In the psychology department at Newcastle University, there is a coffee station where people can help themselves, so long as they leave money in the tray — 50 pence (about $1) for a coffee and 30 for tea. It operates on an honor system.

Alas, not everyone is honorable. “The woman running the station was a little disappointed at the level of contributions,” says Gilbert Roberts, a professor in the department. Psychologists have long been aware of this dismal aspect of human behavior: people are more honest if they know they’re being observed — so when nobody’s watching, they feel they can get away with murder, or at least with a free cup of coffee.

This problem gave Roberts and two colleagues an idea for an experiment. For 10 weeks this spring, they alternately taped two posters over the coffee station. During one week, it was a picture of flowers; during the other, it was a pair of staring eyes. Then they sat back to watch what would happen.

A remarkable pattern emerged. During the weeks when the eyes poster stared down at the coffee station, coffee and tea drinkers contributed 2.76 times as much money as in the weeks when flowers graced the wall. Apparently, the mere feeling of being watched — even by eyes that were patently not real — was enough to encourage people to behave honestly. Roberts says he was stunned: “We kind of thought there might be a subtle effect. We weren’t expecting such a large impact.”