurement

In order to acquire a physiological measure of stress, salivary samples were collected and analyzed for concentrations of both cortisol—a measure of HPA axis engagement, and α-amylase—which indirectly assays noradrenergic activity. The salivary samples were obtained by having participants place an oral swab beneath their tongue for two minutes. In order to control for circadian rhythms and stress induced by travel, participants were only recruited to come into the laboratory between 12:00 PM - 5:00 PM. To ensure that cortisol levels were stable, the first salivary sample was taken ten minutes after the participant arrived at the laboratory. Salivary samples were taken four times during the course of the experiment: 1) Cortisol 1 - at baseline: 10 min after the participant's arrival; 2) Cortisol 2 - 10 min after the stress or control manipulation when cortisol is expected to rise; 3) Cortisol 3 - after completing the first task (approximately 25 minutes after the stress/control manipulation); and 4) Cortisol 4 - after completing the second task (approximately 35 minutes after the stress/control manipulation); see Figure 1C.

Data & Regression Analysis

For the linear regression models we fit participants’ choices to gamble or trust (Choice_t) as a function of feedback on the previous trial (t-1), where 1 denotes a ‘reciprocate’ in the Trust game or ‘win’ in the Lottery game, −1 denotes a ‘defect’ in the Trust game or a ‘loss’ in the Lottery game, and 0 denotes a trial in which the participant chose not to gamble or trust. Participants chose to play at the same rate regardless of whether they were in the stress or control groups (p>.05; in both conditions approximately 70% of the time, see supplement). The parameters were entered into a mixed effects linear regression for each Condition x Game, where the within-subject factors were the intercept and feedback received on the previous trial (Tables 1-​-2)2) which enabled us to explore the effects of stress on an individual's ability to incorporate feedback. See supplemental information for further details on the full regression model (Table S1), and for alternative models, including a reinforcement-learning model and weighted average model. We used the lme4 package in the R programming language to run all regressions.

Table 1

CONTROL Regression coefficients indicating the influence of the outcome of previous trial on amount of money trusted and gambled.

^*Significance at 0.05

TABLE 2

STRESS Regression coefficients indicating the influence of the outcome of previous trial on amount of money trusted and gambled.

^*Significance at 0.05

Results

Social versus Non-social Ambiguous Decisions

Our first question was whether socially ambiguous decisions are valued in a similar way as non-socially ambiguous decisions. Participants in the control condition spent approximately the same amount of money irrespective of whether they were gambling (Lottery game: mean $3.74 ±2.40 SD) or entrusting money to a partner (Trust game: Mean $3.81 ±2.1 SD) (paired samples t-test: t(27)=−.25, p=.80), indicating consistent overall behavior irrespective of context (Fig 2B). Next, we wanted to investigate whether past, irrelevant feedback is incorporated and used in a similar manner during uncertain decisions to gamble and trust. To test this, we modeled decisions to trust and gamble as a function of the type of feedback participants received. We ran a trial-by-trial linear regression where outcome (win/loss in the Lottery game and reciprocate/defect in the Trust game) was used as lagged predictor of choice (how much money gambled/trusted on each trial). In other words, receiving a win/loss on trial 1 was yoked to choice on trial 2. This enabled us to examine how individuals use feedback to inform subsequent choices in both the social domain (trust) and the non-social domain (gamble); see supplemental methods for more details.

Fig 2 NEUROENDOCRINE AND BEHAVIORAL DATA. Mean cortisol levels across the experiment. Subjects in the stress condition exhibited increased cortisol levels at every time point (other than baseline) as compared to subjects in the control condition. Gray bar represents the timing for the stress and control manipulations. Bars represent 1 SEM. B. Participants gambled and trusted at the same rate when not under stress. However, acute stress differentially affected decision-making under uncertainty, such that stress increased gambles but decreased trust. *Significance at 0.05

Results reveal that individuals in the control condition gambled more after experiencing a loss, partaking in the gamblers’ fallacy by believing in the irrational notion that a streak of losses means a greater likelihood of wins for future gambles (Kahneman & Tversky, 1972) (Table 1; Lottery). This pattern was not observed in the social domain, as individuals did not rely on past, unrelated feedback when deciding to trust another (Table 1; Trust). That is, despite observing that their partners either defected or reciprocated their initial move to trust, subjects did not use this social feedback to inform their next choice to trust a new unrelated partner, effectively treating each new decision as an independent choice.

