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The pirate game

298 pointsby dbalanover 9 years ago

23 comments

cousin_itover 9 years ago
Steven Landsburg has pointed out that the problem is a bit underdetermined, and the usual solution is far from unique. For example, &quot;the first pirate gets all the coins and everyone else votes yes&quot; is a subgame-perfect equilibrium which is just as good as the usual solution, because no pirate has an incentive to deviate unilaterally (one vote can&#x27;t change the voting outcome). There are many other equilibria like that. The moral of the story is that games with more than two players seldom have unique solutions, unless you make very strong and explicit assumptions.<p>Another problem which puzzles me even more is dividing a cake among three people by majority vote. Let&#x27;s say Alice and Bob make an agreement where each of them gets half of the cake and Carol is left out. But that&#x27;s unstable, because Carol can offer Bob a different agreement where Bob gets 60%, Carol gets 40%, and Alice is left out. Since Bob gets 10% more than before, he has an incentive to switch. But that&#x27;s unstable as well, because now Alice can make a similar offer to Carol, etc. In the end there&#x27;s no possible setup that everyone will stick with. As far as I know, this problem is still mostly open (though a few advances have been made).<p>To sum up, the idea of &quot;perfectly rational&quot; decision-making is surprisingly difficult to nail down. It&#x27;s been definitively solved only for the case of two-player zero-sum games. When the game is not zero-sum, you get complications like bargaining and equilibrium selection, and when you have more than two players, you get an explosion of complexity without any clear-cut answers.
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aakilfernandesover 9 years ago
I really hate questions like this. Since they mistake rationality for computer-like adherence to a set of rules.<p>——————————————————————<p>Imagine a 100 rounds of ultimatum games. In each round, Alice proposes a split of $1 and Bob can either accept or reject. If Bob rejects, the $1 is burned in a fire. How much money will each end up with?<p>Lets work backwards, as we do in the pirate game.<p>In ROUND100, Alice knows Bob will take whatever non-zero offer she makes. She can offer him 1 cent and Bob will agree.<p>In ROUND99, Alice knows that Bob can will accept whatever non-zero offer she makes in ROUND100, so she can make whatever non-zero offer she wants this round without fear of repercussion. She offers him 1 cent and knows he will agree.<p>Continuing to work backwards, we find that in all 100 rounds, Alice takes 99 cents and offers Bob 1 cent. Alice ends up with $99 and Bob ends up with $1.<p>——————————————————————<p>This is obviously (and experimentally proven) not what would happen in real life. In experimental outcomes, Bob rejects offers that are too low to broadcast that he is “irrational”. As a result, Bob is able to negotiate a much higher outcome (around 40%).<p>So who is more rational? The &quot;rational&quot; Bob who gets $1, or the “irrational” Bob who gets $40?
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ryandvmover 9 years ago
This puzzle should be called &quot;The VC game&quot;. Real pirates were actually far more egalitarian. <a href="http:&#x2F;&#x2F;www.newyorker.com&#x2F;magazine&#x2F;2007&#x2F;07&#x2F;09&#x2F;the-pirates-code" rel="nofollow">http:&#x2F;&#x2F;www.newyorker.com&#x2F;magazine&#x2F;2007&#x2F;07&#x2F;09&#x2F;the-pirates-cod...</a>
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personjerryover 9 years ago
Isn&#x27;t this our current economic status? The rich (A) become and stay rich by scoring votes with key voters (C and E). These key voters are kept in line by intimidation, because they know they could end up worse (like B and D).
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afro88over 9 years ago
There&#x27;s a Korean gameshow called &quot;The Genius&quot; which plays on these kinds of games. It&#x27;s more socially oriented but the contestants and the &quot;narrator&quot; go through the mathematics and strategies a bit. Highly recommended.
