Participants and experimental procedure

The participants (n=50) were randomly assigned to either the experimental or the control group. In the first part of the experiment (Fig. 1a), we informed the participants that we would send each of them 25 Swiss francs in each of the following 4 weeks. We asked the participants in the experimental group to commit to spending the money on other people of their own choice. Examples included taking others out to dinner or buying gifts for others. We asked the participants in the control group to commit to spending the money on themselves. Examples included taking oneself out to dinner or buying oneself a gift.

Figure 1: Experimental design. (a) First, we informed the participants that we would send them money for the next 4 weeks (25 Swiss francs per week). We asked half of the participants to commit to spending this money on other people (experimental group) and half of the participants to commit to spending the money on themselves (control group). In addition, we assessed the participants’ subjective happiness upon their arrival at the laboratory (T1), that is, before they had made the commitment, and after scanning, that is, at the end of the experiment (T2). (b) After the participants had made the commitment, they were asked to select one person to whom they wanted to give a present. Then they performed an independent decision-making task in the MRI scanner. In each trial, the participants were presented with an option that they could accept or reject. Each option was a combination of the benefits for the other person and the participants’ own costs. The magnitude of the benefits and costs varied independently and pseudorandomly from 3 to 25 Swiss francs. Each option was displayed for 4 s, which was the maximum response time. The selected response and the option were displayed together until the jittered ITI began. The participants responded by pressing the left or right arrow button, which corresponded to the ‘yes’ (accept) or ‘no’ (reject) displayed on the screen. The mapping between ‘yes’ or ‘no’ and left or right arrow buttons was randomized across trials. Full size image

After the participants had made a commitment, the second part of the experiment took place; the participants completed an independent decision-making task while we measured their blood-oxygen-level-dependent (BOLD) responses using fMRI (Fig. 1b). In the decision-making task, the participants first selected a person to whom they wanted to give a present (in the experimental group, the selected person was different from those the participant would spend money on in the following weeks). In each trial, the participants were presented with an option that they could accept or reject. Each option was a combination of monetary benefits for the other person and monetary costs to the participant. The magnitude of the benefits and the costs varied independently and pseudorandomly from 3 to 25 Swiss francs. All choice options included some costs, so that acceptance required at least some personal sacrifice on the part of the participant. Hence, we defined ‘generous behaviour’ in this task as the proportion of trials in which a participant accepted a personal cost to benefit someone else. In addition, we assessed individual happiness (using the subjective happiness scale, SHS16) as soon as the participant arrived at the laboratory (Time Point 1) and again after the participant had completed the decision-making task in the scanner (Time Point 2). This allowed us to determine the change in each participant’s self-reported level of happiness as a function of time (Time Point 2 versus Time Point 1) and group assignment (experimental versus control group). Furthermore, to rule out the possibility that familiarity with or affinity for the recipient named in the decision-making task had an impact on our measurement of generous behaviour, we asked the participants to rate how familiar they were with the recipient named in the decision-making task and how much they liked him or her. Two participants were excluded from further analyses because they had accepted all choice options (and rejected none) in the decision-making task, making a subsequent analysis of accept versus reject trials impossible. The remaining sample size was 48 (with 24 participants in each group).

Behavioural effects of benefits and costs

As a manipulation check, we first tested whether generosity increased as a function of the amount of monetary benefits for the other person and decreased as a function of the costs to the participant. As expected, across both groups, participants were more likely to behave generously as the amount of benefits for the other person increased and the costs to the participant decreased (Fig. 2a; average regression coefficient for other’s benefit; one-sample t-test, n=48, b=0.25, t(43)=17.2, P<0.001; own cost b=−0.22, t(43)=17.7, P<0.001). This effect was observable in each group separately and the coefficient for costs did not differ from the coefficient for benefits (experimental group; one-sample t-test, n=24, t(23)=1.72; P=0.1; control group; one-sample t-test, n=24, t(23)=0.42; P=0.68). The similarity of weight given to one’s own costs and to the other’s benefits could be due to the fact that the two were related and that the others were close to the participants, which may have reduced the amount of loss aversion experienced by the participants during the task.