Neuroendocrine Results

To assess the efficacy of the cold pressor (CPT) manipulation, we measured salivary cortisol levels—an index of the hypothalamus-pituitary-adrenal (HPA) system and α-amylase—and index of sympathetic nervous system. Consistent with our prediction that the CPT induces increased cortisol, we found a main effect of Time (F(3,162)=8.5, p<0.001) and Condition (F(1,54)=7.4, p=0.009), as well as a Condition X Time interaction (F(3,162)=12.6, p<0.001, η_p²=.14; Figure 2A). Independent t-tests revealed no differences between conditions at baseline cortisol (t(54)=−.35, p=.73); however, significantly higher cortisol was evident in the stress condition at each time point after the stress/control manipulation (+10 minutes: t(54)=2.5, p=0.01; +20 minutes: t(54)=3.57, p=0.001; + 35 minutes: t(54)=2.98, p=0.004), indicating that the CPT manipulation was successful in increasing participants’ cortisol and engaging greater HPA activation. We ran a similar analysis using α-amylase but did not find any main effects or a Condition x Time interaction (F(3,162)=.56, p=.64), perhaps because the timing of our assessment was not optimal for the relatively rapid response of α-amylase (Maruyama et al., 2012).

Effects of Acute Stress on Social and Non-social Ambiguous Decisions

To examine the effect of acute stress on social choices compared to non-social choices, a 2 (Condition: no stress vs. stress) X 2 (Task: social vs. non-social) repeated measures ANOVA was conducted on the amount of money trusted or gambled. Results reveal an interaction between Task and Condition, (F(1,54)=6.2, p=0.016, η_p²=.10; Figure 2B), such that acute stress enhanced gambles and attenuated trusting behavior, as well as a main effect of Task (F(1,54)=4.63, p=0.036, η_p²=.08). Participants spent the same amount of money in both the Lottery and Trust tasks. However, participants who underwent the stress manipulation exhibited a dissociation between decisions of uncertainty depending on the context of the task. Specifically, stressed participants spent significantly more money gambling in the Lottery game (mean $4.31 ±2.30 SD) than they did trusting their partners in the Trust game (mean $3.27 ±1.96 SD) (paired samples t-test: t(27)=3.1, p=.004). This finding was due to acute stress both enhancing non-social gambling and diminishing social trusting behavior.

Effects of Acute Stress on Sensitivity to Feedback: Trial-by-Trial Analysis

Given the evidence that acute stress has differential effects on social vs. non-social choices under uncertainty, our next aim was to further decompose whether these effects of stress also bias how past, irrelevant feedback is incorporated into future decisions. To do this, we modeled the effect of stress on decisions to trust and gamble as a function of the feedback the participant received. As before, we ran a trial-by-trial linear regression where outcome was used as lagged predictor of choice, see supplemental methods for more details and full model specifications (Table S1).

Acute stress selectively effected how individuals incorporated social feedback. As in the control condition, when under stress, individuals used irrelevant, prior experiences to guide their future choices to gamble (Table 2; Lottery). In other words, stress had no effect on how irrelevant feedback biases choices to gamble. However, decisions to trust were effected by stress. Unlike subjects in the control condition, those in the stress condition used past, irrelevant feedback to guide choices to trust. After receiving negative feedback that a partner defected and did not share back the money, subjects subsequently entrusted more money to a new partner on the next trial (Table 2; Trust), effectively displaying a similar pattern of behavior observed in the gambling task. Simply put, stress compromised participants’ ability to disregard irrelevant, past information when deciding to trust a new partner.

Discussion

There is a wealth of research illustrating that humans are averse to making decisions under uncertainty in non-social contexts (Camerer & Weber, 1992) and that stress has mixed effects on an individual's engagement in non-socially risky and ambiguous decisions (Mather & Lighthall, 2012). Yet, despite the fact that many of our everyday choices of uncertainty are made within a social context, little is known about how individuals value decisions of uncertainty in the social domain compared to the non-social domain. Here, we find that although individuals who are not stressed gambled and trusted at overall similar rates, a trial-by-trial examination revealed that past irrelevant feedback had differential effects depending on context. In the non-social domain individuals exhibited the gamblers’ fallacy, believing incorrectly that a spate of losses will result in a subsequent win, while in the social domain, approximating rational agents, individuals did not use irrelevant past information to guide their subsequent choices to trust another.

