Exploring the Links Between Trait Mindfulness and Emotional and Behavioral Responses in the Ultimatum Game

The current study assessed whether trait mindfulness relates to social decision making as increased acceptance rates towards offers in the Ultimatum Game. Mindfulness has been associated to a reduction in emotional reactivity and an increase in emotion regulation once intense emotions do occur. Therefore, we reasoned that trait mindfulness would predict acceptance in the Ultimatum Game, perhaps even in case of unfair offers. In two online studies we assessed whether trait mindfulness positively predicts acceptance of offers in the Ultimatum Game among community samples (study 1 N = 107; study 2 N = 118). In study 2, we also assessed participants’ emotional reactions to offers prior to their decision to accept or reject. Whereas study 1 indeed showed a significant positive relation between trait mindfulness and acceptance of offers (OR = 2.01, p = .05), study 2 did not show this relation (OR = .91, p = .81). Also, the results of study 2 showed that trait mindfulness may moderate emotional responses to offers (β =  − .06, p = .03). Yet, analyses of the pooled data indicated no relation between trait mindfulness and acceptance of offers (p < .15). Our research provides mixed support regarding the association between trait mindfulness and behavioral acceptance of offers in the Ultimatum Game. We discuss the need for more fine-grained examinations of when and why mindfulness should lead to acceptance of unfairness, and if and when mindfulness would lead to wise responding in social exchange situations.

Both popular and scientific literature posit that the practice of mindfulness may benefit interpersonal behavior by facilitating present moment awareness and non-judgmental attention when interacting with others (e.g., Brown et al., 2007;Dalai Lama & Ekman, 2008). For a large part, social exchanges are shaped by norms and ideas of fairness, and ample research shows that social decision making is strongly guided by adversity to unfairness and inequity (Bolton & Ockenfels, 2000;Fehr & Schmidt, 1999). Yet, how people cope with and respond to perceived unfairness depends on individual and contextual factors (Camerer, 2003;Güth & Kocher, 2014). Trait mindfulness could be an individual difference factor that guides social decision making in light of unfairness.
Economic games like the Ultimatum Game are often used to isolate and examine elements of social decision making in order to gain a better understanding of what drives human decision making in real life (Camerer, 2003). The Ultimatum Game is designed, and has been widely used, to examine social preferences and reactions to unfairness (Güth & Kocher, 2014). Typically, it involves two players (or one player against a computer) of which player A is initially endowed with an amount of credits, and is asked to propose a split between oneself and player B -e.g., 15 credits for player A and 5 credits for player B from a 20 credits total. In turn, player B can accept or reject this offer: when accepted, the credits will be divided according to the proposal, and if rejected neither player A nor B receives anything. Although the rational economic decision would be to accept any nonzero proposal, ample evidence has demonstrated that people usually reject splits that they deem unfair (e.g., Camerer, 2003;Güth & Kocher, 2014). Furthermore, converging evidence has shown that perceived unfairness usually elicits feelings of anger, which, in turn, is a strong predictor of rejection (e.g., Aina et al., 2020;Reed et al., 2020).
Could mindfulness alter reactions towards perceived unfairness? Mindfulness is commonly defined as the regulation of attention to present moment experiences with an attitude of openness and non-judgement (e.g., Bishop et al., 2004;Kabat-Zinn, 1982). The extent to which people generally pay mindful attention to present moment experiences naturally differs between individuals (i.e., trait mindfulness), and each individual experiences fluctuations in mindfulness during each day (i.e., state mindfulness; Brown & Ryan, 2003). Furthermore, mindfulness training has been shown to increase both state and trait mindfulness (e.g., Shoham et al., 2017).
Several mindfulness scholars have theorized that mindfulness could benefit social decision making by de-automatization, dis-identification, and behavior regulation (Berry & Brown, 2017;Leary & Diebels, 2017). By bringing awareness to present moment experiences within social exchange settings, one may gain awareness of emotional and behavioral tendencies, such as anger, irritation, and retaliation, that may otherwise automatically and mindlessly lead to nonsocial behavior. A mindful person might be better aware of such initial response tendencies without identifying with and getting "stuck" on them (Bernstein et al., 2015;Teper & Inzlicht, 2013), thereby decreasing emotional reactivity (e.g., Roemer et al., 2015). For instance, after receiving an unfair offer, instead of identifying with the feeling of anger one might notice the feeling of anger without identifying with it or getting worked up by it (e.g., "I am angry" vs "I notice the experience of anger"). This de-automatization between stimulus and response, and dis-identification from internal experiences and initial tendencies, should allow for more behavioral flexibility to respond in line with longer term goals and values (Bernstein et al., 2019;Karremans et al., 2017). Indeed, empirical evidence has suggested that mindfulness can increase awareness of impulsive tendencies (e.g., Papies et al., 2012), decrease identification with emotions (Farb et al., 2007), and facilitate emotion and behavior regulation (see for reviews e.g. Chiesa et al., 2011;Roemer et al., 2015).
There is some initial research on the relation between mindfulness and responses to unfairness. Kirk et al. (2011) found that expert meditators accepted more unfair offers in the Ultimatum Game as compared to novices. Furthermore, a follow-up study (Kirk et al., 2016) indicated that an 8-week mindfulness-based stress reduction training increased acceptance rates of unfair offers in comparison to a control training. These studies also provided preliminary evidence for a possible neural mechanism underlying the effect. Activation of the anterior insula, as involved in anger and disgust, is usually strongly related to rejection (e.g., Sanfey et al., 2003). Results from the neuroimaging studies by Kirk et al., (2011Kirk et al., ( , 2016 suggest that mindfulness meditation is associated with attenuated activity in the anterior insula, and weaker association between anterior insula activation and rejection of unfair offers, suggesting that mindfulness may temper emotional reactions to unfairness, and allowing one to regulate behavior towards acceptance of offers.
There are theoretical reasons to believe that people high in trait mindfulness may be less likely to respond strongly to unfairness through less emotional reactivity. In the context of the Ultimatum Game, this would imply that higher trait mindfulness is associated with higher acceptance levels, particularly of unfair offers. To examine this basic hypothesis, in two studies we examined the role of trait mindfulness in the willingness to accept offers in the Ultimatum Game. In study 1, we examined the association between the fairness of offers and the decision to accept or reject, and whether trait mindfulness is associated with increased acceptance of offers. With a pre-registered study 2, we further assessed how trait mindfulness might be associated with increased acceptance rates by examining the role of participants' subjective emotional reactions to offers.

