In his 1966 analysis of the Optimal Provision of Public Goods, Alan concluded that a setting in which local governments are left free to make their own independent decisions about the supply of goods with no compensation being paid for spillovers], the complex interactions that occur even in highly simplified situations make impossible to predict a priori whether undersupply or oversupply will generally (p. 19). This and other conclusions of his have evoked considerable comment. As yet, however, no one has questioned his assertion that it is not implied Ithat aggregate oversupply] will necessarily result in all cases lin which spillovers are uncompensated], but only that is a perfectly conceivable result which does not require any obviously unreasonable assumptions to be made (p. 31). It is with the empirical aspects of this assertion that we deal. More specifically, we address the question, How disparate must communities be in their price and income elasticities of demand for the Williams effect to be observed? That is, under what circumstances would uncompensated spillovers of the type of good considered result in greater output than would eventuate if compensation were paid? As Pauly (1970, p. 577), Brainard and Dolbear (1967), and, indeed, (1966, p. 29) indicate, the class of goods examined is public only in a rather special sense: XWhile these goods may satisfy the Samuelson et al. conditions for pure goods within individual communities, they are pure private goods among communities. That is, if a unit of such a good spills over from one community to a second, the first community completely loses the services of that unit. A stylized mosquito control program provides an example. Suppose that a fraction, cc, of the mosquitoes which breed in community 1 migrate to community 2 and, conversely, that the same fraction of the mosquitoes born in community 2 migrate to community 1. This being the case, if community 1 eradicates