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pierrebaiover 9 years ago
A tantalizing idea I&#x27;ve been trying to spell out properly that goes against the usual reasoning behind similar problem is that of putting into question the correctness of causality in hypotheticals.<p>Each such problem I&#x27;ve read about assumes that we can assume causality while reasoning about hypothetical, but strangely, if we let go of that assumptions, then we can arrive at different answers which hinge on otherwise surprising behaviour. I relate this idea to the fact that in logical reasoning, all true statements are, once proved, held to be simultaneously true. That is, given if A then B, with A being true, we don&#x27;t hold B to be true &#x27;after&#x27; A, but to have been always true, given A.<p>In the pirate problem, limiting ourselves to three pirates to shorten my explanation, we end up with the split being: 99 to C, 0 to D, 1 to E. This is because we assume that if only D and E were left, D would keep 100 to himself. Now given that distribution (99,0,1), D should now change his hypothetical proposal to (0,98,2). If we assume that C is not a &#x27;non-causality&#x27; believer, but both D and E to be, then E would vote no to (99,0,1) and D would do the (0,98,2) split. You may argue that D could then do a (0,100,0) split, but that&#x27;s not how a non-causality believer MUST act, because he knows that to get to that point, he must know thet E can logically know that he will do this split. This can be justified by arguing that when a pirates survives, he will enter such gold-splitting game later on. But my argument is subtler than this and doesn&#x27;t require it. It basically become this: all such pirates, posited to be perfectly logical, are interchangeable. Thus they must all reason in the same fashion. Thus, my argument is that <i>true</i> pure logical minds see that the true way to maximize their gold profit is to hold a world view that maximize their profit, even if causality is discarded. Thus both D and E know that tehy can maximize their profit by discarding their belief in causality. That is how D ends up proposing (0,98,2) and E accepts it.<p>Of course, I ended up there by assuming C was not a believer, but given my argument, C must also be ready to throw causality out of the window, otherwise he will end up dead. I believe my argument ends up splitting (49,51,0), but I&#x27;m not sure. Once causality is throw out, it&#x27;s hard to tell, but intuitively, with three pirates, those voting must be given almost equal gold and the remaining pirates must not have a majority.
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asiftover 9 years ago
I love game theory, but I hate that the focus outside of academia is almost always on simplistic single period games. These games can result in some interesting conclusions and teach some valuable concepts, but they are terrible at representing how people behave in the real world.<p>Anyone interested in a more complex and realistic examination of the economics of pirating should consider Peter Leeson&#x27;s book, <i>The Invisible Hook: The Hidden Economics of Pirates</i>.<p>I would also recommend Leeson&#x27;s <i>Anarchy Unbound: Why Self-Governance Works Better Than You Think</i> for some interesting applications of game theory in more realistic historical contexts.
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mrfusionover 9 years ago
This kind of reminds me of the surprise execution paradox.<p><a href="https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;Unexpected_hanging_paradox" rel="nofollow">https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;Unexpected_hanging_paradox</a>
oilywaterover 9 years ago
I remember reading about this game on a private music torrent tracker what.cd and was working on the solution for a couple of hours. I believe there was a whole thread dedicated to these kinds of problems.<p>Quite an entertaining problem, skip the result and try it out yourself, the sense of accomplishment is fulfilling, especially if your mood is shaky.
talmandover 9 years ago
Let&#x27;s see if my logic works. It seems some motivations and negotiations are left out of the proposed solution.<p>If I were C I would have voted no on A&#x27;s proposal and try to make a deal with D and E. The idea is that knowing if A were thrown overboard then B might be more apt to provide a more beneficial coin sharing program in an effort to save his life. After all, he just watched the previous guy get tossed overboard and die. The most I could lose would be 1 coin if B decided to try A&#x27;s method on D and E and they went along with it.<p>The key is that I would have already negotiated a deal with D and E, if A or B doesn&#x27;t share the wealth fairly then we vote to toss them. Once I&#x27;m in charge, I&#x27;ll share the coins equally as possible.
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andyraskinover 9 years ago
I was asked to solve this puzzle once during a project management interview. The solution here says &quot;work backwards,&quot; but I found that &quot;work recursively&quot; was a more helpful (and instructive) way to think about it.
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blobbersover 9 years ago
The results of the pirate game seem a lot like the stock allocation at my company. A billion dollar acquisition and Pirate A figuring out how to distribute the coins... guess where they end up!<p>Note to self: don&#x27;t get thrown overboard.
pratyushagover 9 years ago
I think this is a great game and a portrayal of reality where a small number of people get most of the wealth, a 50% number are middle class with enough to be just okay (or at 1) and with a still a very large number who can barely have a comfortable life (say $4k&#x2F;month household income).
Pxtlover 9 years ago
Shouldn&#x27;t it be E: 2 in the solution?<p>If pirates, all things being equal, prefer violence to peace (which is implied) then E getting 1 from A is less preferable than killing A and B and getting 1 from C.<p>Or is it that E knows that B will be offering next, and B will offer him 0, so it makes sense for him to accept A&#x27;s offer of 1?
