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.