In 1824, a publisher wrote to the first Duke of Wellington with a threat: he was about to publish a salacious memoir by a former mistress of the duke’s. Money could keep Wellington out of those red-hot pages. The Iron Duke, vanquisher of Napoleon, sent the letter back with a message scrawled across it: “Publish and be damned.”

Last week David Letterman outdid the duke. He worked with the Manhattan district attorney’s office and a grand jury to arrest a man who allegedly tried to blackmail him. And then Letterman confessed to his audience of millions that he had had sexual affairs with members of his staff—the very “terrible, terrible things” that, he said, the man had threatened to expose unless he paid him $2 million.

Why does someone like Letterman go on the offensive while others try to buy silence? Eric Dezenhall, a consultant on crisis management, said that while those who have built a reputation based on upholding family values—for instance—could well feel most vulnerable, many of today’s celebrities are all but blackmail proof. “Paris Hilton’s brand is getting into trouble,” he said.

Blackmail is a “wonderfully curious offense”, to use the phrase of Paul H. Robinson, a professor at the University of Pennsylvania Law School and his coauthors in a recent paper. A threat to tell the truth is no crime, and neither is asking someone for money. But if you demand money to prevent the truth from being told, Professor Robinson said, you’ve crossed the line. At its core, he explained, the offense is “a form of wrongful coercion”.

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