Study 1
With study 1, we set out a conceptual replication of the previously found association between mindfulness and acceptance of offers in the Ultimatum Game. Specifically, in study 1 we examined whether trait mindfulness is associated with increased acceptance of offers, and whether the association between the fairness of offers and the decision to accept or reject may be less pronounced for participants relatively high vs low in trait mindfulness. It is important to mention that we initially planned to compare groups of meditators and matched controls. However, due to difficulties in the recruitment process resulting in a very unequal balance of groups (87 meditators vs 34 non-meditators), in the current article we report the results regarding trait mindfulness and acceptance of offers. The analyses on meditators and nonmeditators can be found in the supplementary materials.

Participants
A total of 121 participants (M age = 36.64, SD age = 14.70; female = 85; male = 35; other = 1) participated in this online study that took 15-20 min to complete. Participants were recruited via online discussion fora (e.g., Reddit, Expatforum) as well as via various meditation centers and communities in English-speaking countries (e.g., UK, USA, Australia). At the end of the study, participants could choose to receive reimbursement in Amazon vouchers by (1) receiving a random draw of their accepted trials in Amazon vouchers in the Ultimatum Game Task (1-20 credits) and/or (2) entering a lottery for one of three Amazon vouchers (i.e., 1 × 100, 2 × 50) in the currency of their country.

Procedures
This study was hosted by the Gorilla Experiment Builder (www. goril la. sc). After providing informed consent, participants filled out questions regarding their demographic characteristics. Participants received written instructions about the Ultimatum Game and had three practice rounds in order to familiarize themselves with the game, prior to participating in ten rounds of the Ultimatum Game task. Hereafter, participants filled out the trait mindfulness questionnaire, were debriefed, and received their reimbursement. All procedures were conducted in line with the Ethics Committee of Social Sciences of the Radboud University and adhere to the European Code of Conduct for Research Integrity.

