Creative Commons attribution licensed image courtesy of The Consumerist
The Times Online covers a study by Edinburgh psychologists who found that a wallet with baby pictures was the most likely to be returned. The study showed that only 10% of wallets with baby pictures were not returned, a far higher rate than any other wallet and picture combination. The article notes that a related study found a hardwired response pattern in the brain to baby pictures, meaning that some elements of our reactions might be hard to overcome.
The flaw in the study - at least for those security practitioners who are now pondering putting baby pictures on all of their possessions? The wallets didn't contain any items of real value - no cash or credit cards, meaning that their return rates were likely higher than might have been seen with wallets with a reward for those who don't return them.
As for cute kittens chewing on wallets? I'm not sure if it guarantees more hits...