Acute stress had a divergent effect on decisions under uncertainty, increasing decisions of ambiguity in non-social contexts, but decreasing decisions of ambiguity in social contexts, indicating that there is a qualitative difference between how social and non-social uncertainty is processed. We also find that stress differentially influences whether an individual attends to and uses irrelevant feedback in social contexts relative to non-social contexts. While in non-social contexts individuals exhibited the gamblers’ fallacy regardless of stress levels, in social contexts stress compromised the adaptive behavioral pattern of treating each new choice independently. Rather, stressed individuals were more likely to trust a new partner after receiving negative feedback that a past partner did not reciprocate their trust.

The one study that we are aware of that examines the impact of stress on decisions of uncertainty within the social domain, demonstrates that acute stress increases trusting behavior (von Dawans et al., 2012). Our findings indicate the opposite: acute stress dampens an individual's likelihood of making ambiguously uncertain decisions in social contexts, but heightens how often they engage in ambiguously uncertain decisions in non-social contexts. One critical difference between the findings reported here and this past work is the type of stressor used—the CPT versus Trier Social Stress Test (TSST)—which may have an influence on social decision-making. Unlike the CPT, the TSST induces psychosocial stress by requiring individuals to undergo social evaluation—that is, speaking in front of a panel of evaluative peers. In fact, individuals who experience the TSST report greater emotional and psychological vulnerability, rumination, and an overall decline in mood relative to the CPT (McRae et al., 2006), which could dictate a need to resuscitate (the perception of) fragile social ties. Thus, the effect of increased trust following the TSST is consistent with the theory that individuals are attempting to repair putatively compromised social ties by displaying a greater degree of trusting behavior (von Dawans et al., 2012). However, these behavioral effects cannot be explained by the increase in the neurohormonal response alone, as other non-specific effects of social evaluation likely play a role. This confound presents a problem for interpreting whether increases in trusting behavior are due to the physiological response to stress, or the psychological effects of the social stressor (i.e. rumination and psychological vulnerability), or both. We have eliminated the putatively confounding psychosocial effects on social behavior and report that inducing non-social stress does not increase trusting behavior, but rather attenuates trusting behavior.

By more closely examining how an individual incorporates irrelevant social and non-social feedback to bias future choice, we can further decompose how ambiguity considerations are processed under different contexts. According to classic economic theory, rational agents should not be influenced by past experiences when deciding to take a new, independent gamble or engage in trusting behavior with a new person (Rabin, 2002). That is, every new choice should be treated independently of past, irrelevant experiences. However, there is robust evidence within the literature (Sundali & Croson, 2006; Tversky & Kahneman, 1974), and from the findings here, which indicate that individuals routinely violate this principle in non-social contexts, and systematically engage in what is known as the gamblers’ fallacy. This finding is so robust that stress appears to have no bearing on an individual's belief that a streak of losses in non-social gambles will result in a subsequent win.

In contrast, we found that individuals in the control manipulation did not exhibit this behavior when deciding to trust another. Effectively, these non-stressed individuals correctly treated each decision to trust a new partner as an independent choice, indicating that, unlike in the non-social domain, individuals making ambiguous choices in a social context can successfully do so without relying on earlier irrelevant information. Given that all components of the Trust and Lottery tasks were held constant except for the social interaction component, these divergent behavioral patterns constitute a powerful model for how socially ambiguous choices likely rely on distinct cognitive processes. Furthermore, these divergent behavioral patterns were observed within individuals, such that the same individuals who exhibited the gamblers’ fallacy in the non-social domain, were able to rationally and adaptively make decisions without relying on irrelevant past information in the social domain.

That individuals are better at making independent ambiguous decisions that involve people than they are at making ambiguous decisions devoid of any social component, suggests that a ambiguous uncertainty is differentially valued depending on whether it is embedded in a social context or not. One explanation may be that people do not typically generalize a trait like trustworthiness across all individuals. Indeed, assuming that every individual can be trusted to the same degree would be highly maladaptive. Our data suggests that current relevant information—such as which person you are deciding to invest money in—is more highly weighted, or at least differentially valued than unconnected prior non-social experiences. Although, one important caveat is that any systematic violation of trust should cause an individual to be wary of trusting another. In our task, we attempted to approximate the level of trustworthy behavior observed outside the laboratory, where trustworthiness is highly variable across individuals.