Figure 2: Behavioural data. (a) Mean probability of acceptance across all participants. The probability of acceptance increased as a function of the magnitude of benefits for the other and decreased as a function of the magnitude of costs to oneself. The benefits for the other and costs to oneself varied independently and pseudorandomly from 3 to 25 Swiss francs. (b) The participants in the experimental group, who had committed to spending the money on others, showed significantly more generous behaviour than the control group, who had committed to spending money on themselves. Generous behaviour was defined as the probability of accepting the option presented (t(46)=2.02; P<0.05). Error bars are s.e.m. (c) The participants in the experimental group showed a greater increase in happiness (Happiness(T2)-Happiness(T1)) than the control group did (t(46)=1.87; P<0.05). Error bars are s.e.m. Full size image

A group comparison of the model parameters (weights) showed a significant main effect (ANOVA, n=48, F(3,138)=198.58, P<0.05) and a significant interaction (model parameters × group: F(3,138)=2.9, P<0.05). Post hoc t-tests revealed that neither the coefficient of cost (t-test, n=48, t(46)=0.72, P=0.477) nor of benefit (t(46)=−0.18, P=0.86) significantly differed, whereas the interaction coefficient (t-test, n=48, t(46)=−2.08, P<0.05) and the constant (t-test, n=48, t(46)=2.04, P<0.05) showed significant differences between the groups (Supplementary Table 1 for group mean±s.e.m.). The interaction coefficient was more negative in the experimental group than in the control group, indicating that experimental group participants were more likely than control group participants to accept offers in which costs and benefits were dissimilar, such as when recipients received a small benefit at a high cost.

Control for familiarity and affinity

The two groups did not significantly differ with respect to their ratings of familiarity with and liking of the recipients (Kolmogorov–Smirnov test, n=48, familiarity: P=0.99; liking: P=0.65). Also, the familiarity and liking ratings did not significantly predict the individual differences in generosity (Kolmogorov–Smirnov test, n=24 for each group; experimental group: familiarity r=0.04, P=0.85; liking: r=0.16, P=0.46; control group: familiarity r=−0.12, P=0.58; liking: r=−0.02, P=0.93; all participants: familiarity r=−0.1, P=0.51; liking: r=−0.01, P=0.93). Therefore, we can rule out the possibility that the changes in generous behaviour described below were driven by group differences with respect to the familiarity with or liking of the recipient.

Commitment to be generous increases generosity and happiness

Next, we assessed commitment-induced group differences in generosity and happiness. On average, the experimental group was more likely to make generous choices than the control group, as indicated by a significantly higher acceptance rate (that is, individual differences in generous behaviour; Fig. 2b; t-test, n=48, t(46)=2.02, P<0.05). Furthermore, participants in the experimental group reported a greater increase in happiness than did those in the control group (Fig. 2c; t-test, n=48, t(46)=1.87, P<0.05). However, generous behaviour and changes in happiness were not significantly correlated (Pearson correlation over all participants, n=48, r=0.2, P=0.16; experimental group: n=24, r=0.13, P=0.55; control group: n=24, r=0.16, P=0.46). Although it is difficult to compare results due to the differing study designs, this result is in line with that of a previous experimental study, namely, that participants reported being happier after behaving generously independent of the degree of generous behaviour displayed4. Thus, our behavioural results converge with those of the previous study in two aspects: (1) the increase in generous behaviour was concomitant to an increase in happiness and (2) the magnitude of the increase in happiness was independent of the increase in generosity4.

