Paul J. Zak, a neuroeconomist and director of the Center for Neuroeconomics Studies at Claremont Graduate University, explains the psychology of cons using himself as an example. When he was a teenager, he was taken by the "pigeon drop."
Here's what happened to me. One slow Sunday afternoon, a man comes out of the restroom with a pearl necklace in his hand. "Found it on the bathroom floor" he says. He followed with "Geez, looks nice-I wonder who lost it?" Just then, the gas station's phone rings and a man asked if anyone found a pearl necklace that he had purchased as a gift for his wife. He offers a $200 reward for the necklace's return. I tell him that a customer found it. "OK" he says, "I'll be there in 30 minutes." I give him the ARCO address and he gives me his phone number. The man who found the necklace hears all this but tells me he is running late for a job interview and cannot wait for the other man to arrive.
Huum, what to do? The man with the necklace said "Why don't I give you the necklace and we split the reward?" The greed-o-meter goes off in my head, suppressing all rational thought. "Yeah, you give me the necklace to hold and I'll give you $100" I suggest. He agrees. Since high school kids working at gas stations don't have $100, I take money out of the cash drawer to complete the transaction.
You can guess the rest.
He goes on to explain the psychology of cons. In short, :The key to a con is not that you trust the conman, but that he shows he trusts you."
(Here's a video of the pigeon drop.)