Decision-making under uncertainty is ubiquitous to human life, and thus unsurprisingly, decades of research have explored how humans process uncertainty. Within the laboratory, uncertainty is typically studied within a neutral, non-social context. This approach, however, likely fails to capture the dynamic factors that influence decisions of uncertainty in the real world. Making decisions in uncertain environments during everyday life requires individuals to constantly assess risk and ambiguity under various shifting social contexts and varied emotional states. Here we find that both social context and mild acute stress independently, and jointly, contribute to the processing of uncertainty preferences.

Link to Article

The Influence of Social Context and Acute Stress on Ambiguity Preferences

Decision-making under uncertainty is a pervasive aspect of human behavior. Experimental economics has demonstrated that uncertainty influences learning processes and plays a crucial role in how individuals value options across diverse situations, particularly in scenarios involving trust. Trust, a cornerstone of economic transactions (Arrow, 1974), necessitates the ability to cooperate with individuals beyond one's kin. Learning to trust involves evaluating multiple factors, encompassing risk assessment and ambiguity considerations (Gambetta, 1988). Individuals must gauge the trustworthiness of others while considering their own past experiences with trust and reciprocity. This complex calculus underscores the inherent uncertainty in decisions to trust.

While stress is known to impact decision-making under uncertainty in non-social contexts (Starcke & Brand, 2012), its effects within social contexts, particularly trust-related decisions, remain poorly understood. Traditional models of uncertainty preferences posit that individuals evaluate the desirability and likelihood of outcomes through an expectation-based framework. In social situations, this becomes especially salient in trust decisions, as exemplified by the Trust game (Berg, Dickhaut, & Mccabe, 1995). The Trust game presents a scenario where trusting can yield higher payoffs if reciprocated but carries the risk of total loss if trust is violated, creating a decision analogous to a gamble with unknown probabilities (Knight, 1921).

This study addresses two key questions. First, it investigates whether uncertainty is valued differently in social versus non-social contexts. While research demonstrates ambiguity aversion in non-social settings, where individuals favor small but certain rewards over potentially larger but uncertain ones (Holt & Laury, 2002), less is known about ambiguity preferences in social situations, characterized by fewer known outcomes. To explore this, we employ the Trust game and a matched Lottery task, controlling for all factors except the source of uncertainty. This design allows for a direct comparison of ambiguity preferences in social (trusting others) and non-social (probabilistic gambles) contexts.

Second, we examine how acute stress differentially affects the valuation of uncertainty preferences across these contexts. Existing research on stress and risk-taking in non-social contexts yields inconsistent findings, with stress sometimes increasing risk aversion (Lighthall et al., 2009; Porcelli & Delgado, 2009) and other times decreasing it (Lighthall et al., 2009; Preston et al., 2007; Starcke et al., 2008; van den Bos et al., 2009). Even less is known about stress and ambiguity preferences in both social and non-social domains. While von Dawans et al. (2012) found that socially stressed individuals exhibit more trusting behavior, attributing this to a "tend-and-befriend" response (Taylor et al., 2000), the study's use of a social stressor (public speaking) confounds interpretation. To isolate the neurobiological stress response, we employ a non-social stressor (cold pressor task) to induce acute stress, indexed by cortisol levels (Axelrod & Reisine, 1984), allowing us to examine domain-specific effects of stress on ambiguity preferences.

Furthermore, we explore whether social context and acute stress influence the integration of past experiences when processing ambiguity. In one-shot games, where each decision is independent, prior outcomes should theoretically have no bearing on subsequent choices. However, the gamblers' fallacy demonstrates the influence of recent outcomes on future risk-taking in non-social settings (Oskarsson et al., 2009; Rabin, 2002; Tversky & Kahneman, 1974). We investigate whether a similar phenomenon occurs in social contexts by analyzing whether feedback in the Trust game (reciprocation or defection) influences subsequent trust decisions, mirroring the impact of wins and losses in the Lottery task. This comparison allows us to assess the differential effects of social context and acute stress on the incorporation of feedback into uncertainty judgments.

Materials and Methods

Participants

This study included 56 participants (28 per condition: stress, control) recruited from New York University, with a mean age of 21.67 years. Participants, matched on age and gender, were randomly assigned to conditions and provided informed consent following NYU's ethical guidelines.

