For a more parable-ic version of this, see here.
Suppose I make a precommitment P to take action X unless you take action Y. Action X is not in my interest: I wouldn't do it if I knew you'd never take action Y. You would want me to not precommit to P.
Is this blackmail? Suppose we've been having a steamy affair together, and I have the letters to prove it. It would be bad for both of these if they were published. Then X={Publish the letters} and Y={You pay me money} is textbook blackmail.
But suppose I own a MacGuffin that you want (I value it at £9). If X={Reject any offer} and Y={You offer more than £10}, is this still blackmail? Formally, it looks the same.
What about if I bought the MacGuffin for £500 and you value it at £1000? This makes no difference to the formal structure of the scenario. Then my behaviour feels utterly reasonable, rather than vicious and blackmail-ly.
What is the meaningful difference between the two scenarios? I can't really formalise it.
My conclusion: Calling something blackmail or extortion is what the decision to defect in an ultimatum game feels like from the inside.
The above definition is approximately 2/3rds serious (in the same way that Hansonian cynicism like "X is not about Y" is about 2/3rd serious). That is, it represents what seems to be a salient point about how humans think about the subject and also about the arbitrariness that is inherit to the problem. While there are technically exceptions to the definition (some people will comply with extortion even when labelling it thus) to me at least it seems to capture a significant aspect of the meaning.
I do note that while I am calling the distinction 'arbitrariy', this isn't a criticism. Arbitrariness is inevitable in cases that inherently require some kind of arbitration and things like obvious shelling points and established norms become invaluable. In some cases the decision to defect in an ultimatum game is obvious while in others it is not and part of the way we go about making the distinction is by perceiving the situations to be inherently different despite formally equivalent payoff structures.
This seems like a satisfactory reduction. By focusing on the analogy with Ultimatum Game, we can capture both the idea of cooperating/defecting, and the idea of evaluating fairness/Shelling point, in a way that follows from the structure of the game, not via references to the informal concepts.
(In my previous attempt to unpack "blackmail", the suggestion was that blackmail is the feeling of the more vague "shouldn't cooperate", as an attempt to get away from the annoying passing-the-buck intuition that it has something to do with the re... (read more)