Measures
Trait Mindfulness The Five Facets Mindfulness Questionnaire -Short Form (24 items; α = 0.91 in this sample; Baer et al., 2006) was used to assess trait mindfulness. Five-point Likert scales ranging from 1 (very rarely true) to 5 (very often or always true) were used to rate each item of the FFMQ (e.g., "I perceive my feelings and emotions without having to react to them"). After recoding, mean scores across all items were calculated to create a general trait mindfulness score to use in further analyses.
Ultimatum Game Participants played ten rounds of the Ultimatum Game (Güth et al., 1982). They were told that they were playing against other participants online and that by a random draw they were in the position of the receiver. In reality, the proposals were a predetermined and a randomized set of splits of twenty credits (1: 19, 2:18, 3:17, 4:16, 5:15, 6:14, 7:13, 8:12, 9:11, 10:10 of which the first number was offered towards the participant). Within each trial, participants were offered one of these splits, and asked whether they accepted or rejected this offer (see Fig. 1). Participants were told that if they accepted the offer, both parties received the proposed credits; if they rejected the offer, both parties received nothing. Participants were informed that they would receive a random draw of one of their accepted offers in the Ultimatum Game as Amazon vouchers in the currency of their country (€/£/$ 1-19). In other words, accepting or rejecting an offer had actual consequences for the amount of reward participants would receive.

Demographics
Participants were asked about their demographic characteristics such as age, gender, educational level, and country of residence.

Data Analyses
Analyses were conducted using the statistical software R (R Core Team, 2022;RStudio Team, 2022). To assess whether trait mindfulness is related to acceptance of offers, a generalized logistic mixed-effects (GLMER) analysis was conducted with offer as centered numeric predictor (range: 1-10, with 1 representing the 1:19 split and 10 the 10:10 split), trait mindfulness as a standardized predictor, and the binary response (accept/reject) as the dependent variable. As such, we analyzed all offers from very unfair (1 out of 20) to fair (10 out of 20), such that each credit increase implies an increase in fairness. To account for individual differences in acceptance rate and the repeated number of responses in the Ultimatum Game, a per-participant random intercept and per-participant random slope over offers were included (e.g., Burnham, 2018;Detry & Ma, 2016). p-values were obtained with parametric bootstrapping. Odds ratios (OR) will be reported as effect-size estimates.

Results
A total of 121 individuals completed the study. Six participants were removed from analyses due to technical issues during data collection, which prevented them from participating adequately. Another five participants were removed from analyses because they took less than 10 min to complete the study (M Time = 20:14, SD = 9:50), and we deemed it unlikely that they took the task seriously in so little time, leaving a dataset of 107 for analyses. Descriptive statistics are described in Table 1.
We examined whether trait mindfulness is associated with the increased acceptance of offers, and whether the association between the fairness of offers and the decision to accept/ reject may be less pronounced for those participants high versus low in trait mindfulness. We analyzed all offers from very unfair (1 out of 20) to fair (10 out of 20) and each credit increase in offer thus implies an increase in fairness of the offer. General logistic mixed-effects analysis (GLMER) was used to assess the effect of trait mindfulness, offers in the UG, and their interaction on acceptance of offers in the UG.
Results showed a significant effect of offer (estimate = 1.32, SE = 0.16, z = 8.38, p < 0.001, 95% CI [1.04, 1.67], OR = 3.74) and trait mindfulness (estimate = 0.70, SE = 0.36, z = 1.95, p = 0.05, 95% CI [0.04, 1.41], OR = 2.01) on the acceptance rate of offers. These findings indicate that for every one credit increase in offer, participants were 3.74 times more likely to accept the offer; put differently, participants were overall less likely to accept lower offers. More importantly, the main effect of trait mindfulness implies that participants scoring + 1SD on trait mindfulness were 2.01 times more likely to accept lower (more unfair) offers than participants who scored at the mean level of trait mindfulness. There was no significant interaction between offer and trait mindfulness (estimate = − 0.07, SE = − 0.11, z = − 0.58, p = 0.56, 95% CI [− 0.30, 0.15], OR = 0.93), indicating that the association between offers and acceptance did not differ per level of trait mindfulness. Figure 2 shows the absence of the interaction effect (i.e., the logistic regression curve has the same shape for different levels of trait mindfulness), and the main effect of trait mindfulness: for participants scoring high in trait mindfulness, the logistic regression curve is entirely shifted to the left. Hence, to the extent that participants had higher levels of trait mindfulness, they were more likely to accept offers that were a little more unfair, hereby conceptually replicating earlier work by Kirk et al., (2011Kirk et al., ( , 2016.