Task Procedures

Prior to the experiment, participants received detailed instructions about the Trust and Lottery games, including practice trials. Each game consisted of 36 trials, matched for visual, temporal, and monetary factors. Participants were endowed with $20 ($10 per game) and compensated based on a randomly selected trial from each game.

The Trust Game (social task)

Participants assumed the role of the Investor in a one-shot Trust game (Figure 1A). Each trial involved deciding whether to invest between $0 and $10 (increments of $2) with a different Trustee (represented by a pre-rated photo of a white male face). Participants were informed that investing would quadruple the amount received by the Trustee, who could then choose to reciprocate (splitting the money equally) or defect (keeping all the money). While participants were unaware of the probability distribution, reciprocation was set at 50%.

Stimuli

To enhance ecological validity, participants were led to believe they were interacting with real Trustees who had previously indicated their willingness to share or keep money. Debriefing sessions assessed the credibility of this manipulation.

Timing structure

Each trial involved presenting the Trustee's photo, allowing unlimited decision time (Figure 1B). After the decision, a jittered fixation cross (2-6 seconds) preceded feedback (“Your partner decided to share/keep the money”). A jittered inter-trial interval (2-6 seconds) followed. The trial sequence was pseudo-randomized to avoid consecutive reciprocation or defection streaks.

The Lottery Game (non-social task)

This task mirrored the Trust game's structure but lacked the social interaction element. Participants gambled between $0 and $10 per trial, with a 50% win rate (doubling the gamble) or loss (losing the gambled amount). Participants were unaware of the probability distribution, making the lotteries ambiguous. Stimuli, timing, and feedback were identical to the Trust game, except a computer image replaced the Trustee's photo.

Stress Induction: Cold Pressor Task

Acute stress was induced in the stress condition via the cold pressor task (CPT), requiring participants to submerge their right forearm in ice water (0-4°C) for three minutes. The CPT reliably activates the sympathetic nervous system and HPA axis (Bullinger et al., 1984; Edelson & Robertson, 1986; Kelly & Cooper, 1998; Velasco et al., 1997) without lasting psychological effects (McRae, 2006). Control participants submerged their forearm in room temperature water (32-35°C).

Physiological Stress Measurement

Salivary cortisol and α-amylase levels were collected at four time points: baseline (10 minutes post-arrival), 10 minutes post-manipulation, post-first task, and post-second task (Figure 1C).

Data & Regression Analysis

Linear regression models assessed the influence of previous trial feedback (reciprocate/defect in Trust game, win/loss in Lottery game) on current trial investment/gamble amounts. Mixed effects linear regressions, conducted separately for each Condition x Game, examined the impact of stress on feedback incorporation. Additional analyses, including reinforcement-learning and weighted average models, are detailed in the supplemental materials.

Results

Social versus Non-social Ambiguous Decisions

Control participants exhibited comparable investment patterns in both tasks (Lottery: M = $3.74 ± 2.40 SD; Trust: M = $3.81 ± 2.1 SD; t(27) = -0.25, p = .80), suggesting similar valuation of uncertainty across contexts (Figure 2B). However, a trial-by-trial analysis revealed that past feedback influenced decisions differently. In the Lottery game, control participants gambled more after a loss, exhibiting the gamblers' fallacy (Table 1). Conversely, in the Trust game, prior feedback did not significantly impact trust decisions, suggesting a rational approach where each decision was treated independently (Table 1).

Neuroendocrine Results

The CPT effectively increased cortisol levels in the stress condition, as evidenced by a significant Condition x Time interaction on cortisol levels (F(3,162) = 12.6, p < .001, ηp2 = .14; Figure 2A). Post-manipulation cortisol levels were significantly higher in the stress compared to the control condition at all time points (p < .05). No significant effects were observed for α-amylase.

Effects of Acute Stress on Social and Non-social Ambiguous Decisions

A 2 (Condition) x 2 (Task) repeated measures ANOVA revealed a significant interaction effect on investment/gamble amounts (F(1,54) = 6.2, p = .016, ηp2 = .10; Figure 2B). Stressed participants gambled significantly more in the Lottery game than they invested in the Trust game (t(27) = 3.1, p = .004), indicating a dissociation in uncertainty processing under stress.

