The researchers say the eye pictures were probably influential because the brain naturally reacts to images of faces and eyes. It seems people were subconsciously cooperating with the honesty box when it featured pictures of eyes rather than flowers.

They also say the findings show how people behave differently when they believe they are being watched because they are worried what others will think of them. Being seen to co-operate is a good long-term strategy for individuals because it is likely to mean others will return the gesture when needed.

Details of the experiment, believed to be the first to test how cues of being watched affect people's tendency for social co-operation in a real-life setting, are published today, Wednesday June 28, in the Royal Society journal Biology Letters.

An honesty box is a system of payment which relies on people's honesty to pay a specified price for goods or services - there is no cashier to check whether they are doing so.

For this experiment, lead researcher Dr Melissa Bateson and her colleagues Drs Daniel Nettle and Gilbert Roberts, of the Evolution and Behaviour Research Group in the School of Biology and Psychology at Newcastle University, made use of a long-running 'honesty box' arrangement.

This had been operating as a way of paying for hot drinks in a common room used by around 48 staff for many years, so users had no reason to suspect an experiment was taking place.

An A5 poster was placed above the honesty box, listing prices of tea, coffee and milk. The poster also featured an image banner across the top, and this alternated each week between different pictures of flowers and images of eyes.

The eye pictures varied in the sex and head orientation but were all chosen so that the eyes were looking directly at the observer.

Each week the research team recorded the total amount of money collected and the volume of milk consumed as this was considered to be the best index available of total drink consumption.

The team then calculated the ratio of money collected to the volume of milk consumed in each week. On average, people paid 2.76 as much for their drinks on the weeks when the poster featured pictures of eyes.

Lead author of the study, Melissa Bateson, a Royal Society research fellow based at Newcastle University, said: "Our brains are programmed to respond to eyes and faces whether we are consciously aware of it or not.

"I was really surprised by how big the effect was as we were expecting it to be quite subtle but the statistics show that the eyes had a strong effect on our tea and coffee drinkers."

The findings could have applications in initiatives to curb anti-social behaviour or in law enforcement - perhaps in areas such as payment for public transport, road safety or the general issue of behaviour in public places.

The group now hopes to expand the study to involve a larger sample population.

Dr Bateson said: "Our findings suggest that people are less likely to be selfish if they feel they are being watched, which has huge implications for real life.

"For example, this could be applied to warnings about speed cameras. A sign bearing an image of a camera would have to be actively processed by our brains, as it is an artificial stimulus. Our research and previous studies suggest drivers would react much more quickly and positively to natural stimuli such as eyes and faces."

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