Neural responses underlying extraordinary altruists' generosity for socially distant others

Abstract

Most people are much less generous toward strangers than close others, a bias termed social discounting. But people who engage in extraordinary real-world altruism, like altruistic kidney donors, show dramatically reduced social discounting. Why they do so is unclear. Some prior research suggests reduced social discounting requires effortfully overcoming selfishness via recruitment of the temporoparietal junction. Alternatively, reduced social discounting may reflect genuinely valuing strangers' welfare more due to how the subjective value of their outcomes is encoded in regions such as rostral anterior cingulate cortex (ACC) and amygdala. We tested both hypotheses in this pre-registered study. We also tested the hypothesis that a loving-kindness meditation (LKM) training intervention would cause typical adults' neural and behavioral patterns to resemble altruists. Altruists and matched controls (N = 77) completed a social discounting task during functional magnetic resonance imaging; 25 controls were randomized to complete LKM training. Neither behavioral nor imaging analyses supported the hypothesis that altruists' reduced social discounting reflects effortfully overcoming selfishness. Instead, group differences emerged in social value encoding regions, including rostral ACC and amygdala. Activation in these regions corresponded to the subjective valuation of others' welfare predicted by the social discounting model. LKM training did not result in more generous behavioral or neural patterns, but only greater perceived difficulty during social discounting. Our results indicate extraordinary altruists' generosity results from the way regions involved in social decision-making encode the subjective value of others' welfare. Interventions aimed at promoting generosity may thus succeed to the degree they can increase the subjective valuation of others' welfare.

Keywords: altruism; neuroeconomics; prosocial choice; social discounting; social valuation.

Fig. 1.

Social discounting paradigm. Note. Prior to scanning, participants were provided with instructions that they will make 189 decisions across three independent runs involving six people from a participant's social network of varying social distances (N = 1, 2, 5, 10, 20, 50) plus one stranger (N = 100). The task consisted of 21 blocks with nine trials each, in which they made 9 dichotomous choices about keeping and/or sharing amounts of money with each. For each trial, participants were asked to indicate if they would prefer to keep an amount of money for themselves alone (selfish option) or to keep an amount of money for themselves and also share an amount of money with the Nth person on their list (generous option). The selfish options ranged from keeping $165 to $65 in the order of decreasing $10 increments, and the generous option always kept a set amount of money (either $85, $75, or $65) and sharing that same amount with the Nth person. Each trial lasted 5 s and the interval between trials was randomly jittered with an average of 4 s (jittered ±1 s). The ordering of the blocks was pseudo-randomized. Stimuli consist of text only in which participants chose between two options: “$XX for YOU” or “$XX for YOU and $XX for N=##”, where ## indicated the number of the social distance. Participants were informed that their decisions would result in real outcomes. One of each participant's choices was randomly chosen and implemented such that the participant was paid 10% of the amount they selected to receive themselves, and, if a generous choice was selected, the other person in question also was contacted and disbursed 10% of the amount the participant selected for them.

Fig. 2.

Social discounting in altruists, meditation controls, and wait-list controls. Note. Group labels are listed in descending order of mean discounting slope across groups (logk). Ribbons reflect the 95% CIs around each group mean.

Fig. 3.

Linear mixed-effects model results for group differences in decision difficulty as a function of social distance. Note. Group labels are listed in ascending order of mean slope across groups. Ribbons reflect the 95% CIs around each group mean. For ease of comparison between social discounting in Fig. 2 and difficulty ratings, we reversed-coded the difficulty ratings (i.e. subtracted each score from 7) such that a higher score indicated greater ease rather than greater difficulty for this plot.

Fig. 4.

Neural activation for generous versus selfish decisions across the full sample. Note. Colormap corresponds to the difference in neural activation (FDR-corrected q < 0.05; height threshold t > 6.5 for visualization purposes) between generous and selfish choices across the full sample. Surface corresponds to the following key: left lateral = top left; right lateral = top right; left medial = bottom left; right medial = bottom right.

Fig. 5.

Regions in which altruists exhibited a different relationship between neural activation and social valuation in contrast to all controls. Note. Colormap corresponds to the difference in subjective social value-related activity (FDR-corrected q < 0.05) between altruists and controls. Surface corresponds to the following key: left lateral = top left; right lateral = top right; left medial = bottom left; right medial = bottom right.

Fig. 6.

Amygdala voxels in which altruists exhibited a different relationship between neural activation and social valuation in contrast to all controls. Note. A) Colormap corresponds to the difference in subjective social value-related activity between altruists and controls in a pre-registered a priori region of interest (BA; FDR-corrected q < 0.05). A smaller cluster of voxels in this region also survived multiple-comparison correction across the whole brain (FDR-corrected q < 0.05). B) Plots of mean activation magnitude in these regions as a function of subjective social valuation, group, and social distance. The average plot corresponds to the signal change by value aggregated across all social distances.