Effects of Acute Stress on Sensitivity to Feedback: Trial-by-Trial Analysis

Stress selectively modulated the incorporation of social feedback. While stressed participants, like controls, displayed the gamblers' fallacy in the Lottery task, they exhibited increased trust following negative feedback in the Trust game. This suggests that stress impaired the ability to disregard irrelevant past information in social but not non-social decision-making.

Discussion

This study investigated the impact of social context and acute stress on ambiguity preferences. While non-stressed individuals demonstrated similar overall investment patterns in both social and non-social tasks, a nuanced analysis revealed distinct processing of uncertainty. In the absence of stress, participants treated trust decisions independently, unaffected by previous outcomes, while exhibiting the gamblers' fallacy in non-social gambles.

Acute stress, however, led to a divergence in behavior. It amplified gambling in the Lottery task but attenuated trust in the Trust game, suggesting a qualitative difference in how social and non-social uncertainty are processed under stress. Furthermore, stress selectively impacted the use of past feedback, impairing the ability to disregard irrelevant social information in trust decisions while leaving the gamblers' fallacy intact in the non-social task.

These findings contradict previous research reporting increased trust under stress (von Dawans et al., 2012), likely due to differences in stressor type. Unlike the socially evaluative TSST employed by von Dawans et al., the CPT used in this study isolates the neurobiological stress response, suggesting that the previously observed trust increase may stem from the psychosocial effects of social evaluation rather than stress per se.

The differential impact of past feedback on social versus non-social decisions underscores the distinct cognitive processes underlying these choices. The absence of the gamblers' fallacy in the control group's trust decisions suggests that individuals may be less likely to generalize trustworthiness across individuals, relying instead on current relevant information (e.g., the specific Trustee's photo). This adaptive behavior highlights the importance of context-specific information in social judgments.

This study underscores the need to move beyond traditional, decontextualized paradigms in decision-making research. By incorporating social context and emotional states like stress, we gain a richer understanding of how individuals navigate uncertainty in real-world settings. Our findings highlight the complex interplay of social and emotional factors in shaping uncertainty preferences and emphasize the need for future research to further explore these interactions.

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How Stress Affects Our Trust in Uncertain Situations

We make decisions in uncertain situations every day. Whether we're choosing a career path or deciding to trust someone, uncertainty is a part of life. Economists have shown that uncertainty influences how we learn and make decisions, especially when it comes to trusting others. Trust is essential for any economic transaction (Arrow, 1974), and figuring out who to trust involves considering lots of factors, like potential risks and uncertainties (Gambetta, 1988). For example, a person might judge how trustworthy someone seems while also thinking about whether trusting others has worked out for them in the past.

Choosing to trust is like making a bet where a person doesn't know the odds. A person might gain something if the other person is trustworthy, but they could lose everything if they aren't (Ben-Ner & Putterman, 2001). This is similar to gambling without knowing the chances of winning (Knight, 1921). While we know that stress affects decision-making under uncertainty in situations that don't involve other people (Starcke & Brand, 2012), we don't know much about how it affects our choices to trust.

This study examines two main questions:

1. Do we make decisions differently when uncertainty involves other people versus when it doesn't? We tend to avoid risky choices when we don't know the odds (Holt & Laury, 2002). This study compares how people handle uncertainty in social situations (like deciding to trust someone) versus non-social situations (like gambling).

2. Does stress change how we handle uncertainty in social and non-social situations? Stress can make us more or less likely to take risks, but we don't fully understand why (Lighthall et al., 2009; Pabst et al., 2013; Porcelli & Delgado, 2009). This study investigates whether stress changes our willingness to trust others or take risks in a game. We also look at how stress affects our ability to learn from past experiences in these situations. For example, if someone breaks our trust, does stress make us less likely to trust others in the future?

To answer these questions, the researchers used two tasks: the Trust Game and a similar Lottery Game. Both games involved uncertainty, but the Trust Game included social interaction. By comparing behavior in both games, the study aimed to isolate the effects of social context and stress on decision-making under uncertainty.

Materials and Methods

Participants

56 participants were randomly divided into two groups: a stress group and a control group. Groups were balanced in terms of age and gender. All participants provided consent and were paid for their participation.

Task Procedures

Participants received instructions and practiced both games. They were given $20, with $10 for each game. The order of the games was counterbalanced to control for order effects.

