Donders Institute for Brain, Cognition and Behaviour
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Thesis defense Jeroen van Baar (Donders series 351)

11 January 2019

Promotor: prof. dr. A. Sanfey

Models of Morality in Behavior and Brain

Despite the importance of social decisions in everyday life (e.g. trusting someone, donating money), we know surprisingly little about how people make these choices. In my PhD research, I mainly studied one type of social decision: the choice to honor a stranger’s trust. Across four studies, I discovered that while many people show similarly trustworthy behavior in an economic game, the motivations underlying this behavior are vastly different between people. About 40% of my participants cared most about being fair (economists call this ‘inequity aversion’), 10% was preoccupied with not disappointing their game partner (‘guilt aversion’), and a third group (10%) was only interested in making as much money as possible (‘greed’). A fourth group (40%) behaved in a way never before observed in the lab: they opportunistically switched between inequity aversion and guilt aversion, always choosing the strategy that was cheapest at any given time. Such ‘moral opportunism’ contradicts existing theories from behavioral economics, but can explain behavior in politics and other aspects of everyday life. Functional magnetic resonance imaging (fMRI) showed that guilt aversion and inequity aversion are associated with distinct patterns of brain activity, even if they produce the same decision outcome. As predicted, moral opportunists switched between the patterns of brain activity associated with inequity and guilt aversion. Based on these neural recordings alone, I could predict the moral opportunists’ trustworthiness decisions with over 90% accuracy. In the last chapter of my PhD thesis, I studied the diffusion of responsibility in a group of decision-makers, showing that people feel more personally responsible for decisions made in a group where people disagree with each other. I hope these findings will clarify social choice in daily life.