The computational and neural substrates of moral strategies in social decision-making.

Individuals employ different moral principles to guide their social decision-making, thus expressing a specific 'moral strategy'. Which computations characterize different moral strategies, and how might they be instantiated in the brain? Here, we tackle these questions in the context of decisions about reciprocity using a modified Trust Game. We show that different participants spontaneously and consistently employ different moral strategies. By mapping an integrative computational model of reciprocity decisions onto brain activity using inter-subject representational similarity analysis of fMRI data, we find markedly different neural substrates for the strategies of 'guilt aversion' and 'inequity aversion', even under conditions where the two strategies produce the same choices. We also identify a new strategy, 'moral opportunism', in which participants adaptively switch between guilt and inequity aversion, with a corresponding switch observed in their neural activation patterns. These findings provide a valuable view into understanding how different individuals may utilize different moral principles.