How did a doctor get toilet graffiti artists to clean up their act?

In 1992 a professor named T Steuart Watson discovered a completely effective way to prevent people writing on public toilet walls.

Watson published a report in the Journal of Applied Behavior Analysis, describing both his method and the relentless manner in which he tested it.

Watson, then at Mississippi State University, is now a professor at Miami University of Ohio. He carried out the experiment in three men's toilets. Each chamber had a history writ large, and small, in many different hands. The study says that "during the preceding months, each of the walls had been repainted numerous times due to the proliferation of graffiti".

Each day, Watson and his minions meticulously counted how many marks were on each wall. They tallied each letter, number, or piece of punctuation. Other shapes called for special assessment. The study describes one typically difficult example: "A drawing of a happy face was counted as five marks (one for each eye, one for the nose, one for the mouth, and one for the circle depicting the head)."

The investigators employed professional stealth. "During observations," the report stipulates, "only one observer entered the restroom at a time, and if another person entered to use the facilities, the observer discontinued counting and waited until the bathroom was empty before resuming counting."

New graffiti popped up every day, in every one of the restrooms.

But "after treatment was implemented", Watson reveals, "no marking occurred on any of the walls, and they remained free of graffiti at a three-month follow-up". No marking at all. None. Not a jot. Cleanliness uninterrupted. This was complete, utter success.

The treatment was simple: "Taping a sign on the wall that read, 'A local licensed doctor has agreed to donate a set amount of money to the local chapter of the United Way [a heavily publicised American charity organisation] for each day this wall remains free of any writing, drawing, or other markings'."

"The doctor," reveals the study, "was the author, a licensed psychologist, and the amount of money donated was 5 cents per day per bathroom."

The study lasted 50 days. Thus, with three restrooms in play, the maximum total potential payoff for charity was $2.50 (£2.30) per restroom – an aggregate $7.50 if no one ever made a mark on any wall in any of them.

Why was the treatment so very – nay, completely – effective? Watson speculates that "prior to posting the signs, bare walls appeared to function as discriminative stimuli for graffiti, perhaps because it was not apparent that anyone cared. Posting the signs was evidence that a prominent citizen (a doctor) was prepared to pay for results."

"An alternative explanation," he says, "is that the presence of the observers prompted restroom users to refrain from writing on walls."

• Marc Abrahams is editor of the bimonthly Annals of Improbable Research and organiser of the Ig Nobel prize