Let a large-number of small-individuals contribute to charity. We show that ‘strong aggregate complementarity’ is necessary for multiple equilibria in a competitive equilibrium. Consider two equilibria with low (L) and high (H) levels of giving. Suppose that society is stuck at L and wishes to move to H using welfare-maximizing-public-policy. Subsidies are ineffective when comparative statics at L are ‘perverse’ (subsidies reduce giving). Public policy in the form of temporary direct government grants to charity can engineer a move from L to H. We contribute to the broader question of using public policy to engineer moves between multiple equilibria.