Abstract: Prosocial behavior such as volunteering and cooperation often occurs under uncertainty about others' actions. This research examines how ambiguity shapes prosocial behavior across different strategic environments of social dilemmas.

The first study presents results from a completed laboratory experiment on the Volunteer's Dilemma, introducing Ellsberg-style outcome ambiguity and a language manipulation (native vs. foreign language). The findings show that ambiguity and foreign language use do not directly change average volunteering rates but substantially reshape decision-making. Subjective beliefs about others' willingness to volunteer play a central role, and ambiguity-averse individuals may volunteer to eliminate strategic uncertainty. Foreign language use further attenuates the relationship between risk aversion and volunteering.

The second study extends the analysis to the Prisoner's Dilemma, the Minimum Effort game, and the Volunteer's Dilemma using a within-subject, repeated-interaction design. Preliminary results suggest that the impact of ambiguity is highly context-dependent across strategic structures. Ongoing analysis from the second study will be presented and discussed during the seminar.

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