Abstract: Security is often a non-excludable public good. On the one hand, it benefits the people who buy it; on the other, it also benefits those who live near the people who buy it. It benefits those neighbors even if they refuse to share in the cost of the security themselves. Security also entails economies of scale. In part because of the positive externalities involved, people economize when they purchase security together. Rather than each pay to protect him- or herself, they save resources if they purchase their security together.