Bystander effect emerges from individual psychological prospects

The bystander effect is a social psychological phenomenon in which individuals are less likely to help a person potentially in need if there are others present. Sociologists and psychologists have proposed multiple plausible reasons for the bystander effect, from situational ambiguity and social contagion to diffusion of responsibility and mutual denial. We build a new model of an individual's decision to intervene in a bystander situation based on these social psychological hypotheses, along with ideas borrowed from prospect theory. The model yields an explicit bystander curve, which demonstrates, for the first time, that the bystander effect emerges from social risk perception among non-coordinating individuals in ambiguous bystander situations. Expanding upon this static model, we explore the effect of social learning, where individuals update their perceived risk of intervening after experiencing or witnessing the social repercussions of previous interventions. A novel result of this piecewise-smooth dynamical system model, with threshold-driven switching behavior, is that social learning exacerbates the bystander effect. We validate both the static and dynamic models using a new database of 42 experimental and observational studies across a wide range of bystander situations, demonstrating the importance of both population heterogeneity and social learning rates on the emergence of observed bystander curves. This provides a straightforward and generalizable explanation for the observed phenomenon, which may suggest effective interventions tailored to specific bystander situations.