We performed numerous tests to rule out potential alternative explanations for the increases in generosity and happiness we observed. First, we considered the peak-end effect17, according to which outcomes towards the end of the experiment more strongly influence subjective experience than earlier outcomes. We therefore tested whether one of the groups received significantly better offers in the final trials and whether the final offers were related to increases in happiness. We did not find any significant group differences with respect to the options offered in the final trial (t-test, n=48, t=1.2, P=0.24), the mean over the final five trials (t-test, n=48, t=0.82, P=0.42), the mean over the final 10 trials (t-test, n=48, t=0.33, P=0.74), or the mean over the final 20 trials (t-test, n=48, t=0.59, P=0.56). Furthermore, none of these values predicted the increase in happiness (Pearson correlation; n=48, final trial: r=0.08, P=0.57; mean over the final five trials: Pearson correlation; n=48, r=0.11, P=0.46; mean over the final 10 trials: Pearson correlation; n=48, r=−0.01, P=0.96; mean over the final 20 trials: Pearson correlation; n=48, r=−0.03, P=0.82). Thus, it is unlikely that the peak-end effect played a major role in explaining the observed group differences in happiness.

We also assessed whether other variables could explain the behavioural effects and found that the two groups did not differ in trait empathy18 (t-test, n=48, all P’s>0.3, see Supplementary Table 2 for details) or in prosociality19 (t-test, n=48, t(46)=0.49, P=0.63). Moreover, we could not find a link between these variables and generous behaviour (Pearson correlation; n=48, all P’s>0.2; see Supplementary Table 3 for details). We also found no relationship between increase in happiness and inequality aversion as captured by Fehr-Schmidt model20 model (tested by two-sample t-test; n=48; no group differences in distaste for disadvantageous inequity (t=0.04, P=0.97) or in distaste for advantageous inequity (t=0.88, P=0.38)). Furthermore, we also found no link between inequality aversion and increase in happiness (tested by Pearson correlation; n=48, across groups: advantageous r=−0.06, P=0.67 disadvantageous r=−0.14, P=0.33; experimental group: advantageous r=−0.3, P=0.15 disadvantageous r=−0.09, P=0.67; control group: advantageous r=−0.16, P=0.47 disadvantageous r=−0.14, P=0.51). Thus, groups were well matched in terms of trait empathy, prosociality and inequality aversion, and these factors did not account for the effects of commitment on happiness and generosity.

Commitment to be generous increases TPJ responses

First, we aimed to identify the general effects of the experimental manipulation (that is, commitment to generosity versus control commitment) on decision-related functional neural activity. We modelled BOLD activity at the time of decision separately for decisions to accept and to reject. We expected that a commitment to be generous would increase the recruitment of brain regions engaged in generous behaviour during such choices (that is, accept versus reject decisions). Indeed, a two-sample t-test revealed greater left TPJ activity in the experimental group than in the control group during generous choices (Fig. 3a,b, (−51, −70, 34), n=48, t(46)=4.70, P<0.05, whole brain family-wise error (FWE) corrected). The left TPJ was the only region that showed significantly greater activity in the experimental group than in the control group at this threshold (see Supplementary Fig. 1 for a bar graph depicting the left TPJ activation separately for accept and reject trials for each group; Supplementary Table 4 for whole-brain results; Supplementary Table 5 for correlation with behavioural generosity across groups), and there were no brain regions that showed greater activation in the control than in the experimental group.

Figure 3: Commitment to be generous enhanced TPJ activity during decisions to be generous. (a) Compared to the control group participants, the experimental group participants showed significantly greater TPJ activation ((−51, −70, 34), t(46)=4.70) while accepting versus rejecting a personal cost to benefit another person. (b) Parameter estimates of the accept versus reject contrast, extracted from the TPJ region that showed significant group differences. Error bars are s.e.m. Full size image

Although only left TPJ activation survived the FWE-corrected threshold, our data do not support an argument for lateralization as a direct statistical comparison of left versus right hemisphere, with 3-mm or 5-mm spheres around the left peak coordinates, did not reveal a significant difference between left and right hemispheres (3-mm: t(47)=0.69, P=0.49; 5-mm: t(47)=0.98, P=0.33; paired t-tests; please note that testing at peak coordinates maximizes the chance for finding a difference; in line with this finding, an exploratory analysis at a more lenient threshold also revealed activity in right TPJ: Supplementary Fig. 2).

