Altruism costs-the cheap signal from amygdala

Abstract

When people state their willingness to pay for something, the amount usually differs from the behavior in a real purchase situation. The discrepancy between a hypothetical answer and the real act is called hypothetical bias. We investigated neural processes of hypothetical bias regarding monetary donations to public goods using fMRI with the hypothesis that amygdala codes for real costs. Real decisions activated amygdala more than hypothetical decisions. This was observed for both accepted and rejected proposals. The more the subjects accepted real donation proposals the greater was the activity in rostral anterior cingulate cortex-a region known to control amygdala but also neural processing of the cost-benefit difference. The presentation of a charitable donation goal evoked an insula activity that predicted the later decision to donate. In conclusion, we have identified the neural mechanisms underlying real donation behavior, compatible with theories on hypothetical bias. Our findings imply that the emotional system has an important role in real decision making as it signals what kind of immediate cost and reward an outcome is associated with.

Keywords: amygdala; decision making; fMRI; hypothetical bias.

Fig. 1

Experimental set-up. Participants were randomized to either the real or the hypothetical donation group. The task was either to reject or accept the proposals for a donation toward a charitable cause. The real group was informed that their decision could have a cost (if they chose to accept a proposal) while the hypothetical group was informed that they should answer according to how they would do if the paradigm was about real money. They were explicitly told that neither of their choices would cost them any real money. Control proposals were presented with a picture from one of the donation categories together with a text stating that: ‘this is not a proposal’. The onset-time of events used in the fMRI analysis were set to the onset of presentation of the picture, proposal and choice.

Fig. 2

Hypothetical bias ± 2 s.e. The hypothetical bias is positive and significant for females ( = 0.19, P = 0.009), negative and threshold significant for males ( = −0.14, P = 0.070), and the difference in bias between the genders is significant ( = 0.33, P = 0.003).

Fig. 3

fMRI data showing activations related to the proposals, stake level, rejection and acceptance. (A) Proposals vs non-proposals in the real group, compared with the hypothetical group, yielded a higher activation in amygdala ([−28 0 −24] Z = 2.98, P = 0.042 voxel-level corrected). (B) Low stakes vs high stakes in the real group, compared with the hypothetical group, resulted in greater amygdala activation ([34 4 −20] Z = 2.79, P = 0.070 voxel-level corrected) and (C) dlPFC activation ([−26 42 20] Z = 3.45, P = 0.010 cluster-level corrected). (D) In the real group, compared with the hypothetical group, both rejected proposals, and (E) accepted proposals generated higher amygdala activity (rejected: [−26 −2 −24] Z = 3.32, P = 0.016 voxel-level corrected, accepted: left amygdala: [−20 −6 −18] Z = 2.91, P = 0.056 voxel-level corrected; right amygdala: [22 −4 −16] Z = 2.88, P = 0.060 voxel-level corrected). (F) In addition, accepted proposals in the real group resulted in greater activations in the ventral caudate ([−10 22 −6] Z = 4.03, P = 0.084 voxel-level corrected), and left ACC ([−10 22 30] Z = 4.59, 0.014 cluster-level corrected). (G) Subjects in the real group who accepted most donation proposals activated rACC ([ − 4 50 2] Z = 3.66, P = 0.037 cluster-level corrected) the strongest. (H) Insula activity ([−42 18 −14] Z = 3.94, P = 0.011 cluster-level corrected) was higher in the real group, compared with the hypothetical group, when participants viewed pictures that they later accepted.