Study 2
Study 1 provided support in line with our main prediction that trait mindfulness is positively related to acceptance of offers. More mindful participants were more likely to accept  offers that were more unfair. Study 2 aimed to replicate the main findings and explored the effect of trait mindfulness on emotional reactions to the fairness of offers in the Ultimatum Game as a potential psychological mechanism. Following mindfulness theorizing, we expected that mindfulness might facilitate behavior regulation by attenuating emotional reactions (e.g Berry & Brown, 2017;Leary & Diebels, 2017). Converging evidence indicates that emotional reactions to unfair offers in the Ultimatum Game strongly predict acceptance or rejection (e.g., Aina et al., 2020;Reed et al., 2020). Conversely, emotion regulation strategies (e.g., reappraisal) have been shown to increase acceptance of unfair offers (Grecucci et al., 2013;van 't Wout et al., 2010). As noted, initial neuroimaging research shows that mindfulness may also temper emotional reactions to unfairness (Kirk et al., 2011) and thereby increase acceptance of offers. Here we further explored whether participants high in trait mindfulness would show less intense emotional reactions to unfair offers as compared to participants lower in trait mindfulness. Thus, complementing previous neuroimaging results, we zoom in on the subjective emotional reactions of participants to the fairness of offers to gain a better understanding of the psychological mechanisms of the effect.

Participants
A total of 119 participants (M age = 32.90, SD age = 10.86; female = 66, male = 52, other = 1) took part in this online study. Initially, as in study 1, we wanted to compare existing groups, and aimed to recruit 60 experienced meditators and 60 non-meditators via online fora, apps, facebook groups and by contacting community centers (e.g., meditation centers, arts and crafts centers). Yet, because recruitment went slower than expected and our students had a strict deadline for data collection, we collected data of 12 participants via our initial recruitment plan and recruited an additional 108 participants via the online participation platform Prolific (www. proli fic. co). To this end, we added an attention check at the beginning of the study to assess whether prospective participants were motivated to read our instructions carefully. Those who did not pass the attention check could not partake in the remainder of our study and where not included in the aimed sample size, and we ended with an unequal distribution in groups (meditators = 36, non-meditators = 83).
In the current article we focus on the relation between trait mindfulness and acceptance of offers (the analyses on meditators vs non-meditators can be found in the supplementary materials).

Procedures
The procedure of study 2 was largely the same as that of study 1. Participants played 10 rounds of the Ultimatum Game in the role of the receiver. Within each trial, participants were offered a predetermined and randomized split of 20 credits. In study 2, to examine emotional responses to fairness of the offers, after each offer, participants were first asked how they felt about this offer, followed by the question whether they accepted or rejected this offer (see Fig. 1). Again, we assessed level of trait mindfulness (α = 0.88 in this study), and the study was closed with a debriefing and reimbursement. We slightly changed participant payment such that every offer would count towards their earnings. Prior to starting the Ultimatum Game, participants were informed that they would receive 10% of the offers they accepted in Amazon Vouchers in the currency of their country (€/£/$ 1-5.50).

Measures
We used the same tasks and scales to assess participants' Ultimatum Game behavior and trait mindfulness as in study 1.
Emotional Reaction Participants' emotional reaction to each offer in the Ultimatum Game was measured with an affective slider ("how do you feel about this offer?"; 1 = negative to 100 = positive; Betella & Verschure, 2016) before they indicated accepting or rejecting the offer (see Fig. 1).