TPJ connectivity reflects commitment-dependent generosity

Next, we examined the effects of commitment-induced increases in generosity-related TPJ activity on downstream brain regions. To this end, we performed a psychophysiological interaction (PPI) analysis (Methods section) using the TPJ as a seed region21,22. We hypothesized that activity in the ventral striatum and the OFC would be modulated by TPJ activity (accept versus reject) as a function of individual differences in generous behaviour and the type of commitment (self versus other). To test this hypothesis, we computed the group × acceptance rate interaction for TPJ connectivity during accept versus reject decisions. In line with our prediction, we found significant group differences in how connectivity between the TPJ and the ventral striatum was modulated by generous behaviour (Fig. 4a right: (12, −1, −2), t-test, n=48, t(44)=5.81; left: (−15, 11, −5), t(44)=5.07, P<0.05, small-volume family-wise error (SV—FWE) corrected; Supplementary Table 6 for whole-brain results; Supplementary Table 7 for analysis across groups). The TPJ-striatal connectivity was not modulated by the generosity commitment per se (t(47)=.311, P=0.76). Importantly however, the experimental and the control group showed different patterns of correlation between TPJ-striatal connectivity and generous behaviour (Fig. 4b). Specifically, greater TPJ-ventral striatum connectivity during accept versus reject trials was only associated with a greater acceptance rate in the experimental group, whereas this association was reversed in the control group. Thus, stronger TPJ-ventral striatum connectivity modulation facilitated generosity in the experimental group.

Figure 4: Striatum is modulated by TPJ as a function of generosity and tracks increase in happiness. (a) Striatal regions showing group differences in how generosity modulated connectivity with the TPJ. Inset illustrates TPJ seed region that was identified as the region showing significant group differences in accept versus reject trials (Fig. 3a). The psychological variable of the psychophysiological connectivity analysis was the contrast between accept versus reject trials. A group difference in TPJ-striatal connectivity was observable as a function of acceptance behaviour (right: (12, −1, −2), t(44)=5.81; left: (−15, 11, −5), t(44)=5.07; P<0.05, SV–FWE corrected). (b) We found a positive correlation between TPJ-striatal connectivity and generous behaviour in the experimental group; this correlation was negative in the control group. Thus, across participants, TPJ-striatum connectivity increased with generosity (defined as acceptance rate) in the experimental group, but decreased in the control group. Rank-based correlation analyses, which are robust against outliers, confirmed our results: Kendall’s tie-adjusted tau-b: 0.3, P=0.042, Spearman’s rho: 0.43; P=0.036. (c) Striatal region tracking group-dependent differences in coding increases in happiness during accept versus reject decisions ((−21, 2, −5) t(44)=4.34; P<0.05 FWE-corrected). (d) In the experimental group, we found a negative correlation between increase in happiness and striatal activity: The smaller the differences in striatal activity in accept as compared to reject trials, the greater the increase in happiness. In the control group, this correlation was positive. The ventral striatum activity in the two groups did not significantly differ (t(47)=1.68; P=0.1). Thus, the two groups differed with respect to how striatal activity predicted an increase in happiness, but not with respect to ventral striatum activity per se. (e) Conjunction analysis confirms that the same striatal region (1) tracks the increase in happiness and (2) is also modulated by TPJ connectivity as a function of generous behaviour. Full size image

Given that the TPJ has been shown to modulate value signals in OFC during prosocial decisions8, we hypothesized that commitment-induced generosity would affect also interactions between the TPJ and value-coding OFC regions during generous decisions in our task. In line with this hypothesis, the PPI analysis showed that TPJ-OFC connectivity depended on both generosity and group (Fig. 5a,b; (18, 38, −17), t-test, n=48, t(44)=5.60, P<0.05, SV—FWE corrected; see Supplementary Table 6 for whole-brain results). Furthermore, we tested whether this OFC region also coded the subjective value of the decision option by integrating other’s benefit and own cost and found that it did (Fig. 5, and see section below entitled ‘TPJ modulates OFC region coding value of the choice option’).

