Want to keep your wallet? Carry a baby picture

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.

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