Data Analyses
Following our pre-registration, we first assessed whether we replicated our earlier found effect that trait mindfulness is related to acceptance of offers. A generalized logistic mixedeffects (GLMER) analysis was conducted with offers as centered numeric predictor (range: 1-10, with 1 representing the 1:19 split and 10 the 10:10 split), trait mindfulness as a standardized predictor, and the binary response (accept/ reject) as the dependent variable as in study 1. Again, we analyzed all offers from very unfair (1 out of 20) to fair (10 out of 20), and each credit increase in offer would imply an increase in fairness of the offer. Second, a linear mixedeffects analysis was conducted to assess whether trait mindfulness was related to emotional reactions in the Ultimatum Game. Again, UG offers were included as a numeric predictor and trait mindfulness as a standardized predictor. Emotional reactions were entered as the continuous dependent variable (1, unhappy smiley, to 100, happy smiley). Perparticipant random intercepts and per-item random slopes were included for all analyses to account for individual differences and the repeated number of responses in the Ultimatum Game. Parametric bootstrapping was used to obtain p-values. Odds ratios (OR) and standardized coefficients will be reported as effect-size estimates.

Results
A total of 119 participants completed the study. The data of one participant was omitted from the analyses for taking less than 10 min to complete the study, leaving a sample size of 118 for further analyses. Descriptive statistics are described in Table 1. We examined whether the fairness of offers was associated with emotional reactions to offers, and whether this association was weaker for participants who scored relatively high on trait mindfulness as compared to those low on trait mindfulness. The effect of trait mindfulness, offers in the Ultimatum Game, and their interaction on emotional reactions to offers in the Ultimatum Game were assessed with linear mixed effect modelling (LMER). Results indicated a significant effect of fairness of offer on emotional reaction (estimate = 7.91, SE = 0.31, t = 25.38, p < . 001, 95% CI [7.29,8.53], β = 0.74) and no effect of trait mindfulness (estimate = 0.84, SE = 1.23, t = 0.68, p = 0.50, β = 0.03). Thus, on average, participants reported more negativity to lower offers. These effects were qualified by a significant interaction between fairness of offer and trait mindfulness As in study 1, general logistic mixed-effects analysis (GLMER) was used to assess the effect of trait mindfulness, offers in the Ultimatum Game, and their interaction on acceptance of offers in the Ultimatum Game. Results showed a significant effect of fairness of the offer (estimate = 1.85, SE = 0.23, z = 8.02, p < 0.001, 95% CI [7.29, 8.53], OR = 6.36), but no effect of mindfulness (estimate = − 0.09, SE = 0.36, z = − 0.24, p = 0.81, 95% CI [− 1.57, 3.18], OR = 0.91,) nor an interaction between offer and trait mindfulness on the acceptance rate of offers (estimate = − 0.17, SE = 0.14, z = − 1.18, p = 0.24, 95% CI [− 1.29, − 0.03], OR = 0.85). For every one credit increase in offer, the odds of accepting the offer increased by 6.36. Note that we also described exploratory moderation and mediation analyses in our pre-registration. Even though our direct effect was not significant, we explored the model with and without emotional reactions as a predictor to illustrate the possible involvement of emotional reactions to offers in acceptance of those offers. Emotional reactions significantly predicted acceptance of offers, estimate = 3.50, SE = 0.49, z-value = 7.14, p < 0.001, 95% CI [2.66, 5.10], OR = 33.12. There was no significant interaction between offer and emotional reaction on acceptance of offers, estimate = − 0.17, SE = 0.16, z-value = − 1.07, p = 0.29, 95% CI [− 0.46, 0.20]. Including emotional reactions did neither alter the nonsignificant effects of trait mindfulness (estimate = − 0.19, SE = 0.33, z-value = − 0.58, p = 0.56, 95% CI [− 0.90, 0.54]) nor the interaction between offer and trait mindfulness (estimate = − 0.06, SE = 0.12, z-value = − 0.48, p = 0.63, 95% CI [− 33, 0.18]). Adding emotional reactions as predictor significantly improves the model (χ 2 (2) = 82.502, p < 0.001). Thus, the findings show that trait mindfulness is significantly associated with emotional responses, and emotional responses are significantly associated with accept/reject decisions. These findings suggest that, while the direct effect between trait mindfulness and accept/reject decisions is non-significant, there may be an indirect effect between trait mindfulness and accept/reject decisions via emotional responses. However, the statistical model in r testing for this indirect effect could not converge and we therefore could not formally test whether the indirect effect is statistically significant.
In summary, the results of study 2 suggest that participants high in trait mindfulness may be a little less affected by the fairness of the offer in terms of their emotional response as compared to participants low in trait mindfulness. However, study 2 did not replicate the positive relation between mindfulness and the behavioral acceptance of offers, as found in study 1.

