Long before spam folders, a letter would arrive sealed in wax from a foreign country. A wealthy nobleman, it said, was secretly imprisoned in Spain under a false identity. He could not reveal who he was, and a loyal friend needed a discreet outsider to put up money to bribe the guards. The reward would be a share of a hidden fortune, and sometimes the hand of the prisoner’s beautiful daughter. This was the Spanish prisoner, a confidence trick dating to the late 16th century.

This episode takes the swindle apart piece by piece. It shows how the letter flatters its target, known as the mark, into feeling like the hero of a secret mission, and how one payment becomes many once the trickster reports that further difficulties have arisen. The sunk cost fallacy does the rest: walking away means admitting the money and the fantasy are both gone. From there the line runs straight to the Nigerian 419 email, the advance fee scam, and modern romance fraud.

• Eugene Francois Vidocq, the French criminal turned detective, wrote about the scam in his memoirs in the early 19th century.
• Before wire transfers, marks sent bank drafts or traveled to hand cash to an accomplice posing as a sympathetic priest or a corrupt guard.
• In March 1898 the New York Times ran a warning under the headline An Old Swindle Revived, laying out the trick in detail.
• Six years later, in 1904, a man in McKeesport, Pennsylvania, still sent his money and waited for a payout and a Spanish bride that never came.
• A later variation claimed the prisoner was a distant relative of the mark, turning a stranger’s offer into a private family emergency that public warnings did not seem to cover.