A lost wallet is more likely to be returned to its owner when it contains a large sum of money, new research suggests.

In 38 out of 40 countries studied, levels of honesty increased as the value of the contents rose.

While the findings appear to indicate the inherent altruism of mankind, the researchers suggest that – in fact – people could just be worried about looking like a thief.

The study, published in the journal Science, involved more than 17,300 “lost” wallets in 355 cities between 2013 and 2016.

Each was a transparent business card holder and contained a key, a grocery list and three identical business cards with the owner’s contact details, and was made to make them look like a local resident.

They contained either no money, or the local equivalent of 13.45 US dollars.

Research assistants travelled the world, turning in the wallets to public and private institutions, including hotels, banks and post offices.

They then recorded whether recipients contacted the owner to return the wallet within the next 100 days.

Globally, around 40% of the wallets containing no money were returned, compared to 51% of those which contained cash, the study found.

“In virtually all countries citizens were more likely to return wallets that contained more money,” the researchers said.

“Both non-experts and professional economists were unable to predict this result.”

Mexico and Peru were the only countries where this trend was not seen.

Overall, Switzerland was the most honest country, while the UK came 22nd when it came to reporting rates for the “lost” wallets, according to the research.

The researchers also ran a study in the UK, US and Poland, using a wallet containing the equivalent of 94.15 US dollars in addition to the other two.

Across the three countries, return rates were 72% for wallets containing the largest sum, compared to 61% for the smaller amount and 46% for those containing nothing.

The researchers said the “theft aversion” phenomenon – fear of viewing oneself as a thief – could explain the findings.

“It’s natural to assume that this is simply because people are altruistic and because they care about the owner of the wallet,” , co-author Christian Zund, from the University of Zurich, said.

“However, there’s a lot of research that suggests that even though people care about others, they still tend to care more about themselves.”

The researchers were surprised by the findings of the study, which cost around 600,000 US dollars to carry out.

Alain Cohn, one of the authors and assistant professor of economics at the University of Michigan, said: “We mistakenly assume that our fellow human beings are selfish.

“In reality, their self-image as an honest person is more important to them than a short-term monetary gain.”

Sally Wardle is Press Association Health and Science Correspondent.