Pooled Analysis
As our results of study 1 did not replicate in study 2, it is yet inconclusive whether trait mindfulness affects behavior in the Ultimatum Game. To increase statistical power and draw firmer conclusions, we pooled the data of study 1 and study 2 and ran the analysis on the aggregated data. We assessed the effect of study (sum-to-zero; study1 = 1, study 2 = − 1), trait mindfulness, offer, and the interaction between offer and trait mindfulness as standardized predictors on the binary decision to accept/reject in the Ultimatum Game as dependent variable. Results showed a significant effect of study Hence, overall participants were more likely to accept offers in study 2 and participants were more likely to accept offers that were higher. So even though we found a small significant effect of trait mindfulness on behavior in the Ultimatum Game in study 1, this effect might not be robust when regarding a larger sample size.

Trait Mindfulness Facets
The Five Facets Mindfulness Questionnaire (Baer et al., 2006), as used in this study, posits that trait mindfulness is a multidimensional construct consisting of five facetsobserving, describing, acting with awareness, nonjudgement of thoughts and feelings, and non-reactivity to thoughts and feelings. Whereas the overarching construct is often used to examine the relationship between trait mindfulness and social decision making, assessing the separate facets may allow for greater insights into which specific mindfulness facets explain a possible relationship. Although we did not formulate a priori hypotheses on the role of the different mindfulness facets on the decision to accept/reject, we explored the role of each facet in the likelihood to accept or reject offers on the pooled data of studies 1 and 2.
Separate GLMER analyses were run with each of the mindfulness facets -non-reactivity, nonjudgement, acting with awareness, observing and describing -offer, and the interaction between offer and the mindfulness facet as standardized independent variables and accept/reject as binary dependent variable on the pooled data of study 1 and study 2. As such, study was also included as fixed factor. Analyses were Bonferroni-corrected for multiple testing.
Results are presented in Table 2. Across the two studies, there was a significant interaction effect between non-reactivity and offer on acceptance of offers, such that for those participants who scored high on non-reactivity, the decision to accept/reject was less strongly predicted by the height of offers. There were no main or interaction effects for the other mindfulness facets.
Furthermore, we explored the relation between each mindfulness facet and emotional reaction to offers in the UG in study 2. Separate linear mixed effects models were used to assess the effect of each mindfulness facet, offer and their interaction on emotional reaction to the offers. Again, the analyses were Bonferroni-corrected for multiple testing. Results only indicated a significant interaction effect of offers x acting with awareness on emotional reaction to UG offers (Table 3). Those participants who scored + 1 SD on act with awareness had a weakened relation between offer and emotion reaction to the offer as compared to participants who scores − 1 SD on act with awareness.
In short, explorations of the trait mindfulness facets in the pooled data showed significant interaction effects of the facet non-reactivity on acceptance of offers, and an interaction effect of offer with acting with awareness on emotional reactions to unfair offers in the Ultimatum Game.

