Diffusion of responsibility is a sociopsychological phenomenon whereby a person is less likely to take responsibility for action or inaction when other bystanders or witnesses are present. Considered a form of attribution, the individual assumes that others either are responsible for taking action or have already done so.

The diffusion of responsibility refers to the decreased responsibility of action each member of a group feels when they are part of a group. For example, in emergency situations, individuals feel less responsibility to respond or call for help if they know that there are others also watching the situation - if they know they are a part of the group of witnesses. In other group settings (in which a group is appointed to complete a task or reach a certain goal), the diffusion of responsibility manifests itself as the decreased responsibility each member feels to contribute and work hard towards accomplishing the task or goal. The diffusion of responsibility is present in almost all groups, but to varying degrees, and can be mitigated by reducing group size, defining clear expectations, and increasing accountability.

Assumption of responsibility tends to decrease when the potential helping group is larger, resulting in little aiding behavior demonstrated by the bystander(s). Causes range from psychological effects of anonymity to differences in sex. Implication of behaviours related to diffusion of responsibility can be threatening as there have been increases in moral disengagement and helping behaviour.