The Trust Game (Social Task)

The Trust Game involves two players: an Investor and a Trustee. The Investor decides whether to keep their money or share some with the Trustee. If shared, the money is multiplied. The Trustee then chooses to either split the multiplied money with the Investor (reciprocate) or keep it all (defect).

Participants were told they would play 36 rounds, each with a different Trustee. Before each round, they saw a photo of the supposed Trustee. They could choose how much to invest, knowing they could double their money if the Trustee reciprocated or lose it all if the Trustee defected. In reality, the Trustee's choices were determined by a computer program that ensured a 50% chance of reciprocation. Participants didn't know the odds and were told that the Trustees were real people who had been interviewed earlier.

The Lottery Game (Non-Social Task)

The Lottery Game was identical to the Trust Game, except there were no social elements. Participants decided how much to gamble on each trial, with a 50% chance of winning (doubling their money) or losing.

Stress Induction: Cold Pressor Task

The stress group submerged their arm in ice water for three minutes, a procedure known to activate the body's stress response (Bullinger et al., 1984; Edelson & Robertson, 1986; Kelly & Cooper, 1998; Velasco et al., 1997). The control group submerged their arm in room temperature water.

Physiological Stress Measurement

Saliva samples were collected at different times to measure cortisol, a stress hormone.

Data & Regression Analysis

Researchers analyzed how previous outcomes (wins/losses, reciprocation/defection) influenced participants' subsequent choices in both games. This allowed them to assess whether people fall prey to the "gambler's fallacy" (believing that past losses predict future wins) and whether this tendency is different in social versus non-social situations. They also looked at whether stress changed how people used this past information.

Results

Social versus Non-social Ambiguous Decisions

People in the control group (not stressed) spent similar amounts of money in both games. However, they showed different patterns of behavior depending on whether the uncertainty involved another person or not. In the Lottery Game, they were more likely to gamble after a loss, demonstrating the gambler's fallacy. In the Trust Game, they didn't rely on past outcomes when deciding to trust a new partner. Each decision was made independently.

Neuroendocrine Results

The cold pressor task effectively increased cortisol levels in the stress group, confirming its ability to induce a physiological stress response.

Effects of Acute Stress on Social and Non-social Ambiguous Decisions

Stress affected decisions differently in the two games. Stressed participants gambled more in the Lottery Game but trusted less in the Trust Game. This suggests that stress changes how we evaluate uncertainty in social situations compared to non-social ones.

Effects of Acute Stress on Sensitivity to Feedback

Stress also affected how people used past information. While the gambler's fallacy was present in the Lottery Game regardless of stress, it only appeared in the Trust Game for the stressed group. This means that stress impaired their ability to treat each new trust decision independently. Instead, they were more likely to trust someone new even if a previous partner had betrayed their trust.

Discussion

This study highlights important differences in how we handle uncertainty in social and non-social situations. When not stressed, people seem to be better at making independent decisions involving trust compared to gambling. This suggests that we process uncertainty differently depending on the context.

Stress disrupts this ability. It makes us more likely to rely on irrelevant past experiences when deciding whether to trust someone. This finding has implications for understanding how stress can negatively affect our social behavior.

The study emphasizes the importance of considering both social context and stress when studying decision-making under uncertainty. Future research can build on these findings to further explore the complex interplay between our social world, our emotions, and our choices.

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How Stress Changes the Way We Make Decisions About Trust and Gambling

Life is full of uncertainties. We face them every day when we make decisions, and they play a big role in how we learn and value our choices. One area where uncertainty is a huge factor is in deciding whether to trust someone. Trust is a fundamental part of how our economy works (Arrow, 1974). Think about it – we constantly interact and cooperate with people we aren't related to. Figuring out who to trust involves weighing a bunch of factors, like how trustworthy the other person seems and whether our past experiences with trust have paid off (Gambetta, 1988). It's like making a decision in the dark.

While we know stress can affect how we make choices in situations with unknown outcomes (Starcke & Brand, 2012), we don't really know how stress affects those choices in social situations where we interact with others. This article explores this gap in our understanding.

Current theories suggest that when faced with uncertainty, people try to figure out the likelihood of different outcomes. This is especially important when deciding whether to trust someone, a dynamic explored in something called the Trust game (Berg, Dickhaut, & Mccabe, 1995).

