Starting small toward voluntary formation of efficient large groups in public goods provision.

Highlights: [•] We test a mechanism where groups can grow and shrink via voluntary voting. [•] The mechanism successfully induces efficient outcome in group size and contribution. [•] This is driven by economies of scale combined with concern about being left out. [•] ‘Redemption’ is not rare; many people become good citizens after initially not. [Copyright &y& Elsevier]