A real-life experiment in engineering green behavior unfolded recently in the nation's capital.

Washington, D.C., imposed a five-cent tax on every disposable bag, paper or plastic, handed out at any retail outlet in the city that sells food, candy or liquor, effective Jan 1. But more important than the extra cost was something more subtle: No one got bags automatically anymore. Instead, shoppers had to ask for them—right in front of their fellow customers.

The...