Social discounting involves modulation of neural value signals by temporoparietal junction

Abstract

Most people are generous, but not toward everyone alike: generosity usually declines with social distance between individuals, a phenomenon called social discounting. Despite the pervasiveness of social discounting, social distance between actors has been surprisingly neglected in economic theory and neuroscientific research. We used functional magnetic resonance imaging (fMRI) to study the neural basis of this process to understand the neural underpinnings of social decision making. Participants chose between selfish and generous alternatives, yielding either a large reward for the participant alone, or smaller rewards for the participant and another individual at a particular social distance. We found that generous choices engaged the temporoparietal junction (TPJ). In particular, the TPJ activity was scaled to the social-distance-dependent conflict between selfish and generous motives during prosocial choice, consistent with ideas that the TPJ promotes generosity by facilitating overcoming egoism bias. Based on functional coupling data, we propose and provide evidence for a biologically plausible neural model according to which the TPJ supports social discounting by modulating basic neural value signals in the ventromedial prefrontal cortex to incorporate social-distance-dependent other-regarding preferences into an otherwise exclusively own-reward value representation.

Keywords: connectivity; fMRI; neuroeconomics; prosocial choice; social discounting.

Fig. 1.

Participants received task-relevant information sequentially. First, social distance information was given on a scale consisting of 101 icons (100 icons representing 100 social distance levels plus one icon, shown in purple on the left end, representing the participant himself). The social distance information for a specific trial was indicated by a yellow icon and, additionally, presented numerically as a number on top of the yellow icon (here: social distance 10). Participants chose between a selfish (here: €125 only for themselves) and a generous option (here: €75 for the participant and €75 for a recipient on the specific social distance). The generous and selfish options were then presented sequentially and in random order. All ISIs had a mean duration of 4 s (jittered by ±1 s). Participants indicated their preference during the decision period within a maximum time frame of 6 s. The trials were separated using a fixation cross with a mean ITI of 6 s (jittered by ±1 s). Note that this figure has been adjusted for illustration purposes; stimulus size and screen format are not to scale with the presentation dimensions used during fMRI scanning. In addition, the figure displays only 21 icons, instead of 101 icons shown during scanning, to facilitate perceptibility.

Fig. 2.

We determined, for social distance levels 1, 2, 3, 5, 10, 20, 50, and 100, the individual payoff magnitudes at which a participant was indifferent between the selfish (yielding a larger reward for the participant) and the generous alternative (yielding a smaller reward for the participant plus a reward for the other person). The amount foregone, i.e., the difference in own-reward magnitude between the selfish and generous option at indifference point, indicates the willingness to sacrifice a reward to give to another person at a specific social distance. The amount foregone can be interpreted as a social premium that reflects the utility a participant attaches to increasing a recipient’s payoff. A standard hyperbolic model was fit to the individual social-distance–dependent amounts foregone to reconstruct the participant’s ORU function. The figure shows the best-fitting hyperbolic function to the median amounts foregone across all participants.

Fig. 3.

Brain activations during social discounting. (A) BOLD responses in the VMPFC were stronger during generous than during selfish decisions [0, 47, −20; t₍₂₂₎ = 5.47, P = 0.028, whole-brain FWE corrected; displayed at P < 0.005, uncorrected, k ≥ 10 voxel]. (B) Generous decisions elicited activation in the posterior part of the rTPJ. (C) Beta estimates within the rTPJ . The rTPJ was more activated during generous than selfish decisions. (D) Activity in the rTPJ was more strongly modulated by the temptation to be selfish during generous than selfish decisions. Error bars indicate ±1 SE.

Fig. 4.

(A) ROI in the rTPJ (51, −49, 34; 10-mm sphere) as the seed region for the PPI. (B) Positive functional connectivity of the rTPJ with VMPFC during generous decisions. The PPI analysis revealed that connectivity between the VMPFC and the rTPJ was stronger during prosocial than selfish choices [t₍₁₄₎ = 6.61, P = 0.031, whole-brain FWE corrected; displayed at P < 0.005, uncorrected, k ≥ 10 voxel].