We set up an evolutionary game-theoretic model aimed at addressing the issue of local public good provision via direct commitment of voluntary forces (private donors and nonprofit providers) only. Two classes of agents are assumed to strategically interact within a ‘double critical mass’ model and we investigate the critical factors affecting the dynamic outcomes of such interaction. Further, we explore the conditions under which (what we term) ‘evolutionary crowding-out/in’ occurs, depending on agents’ degree of opportunism, social comparison and positive selective incentives (such as subsidies given by the government to ‘virtuous’ citizens or nonprofits only).