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archibaldJover 9 years ago
Reminded me of the game theory course by Stanford on Coursera. It was in the exercise on [backward induction](<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Backward_induction" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Backward_induction</a>), if I remember correctly.
mcvover 9 years ago
It&#x27;s a nice demonstration of how the lack of trust ultimate screws everybody except the leader.
drelihanover 9 years ago
I was asked this question in an interview 10+ years ago. I thought it was a really good question to promote logical discussions. Lots of branches and extensions you can add to see how people think. Really fun.
dsegoover 9 years ago
If you like this type of puzzle, I can recommend the book: “How Would You Move Mount Fuji?: Microsoft&#x27;s Cult of the Puzzle”.
quickpostover 9 years ago
This game reminds me of some employee stock option negotiations I&#x27;ve been on the lessor end of...
efnxover 9 years ago
These outcomes assume you can live rich (having ~98 coins) in the same space as poor pirates without them stabbing you and taking your coins! Real pirates would distribute more evenly for fear of death after distribution.
ALeeover 9 years ago
Well now I have to choose a new interview question.
logfromblammoover 9 years ago
Now that you have that down, allow the pirates to secretly bribe each other out of their own savings before the vote. If multiple votes occur, previous bribes are added to a pirate&#x27;s savings. If a pirate is thrown overboard, his remaining personal savings are looted and added to the pool of coins to be divided. The personal savings of each pirate are secret from all the other pirates, but all pirates always have more in savings than the next pirate in order of decreasing seniority.<p>According to the pirate&#x27;s code, a bribe is a binding contract, but only to the extent that breaching the terms of the contract results in returning the amount of the bribe <i>after</i> the breach occurs. Otherwise, the bribed pirate is thrown overboard. A pirate may therefore spend the bribe money before reneging, if reneging will still yield enough money to pay back the bribe afterward.<p>Pirate P[x] has savings s[x], and s[x] &gt; s[x+1].<p>The degenerate case where n=1 is easy to deduce. P[1] proposes {pool}, votes yes, and wins.<p>The case where n=2 is also easy. P[1] proposes {pool, 0}, votes yes, and wins.<p>When n=3, it gets more complicated. P[1] needs one more vote to win. If P[2] is able to propose a split, he will be proposing {0, pool + s[1], 0}. So the cost of P[2]&#x27;s vote is at least pool + s[1] + 1, which is normally impossible to achieve for P[1]. But since P[2] does not know how much s[1] is, other than that it is more than s[2], P[1] might be able to risk it. But since P[1] doesn&#x27;t know s[2], bluffing a lower amount for s[1] is risky, as if the bluff amount is lower than s[2] + 1, then P[2] will immediately know to vote no. The cost of P[3]&#x27;s vote is at least 1. P[1] and P[2] will therefore be bidding competitively for P[3]&#x27;s vote. P[1] has the choice of offering value to P[3] publicly in the proposal, or secretly as a bribe. Either way, P[1] is likely to propose 0 for P[2].<p>So if P[1] proposes { pool, 0, 0 }, and bribes P[3] to vote yes with s[1], P[2] might try bribing P[3] with any amount from 1 to s[2] to vote no, with no effect. The net effect is { +pool -s[1], 0, +s[1] }.<p>Remember also that any bribe P[1] pays to P[2] <i>could</i> be added to a bribe from P[2] to P[3]. If P[1] bribes 1 to P[2] to vote yes, and s[1]-1 to P[3] to vote yes, and P[2] bribes s[2]+1 to P[3] to vote no, then if P[2] and P[3] both vote no, P[1] is thrown overboard for losing the vote, and P[2] is thrown overboard for reneging on a bribe and not having the coin to pay it back, so P[3] gets everything. If P[3] suspects that P[1] may have bribed P[2] to vote yes, and knows that P[2] bribed him to vote no--which would be useless unless P[2] himself intended to vote no--then P[3] may vote no on the possibility of getting pool + s[1] + s[2], which would otherwise be impossible for him. So knowing this, P[1] may be confident that any bribe to P[2] to vote yes <i>would not</i> reasonably be re-bribed to P[3] to vote no.<p>So P[1] could bribe 1 to P[2] to vote yes, tell P[3] that he had bribed P[2], tell P[2] that he had told P[3], and then propose {pool - 1, 0, 1}. It would be useless for P[2] to bribe P[3] unless he intended to reneg, he can&#x27;t add P[1]&#x27;s bribe to his own, and he knows that s[1] - 1 &gt;= s[2]. With the 1 from the pool, he could not match such a theoretical bribe anyway. So the net effect is now { +pool -2, 1, 1 }.<p>I&#x27;m not exactly sure if that is a stable solution or not.<p>I think maybe that P[3] might be able to get more by bribing P[1] to vote no, or P[2] to vote yes.
vacriover 9 years ago
If the pirates were that strictly rational and interested in their own survival, they wouldn&#x27;t be pirates. They&#x27;d have a nice, safe job, free from threat of death or injury.
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