The Experiment

In this game, trusting someone can lead to more money if they reciprocate, but there's also the risk of losing everything if they decide not to. This makes trusting a very uncertain decision (Ben-Ner & Putterman, 2001), much like a gamble with unknown odds (Knight, 1921).

This study focuses on two key questions:

1. Do we value uncertainty differently in social and non-social situations?

2. Does stress change how we value these uncertainties in social and non-social situations?

This research also looks at whether stress and social context affect how we factor in past experiences when dealing with uncertainty. Ideally, in one-shot games where each round is independent, past experiences shouldn't matter. However, we know that in non-social settings, people often fall into the "gambler's fallacy" – letting past outcomes influence their future choices even when those choices are unrelated (Oskarsson, Van Boven, McClelland, & Hastie, 2009; Rabin, 2002; Tversky & Kahneman, 1974). This study investigates whether this also happens in social contexts, where a decision to trust someone new might be swayed by whether previous, unrelated people were trustworthy.

By comparing wins and losses in a lottery game to outcomes in the Trust game, researchers can determine whether stress and social context affect how we integrate feedback into our decisions under uncertainty.

Results

People in the control group (not stressed) spent similar amounts of money in both games. However, they showed different patterns of behavior depending on whether the uncertainty involved another person or not. In the Lottery Game, they were more likely to gamble after a loss, demonstrating the gambler's fallacy. In the Trust Game, they didn't rely on past outcomes when deciding to trust a new partner. Each decision was made independently.

Stress affected decisions differently in the two games. Stressed participants gambled more in the Lottery Game but trusted less in the Trust Game. This suggests that stress changes how we evaluate uncertainty in social situations compared to non-social ones.

Stress also affected how people used past information. While the gambler's fallacy was apparent in the Lottery Game regardless of stress, it only appeared in the Trust Game for the stressed group. This means that stress impaired their ability to treat each new trust decision independently. Instead, they were more likely to trust someone new even if a previous partner had betrayed their trust.

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Do We Trust Differently When We're Stressed?

We make choices every day, and sometimes those choices are uncertain, like taking a chance. Scientists who study how we make choices found that we often don't like taking chances when we don't know what will happen. This is especially true when we have to decide whether to trust someone. Trusting someone is important for working together, but it can also be risky. We have to figure out if someone seems trustworthy and if trusting people in the past has worked out for us.

Scientists wanted to know if we think about uncertain choices differently when they involve people compared to when they don't. They also wanted to know if feeling stressed changes how we make these choices. To find out, they asked people to play two games: a "Trust Game" and a "Lottery Game."

The Trust Game: A participant gets $10. They can keep it safe or share some with another person. If they share, the money grows four times bigger. But the other person can choose to share the bigger amount with them or keep it all for themselves. Will they trust them?

The Lottery Game: A participant gets $10 and can use some of it to buy lottery tickets. If they win, they get double the money they spent on tickets. If they lose, they lose the money they spent. Will they try their luck?

These games helped scientists understand how people make choices when they don't know the outcome. The Trust Game involved trusting another person, while the Lottery Game was just about chance.

Half of the people in the study did a stressful activity before playing the games: holding their arm in ice-cold water. This helped the scientists see if stress changes how we make decisions. The other half just put their arm in room temperature water.

Here's what they found:

• Social vs. Non-social Decisions: People who weren't stressed played both games in similar ways, using about the same amount of money. This means that whether they were trusting a person or taking a chance in a lottery, they made similar choices.

• Does Stress Change Things?: Stress made a big difference. Stressed people took more chances in the Lottery Game but were less trusting in the Trust Game. This shows that stress makes us think about uncertain choices differently depending on if they involve people.

• Learning from Experience: In the Lottery Game, people who lost money often took a bigger chance on the next try, thinking they were more likely to win after a loss. This isn't true, but it's a common mistake people make. In the Trust Game, people who weren't stressed didn't let past experiences change their decisions. But, stressed people who were tricked in the Trust Game often trusted even more on the next try, making the same mistake as in the Lottery Game.

What does it all mean?

This study shows that our brains might have different ways of dealing with uncertainty when we are stressed and when we have to trust others. When we're stressed, we might make riskier choices when we don't know the outcome, especially when it comes to trusting people. This helps us understand how stress affects our choices in everyday life.

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