(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.