Imagine it's the future, and everything has gone according to plan. Humanity has worked out its own utility function, f0, and has worked out a strategy S0 to optimize it.
Humanity has also run a large number of simulations of how alien worlds evolve. It has determined that of those civilizations which reach the same level of advancement - that know their own utility function and have a strategy for optimizing it - there is an equal probability that they will end up with each of 10 possible utility functions. Call these f0...f9.
(Of course, these simulations are coarse-grained enough to satisfy the nonperson predicate).
Humanity has also worked out the optimal strategy S0...S9 for each utility function. But they just happen to score poorly on all of the others:
fi(Si) = 10
fi(Sj) = 1 for i != j
In addition, there is a compromise strategy C:
fi(C) = 3 for all i.
The utility functions, f0 through f9, satisfy certain properties:
They are altruistic, in the sense that they care just as much about far-away aliens that they can't even see as they do about members of their own species.
They are additive: if one planet implements Sj and another implements Sk, then:
fi(Sj on one planet and Sk on the other) = fi(Sj) + fi(Sk).
(This is just to make things easier - the problem I'm describing will still apply in cases where this rule doesn't hold).
They are non-negotiable. They won't "change" if that civilization encounters aliens with a different utility function. So if two of these civilisations were to meet, we would expect it to be like the humans and the babyeaters: the stronger would attempt to conquer the weaker and impose their own values.
In addition, humanity has worked out that it's very likely that a lot of alien worlds exist, i.e. aliens are really really real. They are just too far away to see or exist in other Everett branches.
So given these not entirely ridiculous assumptions, it seems that we have a multiplayer prisoner's dilemma even though none of the players has any causal influence on any other. If the universe contains 10 worlds, and each chooses its own best strategy, then each expects to score 19. If they all choose the compromise strategy then each expects to score 30.
Anyone else worried by this result, or have I made a mistake?
Edit: This comment is retracted. My comment is wrong, primarily because it misses the point of the post, which simply presents a usual game theory-style payoff matrix problem statement. Thanks to Tyrrell McAllister for pointing out the error, apologies to the readers. See this comment for details. (One more data point against going on a perceptual judgement at 4AM, and not double-checking own understanding before commenting on a perceived flaw in an argument. A bit of motivated procrastination also delayed reviewing Tyrrell's response.)
Who is following these strategies? The only interpretation that seems to make sense is that it's humanity in each case (is this correct?), that is S2 is the strategy that, if followed by humanity, would optimize aliens #2's utility.
In this case, the question is what do the f_i(S_j) mean. These are expected utilities of a possible strategy, but how do you compute them? CDT, TDT and UDT would have it differently.
In any case, it's conventional to mean by "expected utility of a possible decision" the value that you'll be actually optimizing. With CDT, it's computed in such a way that you two-box on Newcomb as a result, in TDT and UDT the bug is fixed and you one-box, but still by optimizing expected utility (computed differently) of the decision that you'd make as a result. Similarly for PD, where you one-box in UDT/ADT not because you take into account utilities of different agents, but because you take into account the effect on your own utility mediated by other agent's hypothetical response to your hypothetical decision, that is you just compute your own expected utility more accurately, and still just maximize only your own utility.
Cooperation in PD of the kind TDT and UDT enable is not about helping the other, it's about being able to take into account other's hypothetical cooperation arising in response to your hypothetical cooperation. Altruistic agents already have their altruism as part of their own utility function, it's a property of their values that's abstracted away at the level where you talk about utilities and should no longer be considered at that level.
So the answer to "What should you maximize?" is, by convention, "Your own expected utility, period." This is just what "expected utility" means (that is, where you can factor utility as expected value of a utility function over some probability distribution; otherwise you use "utility" in this role). The right question should be, "How should you compute your expected utility?", and it can't be answered given the setup in this post, since f_i are given as black boxes. (Alternatively, you could give a way of estimating utility as a black box, and then consider various ways of constructing an estimate of expected utility out of it.)
[...]
The post calls the functions f_i "utility functions", not "expected utility functions". So, I take Giles to be pursuing your "alternative" approach. However, I don't think... (read more)