Panera Bread has been experimenting with "pay what you can" restaurants for about a year, at three of its 1500 US locations. So far, the model seems to be working.

Most patrons, it finds, drop the entire retail cost, or more, into the voluntary donation box, in essence subsidizing a meal for somewhat who can't pay the full amount. Panera says about 60 percent leave the suggested amount; 20 percent leave more; and 20 percent leave less. The largest single payment so far? One person paid $500 for a meal.

Few people seem to be taking unfair advantage of the system. Most know that wouldn't be fair. Not paying when you could "is like parking in a handicapped spot," Mr. Shaich says. "The lesson here is most people are fundamentally good."