The most common way academics study regulation is to seek models under which such regulation could be efficient (or moral), and to seek empirical data on details of current local regulatory practice to distinguish such models. But this standard approach tends to neglect both models where people personally gain by supporting inefficient (or immoral) regulation, and the patterns of such regulations across diverse cultures, times, and topics. Some other day I’ll elaborate on this general point. Today I’ll apply my own advice to blackmail, and consider the history of blackmail law.
Andrew Gelman brought up the issue of economists assuming efficiency in some cases and then doing the opposite in others here.
I didn't notice many people saying blackmail laws were intended to encourage gossip, though that was a possibility you mentioned.
There are mechanisms by which some types of blackmail are “legal”. The problems that Rupert Murdoch has found himself in due to the egregious invasions of privacy in the UK are blackmail of a type. Murdoch's employees hacked into the phones of crime victims and interfered with ongoing criminal investigations to try and get information with which to titillate their readers and sell more papers. People who found out that their phones were hacked were able to extract large settlements one provision of which was to keep that information silent.
Keith Olbermann feels that he was blackmailed by Rupert Murdoch.
If you agree in a legal settlement that the payment is not “blackmail”, then it isn't “blackmail”. The wealthy can afford to hire lawyers as intermediaries so they can pretend to have “plausible deniability”, that is unless they testify under oath that they didn't know what their lawyers had told them and try to throw their lawyers under the bus (as Murdoch has done).
If there was really a desire to make blackmail illegal, an easy way to do it would be to ban secret legal agreements, that is to make legal agreements to require non-disclosure of certain types of information non-enforceable. There already is this in terms of some types of information for some individuals. In some jurisdictions health care workers are mandated reporters for suspected child abuse.
If you made lawyers into mandated reporters for suspected child abuse, then they wouldn't be able to draft secrecy agreements to protect pedophile priests without disclosing the suspected abuse to civil authorities.
But this won't happen. The whole purpose of the law is to protect elites.