 The problem posed by a public good is that the optimal course of action for each individual, from the standpoint of her egoistic rationality, is for her not to contribute to the provision of the good, even though everyone would be better off if they all did so (see Olson 1965). Take any resident in the town I just described. From the standpoint of her own self-interest, she should not spray her lawn: If the other residents spray their lawns, she would get the benefit without paying the cost. And if the other residents do not spray their lawns, she would save herself the cost of spraying her lawn. It follows that as long as residents are moved only by their own self-interest, they will not produce the good of a mosquito free summer.