Figure 5: OFC codes subjective value of options and is modulated by TPJ. (a) TPJ-OFC connectivity reflecting group-dependent generosity (TPJ seed definition and PPI analysis as described in Fig. 4a). We observed a group difference in TPJ connectivity in a medial OFC region ((18, 38, −17), t(44)=5.60, P<0.05, SV–FWE corrected). Importantly, across participants, this connectivity was modulated by individual generosity (defined by acceptance rate) in a group-dependent manner. (b) In the experimental group, the participants who showed greater TPJ-OFC connectivity during the acceptance of an offer also showed more generous behaviour on average. In the control group, the opposite pattern was observed. Outliers-resistant rank-based correlation analyses confirmed the findings in the experimental group: Kendall’s tie-adjusted tau-b: 0.32, P=0.029, Spearman’s rho: 0.47; P=0.02. (c) We performed an additional analysis with trial-wise changes in subjective value as a parametric regressor. We identified an OFC cluster in which activity reflects the subjective value of the presented option in each trial ((0, 62, −11) t(45)=4.63; P<0.05, SV–FWE corrected). (d) Conjunction analysis confirms that the OFC region tracking subjective value is the very same region being modulated by the TPJ during acceptance decisions (conjuntion of a,c). Full size image

Ventral striatum activations predict changes in happiness

The above analyses show that the functional interaction between the TPJ and the ventral striatum is positively associated with generosity in the experimental group. We further predicted that the ventral striatum would play a key role in linking generous behaviour to happiness. We reasoned that activity in the striatum might link commitment-induced generosity and happiness by showing both (1) generosity-related and (2) happiness-related modulation, even though these two concepts have no significant shared variance. We therefore tested whether activity in the ventral striatum correlated with group-dependent changes in happiness. In other words, we tested the group × change-in-happiness interaction during accept versus reject decisions. We found that activity in the ventral striatum was significantly related to changes in happiness in a group-dependent manner (Fig. 4c, (−21, 2, −5) t-test, n=48, t(44)=4.34, P<0.05, SV—FWE corrected; Supplementary Table 8 for whole-brain results). Specifically, higher striatal activity during accept versus reject decisions was associated with greater increases in happiness in the control group (Fig. 4d). Conversely, in the experimental group, the participants with lower striatal activity reported greater increases in happiness. As a final step, we performed a conjunction analysis that confirmed the convergence of (1) group-specific generosity-predicting TPJ-ventral striatum connectivity and (2) group-specific coding of individual increases in happiness in the very same striatal region (Fig. 4e).

TPJ modulates OFC region coding value of the choice option

The preceding analysis revealed no association between OFC activity and happiness (t-test, n=48, t(47)=−.825, P=0.414), suggesting that the OFC is not involved in linking generosity and happiness. However, it is conceivable that the OFC links generosity and subjective value. Therefore, we investigated the role of the OFC region that communicated with the TPJ in a group-specific manner. Since several studies have found that the OFC plays a role in coding the subjective value of choice options and social rewards, we conducted a parametric modulation analysis to test whether OFC activity codes the net subjective value of the presented choice option in each trial. We found that this was indeed the case: Over all participants, the OFC reflected the subjective value of the trial option (Fig. 5c; (0, 62, −11); t-test, n=48, t(45)=4.63, P<0.05, small-volume (SV)–family-wise error (FWE) corrected; Supplementary Table 9 for whole-brain results). Moreover, a conjunction analysis confirmed that the very same OFC region was also modulated by the TPJ in the connectivity analysis (Fig. 5d). In other words, the OFC region that coded the net subjective value of the option was functionally coupled with the TPJ in a manner that depended on commitment type and predicted generosity. Taken together, these results suggest that the generosity commitment modulated activity in the TPJ, which changed its connectivity with OFC coding the subjective value of the choices.