It is subgame perfect equilibrium that rules out “never cheat, always punish cheating” (the set of all SPE of a sequential game is a subset of the set of all NE of that game).
How strictly do you (or the standard approach) mean to rule out options that aren’t good on all parts of the game? It seems like sometimes you do want to do things that are subgame suboptimal.
Edit: or at least be known to do things, which unfortunately can require actually being prepared to do the things.
Well, the classical game theorist would reply that they’re studying one-off games, in which the game you’re currently playing doesn’t affect any payoff you get outside that game (otherwise that should be made part of the game), so you can’t be doing the punishment because you want to be known to be a punisher, or the game that Robin specified doesn’t model the situation you’re in. The classical game theorist assumes you can’t look into people’s heads, so whatever you say or do before the cheating, you’re always free to not punish during the punishment round (as you’re undoubtedly aware, mutual checking of source code is prohibited by antitrust laws in over 185 countries).
The classical game theorist would further point out that if you do want model that punishment helps you be known as a punisher, then you should use their theory of repeated games, where they have some folk theorems for you saying that lots and lots of things can be Nash equilibria e.g. in a game where after each round there is a fixed probability of another round; for example, cooperation in the prisoner’s dilemma, but also all sorts of suboptimal outcomes (which become Nash equilibria because any deviator gets punished as badly as the other players can punish them).
I should point out that not all classical game theorists think that SPE makes particularly good predictions, though; I’ve read someone say, I think Binmore, that you expect to virtually always see a NE in the laboratory after a learning period, but not an SPE, and that the original inventor of SPE actually came up with it as an example of what you would not expect to see in the lab, or something to that tune. (Sorry, I should really chase down that reference, but I don’t have time right now. I’ll try to remember to do that later. ETA: Ok, Binmore and Shaked, 2010: Experimental Economics: Where Next?Journal of Economic Behavior & Organization, 73: 87-100. See the stuff about backward induction, starting at the bottom on p.88. The inventor of SPE is Reinhard Selten, and the claim is that he didn’t believe it would predict what you see it in the lab and “[i]t was to demonstrate this fact that he encouraged Werner Güth (...) to carry out the very first experiment on the Ultimatum game”, not that he invented SPE for this purpose.)
so whatever you say or do before the cheating, you’re always free to not punish during the punishment round
Interesting. This idea, used as an argument for SPE, seems to be the free will debate intruding into decision theory. “Only some of these algorithms have freedom, and others don’t, and humans are free, so they should behave like the free algorithms.” This either ignores, or accepts, the fact that the “free” algorithms are just as deterministic as the “unfree” algorithms. (And it depends on other stuff, but that’s not the fun bit)
(as you’re undoubtedly aware, mutual checking of source code is prohibited by antitrust laws in over 185 countries).
Hm, I may not quite have gotten the point across: I think you may be thinking of the argument that humans have free will, so they can’t force future versions of themselves to do something that would be against that future version’s given its information, but that isn’t the argument I was trying to explain. The idea I was refering to works precisely the same way with deterministic algorithms, as long as the players only get to observe each others’ actions, not each others’ source (though of course its proponents don’t think in those terms). The point is that if the other player looks at you severely and suggestively taps their baseball bat and tells you about how they’ve beaten up people who have defected in the past, that still doesn’t mean that they’re actually going to beat you up—since if such threats were effective on you, then making them would be the smart thing to do even if the other player has no intention of actually beating you up (and risk going to jail) if for some reason you end up defecting. (Compare AI-in-the-box...) (Of course, this argument only works if you’re reasonably sure that the other player is a classical game theorist; if you think you might be playing against someone who will, “irrationally”, actually punish you, like a timeless decision theorist, then you should not defect, and they won’t have to punish you...)
Now, if you had actual information about what this player had done in similar situations in the past, like police reports of beaten-up defectors, this argument wouldn’t work, but then (the standard argument continues) you have the wrong game-theoretical model; the correct model includes all of the punisher’s previous interactions, and in that game, it might well be a SPE to punish. (Though only if the exact number of “rounds” is not certain, for the same reason as in the finitely iterated Prisoner’s Dilemma: in the last round the punisher has no more reason to punish because there are no future targets to impress, so you defect no matter what they did in previous rounds, so they have no reason to punish in the second-to-last round, etc.)
I think you may be thinking of the argument that humans have free will, so they can’t force future versions of themselves to do something that would be against that future version’s given its information
That is not what I was thinking of. Here, let me re-quote the whole sentence:
The classical game theorist assumes you can’t look into people’s heads, so whatever you say or do before the cheating, you’re always free to not punish during the punishment round
The funny implication here is that if someone did look into your head, you would no longer be “free.” Like a lightswitch :P And then if they erased their memory of what they saw, you’re free again. Freedom on, freedom off.
