A couple of months ago, I met up with one of my friends for a morning coffee. As soon as I arrived, he asked me not to buy anything. “I just realized I’ve lost my wallet,” he said. “Let’s go look for it.” Luckily enough, we found it at the police station nearby, where someone had returned it. Everything was intact — money, cards, and IDs. My friend evaded a disaster.

Losing your wallet can be very costly. For starters, an ill-motivated person could take your money and use your cards for some transactions. Even more problematic is having to report your lost ID and get a new one — it’s anywhere between a hassle and a disaster. There’s no official statistic, but it’s probably safe to say that millions of people lose their wallets every year.

Some of them are returned, and some of them aren’t. A group of researchers wanted to see if and why people around the world returned wallets, and if the amount of money in it makes a difference. So they did what every sensible person would do: they sent a graduate student all around the world with 17,000 wallets, loads of cash, and about 400 spare keys.

Everywhere the graduate student went, the world around him became a laboratory. Unbeknownst to them, staff members from museums, banks, and other institutions became part of his research on civic honesty. They were presented with an allegedly lost wallet, with an amount of money in it.

The setup was simple. A young European tourist walked into an institution with a wallet containing a shopping list, a key and a few duplicate business cards written in the local language. The tourist handed the wallet to a member of staff saying “I found this on the street near the entrance. Someone must have lost it. Can you take care of it?”. Then, he would promptly leave.

In some cases, the wallet would have no money. In others, it would have the equivalent of $15 in local currency. In some cases, it had up to $100.

The results were surprising: wallets with money were more likely to be returned than the ones with no money. Furthermore, the more money the wallet had, the more likely it was to be returned.

This came as a shock to researchers, who were expecting to see quite the opposite. A separate survey of 300 economists found that the scholars were expecting people to return more wallets with little or no money and keep the ones with more money.

Interpreting the findings is not easy. It’s always tricky to draw conclusions from studies like this — but at the very least, this study goes to show that people’s reactions to a moral dilemma are not always predictable, and could be quite surprising.

So, that just leaves one question: what would you do if you found a lost wallet on the sidewalk?