Americans are determined that massacres such as happened in Newtown, Conn., never happen again. But how? Many advocate more effective treatment of mentally-ill people or armed protection in so-called gun-free zones. Many others demand stricter control of firearms.

We aren't alone in facing this problem. Great Britain and Australia, for example, suffered mass shootings in the 1980s and 1990s. Both countries had very stringent gun laws when they occurred. Nevertheless, both decided...