And when mistakes are made, “‘Don’t keep it,’ is the firm advice,” Johnson said. “Eventually, the bank will come back to the customer. First they’ll reverse the transaction but also potentially generate a police report after effective research, meaning the bank will contact the customer … and ask the logical questions: Did they notice that it was inadvertently deposited, why didn’t they alert the bank, why didn’t they return the funds. It creates a whole confluence of events that are not attractive.”

One Georgia teen learned this the hard way. After spending $30,000 in his bank account that didn’t belong to him, the young man earned a 10-year sentence, according to local news reports. A teller had deposited a check from a client with the same last name into the teen’s account; he spent most of the funds on a BMW.

Lieberman, a former neuroscientist, speculated on how a person might make such a bad decision. The same part of the brain that gives us a rush when, say, we win a hand of cards, kicks in when we confront a scenario like unexpected money, Lieberman suggested. The limbic system is like the rewards circuitry. “There’s a lot of dopamine signaling … your heart rate goes up, your pupils dilate, you start sweating. That’s when you make bad decisions. A lot of this is evolutionary response.”