And though that is a fine idea to define, to mix it up with an algorithmic use of “freedom” seems to just be used to argue “by definition.”
Ok, sorry I misread you. “Free” was just my word rather than part of the standard explanation, so alas we don’t have anybody we can attribute that belief to :-)
How strictly do you (or the standard approach) mean to rule out options that aren’t good on all parts of the game? It seems like sometimes you do want to do things that are subgame suboptimal.
Edit: or at least be known to do things, which unfortunately can require actually being prepared to do the things.
Well, the classical game theorist would reply that they’re studying one-off games, in which the game you’re currently playing doesn’t affect any payoff you get outside that game (otherwise that should be made part of the game), so you can’t be doing the punishment because you want to be known to be a punisher, or the game that Robin specified doesn’t model the situation you’re in. The classical game theorist assumes you can’t look into people’s heads, so whatever you say or do before the cheating, you’re always free to not punish during the punishment round (as you’re undoubtedly aware, mutual checking of source code is prohibited by antitrust laws in over 185 countries).
The classical game theorist would further point out that if you do want model that punishment helps you be known as a punisher, then you should use their theory of repeated games, where they have some folk theorems for you saying that lots and lots of things can be Nash equilibria e.g. in a game where after each round there is a fixed probability of another round; for example, cooperation in the prisoner’s dilemma, but also all sorts of suboptimal outcomes (which become Nash equilibria because any deviator gets punished as badly as the other players can punish them).
I should point out that not all classical game theorists think that SPE makes particularly good predictions, though; I’ve read someone say, I think Binmore, that you expect to virtually always see a NE in the laboratory after a learning period, but not an SPE, and that the original inventor of SPE actually came up with it as an example of what you would not expect to see in the lab, or something to that tune. (Sorry, I should really chase down that reference, but I don’t have time right now. I’ll try to remember to do that later. ETA: Ok, Binmore and Shaked, 2010: Experimental Economics: Where Next? Journal of Economic Behavior & Organization, 73: 87-100. See the stuff about backward induction, starting at the bottom on p.88. The inventor of SPE is Reinhard Selten, and the claim is that he didn’t believe it would predict what you see it in the lab and “[i]t was to demonstrate this fact that he encouraged Werner Güth (...) to carry out the very first experiment on the Ultimatum game”, not that he invented SPE for this purpose.)
Interesting. This idea, used as an argument for SPE, seems to be the free will debate intruding into decision theory. “Only some of these algorithms have freedom, and others don’t, and humans are free, so they should behave like the free algorithms.” This either ignores, or accepts, the fact that the “free” algorithms are just as deterministic as the “unfree” algorithms. (And it depends on other stuff, but that’s not the fun bit)
:D
Hm, I may not quite have gotten the point across: I think you may be thinking of the argument that humans have free will, so they can’t force future versions of themselves to do something that would be against that future version’s given its information, but that isn’t the argument I was trying to explain. The idea I was refering to works precisely the same way with deterministic algorithms, as long as the players only get to observe each others’ actions, not each others’ source (though of course its proponents don’t think in those terms). The point is that if the other player looks at you severely and suggestively taps their baseball bat and tells you about how they’ve beaten up people who have defected in the past, that still doesn’t mean that they’re actually going to beat you up—since if such threats were effective on you, then making them would be the smart thing to do even if the other player has no intention of actually beating you up (and risk going to jail) if for some reason you end up defecting. (Compare AI-in-the-box...) (Of course, this argument only works if you’re reasonably sure that the other player is a classical game theorist; if you think you might be playing against someone who will, “irrationally”, actually punish you, like a timeless decision theorist, then you should not defect, and they won’t have to punish you...)
Now, if you had actual information about what this player had done in similar situations in the past, like police reports of beaten-up defectors, this argument wouldn’t work, but then (the standard argument continues) you have the wrong game-theoretical model; the correct model includes all of the punisher’s previous interactions, and in that game, it might well be a SPE to punish. (Though only if the exact number of “rounds” is not certain, for the same reason as in the finitely iterated Prisoner’s Dilemma: in the last round the punisher has no more reason to punish because there are no future targets to impress, so you defect no matter what they did in previous rounds, so they have no reason to punish in the second-to-last round, etc.)
(BTW: reference added to grandparent.)
That is not what I was thinking of. Here, let me re-quote the whole sentence:
The funny implication here is that if someone did look into your head, you would no longer be “free.” Like a lightswitch :P And then if they erased their memory of what they saw, you’re free again. Freedom on, freedom off.
And though that is a fine idea to define, to mix it up with an algorithmic use of “freedom” seems to just be used to argue “by definition.”
Ok, sorry I misread you. “Free” was just my word rather than part of the standard explanation, so alas we don’t have anybody we can attribute that belief to :-)