Discussion
It has been widely suggested that mindfulness could foster more positive interpersonal interactions (e.g., Brown et al., 2007;Davidson, 2010). The present research sought to contribute to this young literature by examining the relation between trait mindfulness and reactions to unfairness, focusing particularly on the relation between trait mindfulness, acceptance of offers and emotional reactions to the fairness of offers in the Ultimatum Game. Two conceptual replication studies provided mixed results. Study 1 showed a positive relation between trait mindfulness and acceptance of offers, such that those participants higher in mindfulness were more likely to accept more unfair offers in the Ultimatum Game. Results of a pre-registered study 2, however, did not replicate the association between trait mindfulness and acceptance of offers. To increase confidence in the findings, we pooled the data of studies 1 and 2. The analysis on the aggregated data showed neither a main effect of trait mindfulness nor an interaction effect between trait mindfulness and the fairness of the offer on the decision to accept/reject in the Ultimatum Game.
Following previous theorizing on mindfulness and social decision making, we argued that mindfulness might facilitate acceptance by de-automatization, dis-identification, and behavior regulation (e.g., Berry & Brown, 2017;Leary & Diebels, 2017). By bringing present moment non-judgmental awareness to social exchange situations, one may notice initial emotional and behavioral tendencies without getting too caught up in them, which in turn may allow for the regulation of these tendencies and more behavioral flexibility (see for reviews e.g. Chiesa et al., 2011;Roemer et al., 2015). Interestingly, and consistent with this reasoning, study 2 showed that trait mindfulness attenuated emotional responses to offers in the Ultimatum Game: whereas there was a strong relation between fairness of offers and emotional valence among participants relatively low in trait mindfulness, this relation was less pronounced among participants high in trait mindfulness. Hence, the behavioral decisions of participants high in trait mindfulness were less likely to be guided by their initial emotional responses.
Nonetheless, analyses on our pooled data showed no effect of trait mindfulness on the decision to accept/reject in the Ultimatum Game. Based on these findings, it seems best to conclude that the association between trait mindfulness and responses to unfairness in the Ultimatum Game is not as robust as theorizing and previous empirical findings may suggest. It could be that individual differences in trait mindfulness might not be powerful enough to drive meaningful behavioral differences when encountering unfairness. It may take considerable mindfulness practice to be able to deal with such emotional social exchange settings in a mindful way (cf. Grossman & Van Dam, 2011). Previous research by Kirk et al., (2011Kirk et al., ( , 2016 did show that expert meditators and trained mindfulness meditators were more likely to accept unfair offers in the Ultimatum Game as compared to control participants. Indeed, the effects of trait mindfulness and mindfulness training do not always align, and the current findings raise broader questions about how trait mindfulness compares to the effects of mindfulness training (Grossman & Van Dam, 2011;Van Dam et al., 2018). Also, the dissociation between how trait mindfulness attenuated emotional reactions, but had no robust effect on behavior in the Ultimatum Game contributes to the debate about the meaning of trait mindfulness questionnaires. Whereas converging evidence has suggested that trait mindfulness relates to self-reported indices of social emotions and behavior (e.g., Beitel et al., 2005;Dekeyser et al., 2008), various studies showed no support for effects of trait mindfulness on real social behavior (e.g., Kaplan et al., 2018;Schindler & Friese, 2022). Future research should further assess the validity of trait mindfulness and whether trait mindfulness is associated with meaningful differences in social behavior in the general population. Additionally, exploration of the mindfulness facets provided some interesting results. Analyses on the pooled data revealed that non-reactivity was associated with a somewhat weaker association between the height of the offer and the decision to accept/reject, suggesting that people high in nonreactivity were less likely to reciprocate offers. These findings are in line with the theoretical reasoning that mindfulness may help not responding on initial impulses that often are led by reciprocity motives (e.g., Kelley & Thibaut, 1978;Knoch et al., 2006). We also found that acting with awareness was associated with less negative emotional responses to unfair offers. While interesting, an important goal for future research is to (1) further assess the robustness of the effects of non-reactivity and acting with awareness in responding to unfairness, and (2) experimentally disentangle the components awareness and non-reactance, and their respective role in responding to unfairness.
An interesting question concerning the relationship between mindfulness and acceptance of unfair offers is whether mindfulness is associated with "wise" responding (for a discussion, see Karremans et al., 2020). Accepting unequal splits and thus unfair offers may be conceptualized as cooperation as it maximizes profits for both parties involved (e.g., Kirk et al., 2016). Yet accepting such unequal or unfair splits may also elicit exploitation in repeated social exchange situations, or put differently, it might make someone a "social doormat" (cf. Luchies et al., 2010). Different motivations may drive the decision to accept or reject offers in the Ultimatum Game. While acceptance of unequal splits may be construed as the cooperative response in a singleshot interaction, punishment of unfairness may be regarded as the pro-social response within repeated social exchanges between members of a group. Indeed, while altruistic punishment may be costly to the individual, it may be used as a social signal that the proposer should make more fair offers in the future towards the benefit of the group (Fehr & Fischbacher, 2003). Hence, what does it mean if mindful individuals are more likely to accept unfairness? And what is wise responding in such situations? Such questions are relevant to debates about (the lack of Buddhist) ethics in mindfulness practice and research (e.g., Greenberg & Mitra, 2015;Monteiro et al., 2015). Relatedly, a study by Poulin et al. (2021) showed that mindfulness meditation decreased prosocial helping when participant had an individualistic motivation. Hence, the link between mindfulness and prosocial behavior might not be as straightforward as previously assumed and may depend on salient personal goals and values (Schindler & Friese, 2022).

