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Consumers sometimes make choices that impose greater external costs on those who do not make the same choice. This paper examines how the selectivity of negative externalities in such situations affects the competitive equilibrium and the desirability of an externality-reducing public policy. Selective negative externalities create network externalities, but outcomes may differ greatly from typical network effects. Price effects may cause the imposing product's sales to decline with the size of the negative externality. Consequently, a positive competitive effect may overwhelm the externality's negative direct effects on welfare, such that a policy that enlarges the externality may improve welfare.