What would you do if you found a wallet on the street? Leave it? Take it to a police station? Post it back to the owner? Keep it, even?

The answer, scientists have found, depends rather more on evolution than morality.

Hundreds of wallets were planted on the streets of Edinburgh by psychologists last year. Perhaps surprisingly, nearly half of the 240 wallets were posted back. But there was a twist.

Richard Wiseman, a psychologist, and his team inserted one of four photographs behind a clear plastic window inside, showing either a smiling baby, a cute puppy, a happy family or a contented elderly couple. Some wallets had no image and some had charity papers inside.

When faced with the photograph of the baby people were far more likely to send the wallet back, the study found. In fact, only one in ten were hard-hearted enough not to do so. With no picture to tug at the emotions, just one in seven were sent back.

According to Dr Wiseman the result reflects a compassionate instinct towards vulnerable infants that people have evolved to ensure the survival of future generations. “The baby kicked off a caring feeling in people, which is not surprising from an evolutionary perspective,” he said.

Scientists argue that it would be difficult to genetically code for feeling empathy exclusively towards your own child and much easier to code for feeling empathy towards all children. If you find a baby alone, there is a good chance it belongs to you, making it an effective evolutionary trait, said Dr Wiseman.

In the study, 40 wallets were sent out in each photograph category as well as 40 containing a card suggesting that the owner had recently made a contribution to charity. A control batch contained no additional item.

All of the wallets were stuffed with the same set of everyday items, including raffle tickets, discount vouchers, and membership cards. None of them contained money, however.

The wallets were mixed up randomly, and over a couple of weeks were secretly dropped on the streets in areas of high footfall, but well away from postboxes, litter bins, vomit, and dog faeces.

The researchers planted each wallet about a quarter of a mile apart to ensure that people would be unlikely to find more than one.

The baby photograph wallets had the highest return rate, with 88 per cent of the 40 being sent back. Next came the puppy, the family and the elderly couple, with 53 per cent, 48 and 28 respectively. At 20 per cent and 15, the charity card and control wallets had the lowest return rates.

Overall, 42 per cent of the wallets were posted back — more than the team had anticipated. “We were amazed by the high percentage of wallets that came back,” said Dr Wiseman.

Scientists have also found evidence for a baby instinct in brain scanning experiments. A recent study at the University of Oxford examined how people responded when they were shown photographs of baby or adult faces.

Even though all of the photographs were matched for attractiveness, activity in the section of the brain associated with empathy was much more responsive to the baby faces than to adult faces. The response happened too fast to be consciously controlled, according to the study.

Whatever the scientific explanation, the practical message is clear, said Dr Wiseman. “If you want to increase the chances of your wallet being returned if lost, obtain a photograph of the cutest baby you can find, and ensure that it is prominently displayed,” he said.

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