When the cashier is gone, do you still pay?

You can find all the usual items at The Vault coffee shop in North Dakota. What you won't find is an employee.

The shop's owners think their store may be the world's only unmanned self-service coffee shop. It has a credit card reader and a money slot for checks and cash. People can brew themselves coffee or tea at the shop's single-cup brewing machine and grab a pastry.

Owners David and Kimberly Brekke installed cameras in the shop in Valley City, mainly to provide a safe atmosphere, they say. Perhaps the cameras are keeping people in line, or maybe customers simply have a whopping helping of honesty. Either way, the Brekkes say business is great.

"Generally speaking, the people of Valley City are more generous than dishonest," the owners write on their website. So far, the shop receives about 15 percent more than its asking prices.

Yes, there are still some businesses in the country that rely on the honor system. Self-service fruit stands dot the rural areas of California and encourage people to take bags of kiwis or oranges and leave the cash in a nearby locked box.

In one Phoenix suburb, a 24-hour farm stand sells $1 tomatoes and cucumbers and a dozen refrigerated eggs for $7. "People are taken aback," the head farmer at the Farm at Agritopia told The Arizona Republic. "They're excited to see our honor system. They're surprised that people trust other people."

A Massachusetts company that makes the Crane Crest brand of gourmet salad dressings asks customers to leave order requests on its answering machine. Employees take the orders and ship the jars of dressing along with a handwritten invoice and a self-addressed envelope. The company says that only three or four bills go unpaid every year.

So is a little trust all we need to shell out the proper amount for a cup of coffee? Researchers say that the currency of trust is a powerful one in human interactions. An economist and brain researcher at Claremont Graduate University told NPR that mutual trust evokes positive feelings that are tied to the brain's natural release of the peptide oxytocin. The same peptide is in play when a mom bonds with her newborn. It's a high of sorts that helps people connect.

Other experts think something beyond the need to connect is at work. The psychology department at Newcastle University in England found in 2006 that it was getting ripped off at its self-serve station that sold coffee for $1 and tea for 80 cents. So professors there naturally turned the station into an experiment. For one week, they plastered a poster of flowers over the station, The New York Times reports. The next week, they switched to a poster showing a pair of staring eyes.

During the days when the eyes stared them down, coffee drinkers gave 2.76 times as much money as the days when they were greeted by flowers. Just the suggestion that they were being watched prompted users to be more honest, the professors said.

So does the honor system work, as long as there is a pair of eyes nearby, perhaps? Panera Bread (PNRA) has run its own honor system for years, starting with a non-profit cafe in St. Louis that asked customers to pay what they wanted. The Panera Cares cafes still employ cashiers, but they simply hand customers a receipt showing what their food would have cost in a regular Panera. Customers are asked to deposit cash in donation boxes to help people in need.

"There's no pressure on anyone to leave anything," former CEO Ron Shaich told USA Today. "But if no one left anything, we wouldn't be open long."

Panera estimates that 60 percent of customers leave the suggested donation, around 20 percent leave more, and 20 percent leave less or nothing at all. The company says it brings in about 70 percent to 75 percent of what it would normally receive at one of its regular cafes. Panera has opened five of the non-profit cafes so far.

So why aren't there more self-serve businesses? Because it just takes one bad apple to spoil the bunch, as the saying goes. One social psychologist who has studied honesty for decades told NPR that people generally break into the following groups: 25 percent are consistently honest, 25 percent are mostly honest, 25 percent are dishonest and 25 percent are erratic.

He summed it up this way: "Even if most people do the right thing or add extra money to an honor till, eventually somebody's going to come along who tries to take all the cash."