Limitations and Future Research
A few limitations should be mentioned. First, as we used a trait mindfulness questionnaire to assess the relation between mindfulness and acceptance of unfairness, our results may be confounded with other personality characteristics and we cannot make conclusions about causality. To further understand the boundary conditions and causality of the effect, it would be necessary to assess the presumed benefit of trait and state mindfulness, and mindfulness training on various outcome measures using longitudinal designs (e.g., Van Dam et al., 2018).
Second, as we first measured behavior in the Ultimatum Game and asked responses to the trait mindfulness questionnaire afterwards, there is a chance of reverse causality. Yet, given that the mindfulness questionnaire was located in the middle of a battery of questionnaires in both studies, we deem it unlikely that participants might have deduced their degree of mindfulness from their game behavior, but we cannot rule out this possibility.
Third, the sample for study 1 was recruited via (meditation) community centers, whereas the sample for study 2 was mostly recruited via the recruitment platform Prolific. Hence, it is not surprising that the mean level of trait mindfulness is higher in study 1 as compared to study 2 (see supplementary materials). Nonetheless, it does not explain why the overall acceptance level was higher in study 2 as compared to study 1. A potential explanation may lie in the fact that in study 2 we asked participants to indicate their emotional reactions towards Ultimatum Game offers prior to measuring their decision to accept or reject. Possibly, measuring the presumed process (emotional reactions) may have interfered with the behavioral response (i.e., accepting or rejecting). Consistent with previous research, a delayed response could result in more prosocial responses as compared to an immediate response (Lieberman et al., 2007(Lieberman et al., , 2011. Lastly, we measured emotional reactions to offers in the UG with one visual analogue scale ranging from very negative to very positive in study 2. We assumed that the extreme ends of the scale would represent more intense emotions than scores close to the midpoint of the scale. This reasoning is in line with how attitude scales measure both the valence and intensity of the evaluation (e.g., Eagly & Chaiken, 1993) and how mood scales can measure the valence and intensity of moods (e.g., van Rijsbergen et al., 2012). Nonetheless, with such a unidimensional scale, we are unable to disentangle the intensity from the valence of emotions. Moreover, future research should further assess whether different specific emotions may be regulated differently by mindfulness.
Generally, research shows a positive relation between mindfulness and social behavior (e.g., Berry et al., 2020;Donald et al., 2018). The current research aimed to corroborate and extend previous (neuroimaging) studies on the role of trait mindfulness in Ultimatum Game behavior with a conceptual replication. The mixed results presented here indicate that the relation between mindfulness and social decision making -specifically unfairness -may not be as straightforward as previous theorizing and empirical findings have suggested. The present findings stress the need for more fine-grained examinations of when and why mindfulness would lead to acceptance of unfairness, and if and when mindfulness would lead to wise responding in social exchange situations.
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