New York, June 21 -- People are more likely to return a lost wallet if it contains money - and the more cash, the better.

That's the surprising conclusion from researchers who planted more than 17,000 "lost wallets" across 355 cities in 40 countries, and kept track of how often somebody contacted the supposed owners.

The presence of money - the equivalent of about $13 in local currency - boosted this response rate to about 51%, versus 40% for wallets with no cash. That trend showed up in virtually every nation, although the actual numbers varied.

Researchers raised the stakes in the U.S., the United Kingdom and Poland. The response jumped to 72% for wallets containing the equivalent of about $94, versus 61% for those containing $13. If...