A common background assumption on LW seems to be that it's rational to act in accordance with the dispositions one would wish to have. (Rationalists must WIN, and all that.)

E.g., Eliezer:

It is, I would say, a general principle of rationality - indeed, part of how I define rationality - that you never end up envying someone else's mere choices.  You might envy someone their genes, if Omega rewards genes, or if the genes give you a generally happier disposition.  But [two-boxing] Rachel, above, envies [one-boxing] Irene her choice, and only her choice, irrespective of what algorithm Irene used to make it.  Rachel wishes just that she had a disposition to choose differently.

And more recently, from AdamBell:

I [previously] saw Newcomb’s Problem as proof that it was sometimes beneficial to be irrational. I changed my mind when I realized that I’d been asking the wrong question. I had been asking which decision would give the best payoff at the time and saying it was rational to make that decision. Instead, I should have been asking which decision theory would lead to the greatest payoff.

Within academic philosophy, this is the position advocated by David Gauthier.  Derek Parfit has constructed some compelling counterarguments against Gauthier, so I thought I'd share them here to see what the rest of you think.

First, let's note that there definitely are possible cases where it would be "beneficial to be irrational".  For example, suppose an evil demon ('Omega') will scan your brain, assess your rational capacities, and torture you iff you surpass some minimal baseline of rationality.  In that case, it would very much be in your interests to fall below the baseline!  Or suppose you're rewarded every time you honestly believe the conclusion of some fallacious reasoning.  We can easily multiply cases here.  What's important for now is just to acknowledge this phenomenon of 'beneficial irrationality' as a genuine possibility.

This possibility poses a problem for the Eliezer-Gauthier methodology. (Quoting Eliezer again:)

Rather than starting with a concept of what is the reasonable decision, and then asking whether "reasonable" agents leave with a lot of money, start by looking at the agents who leave with a lot of money, develop a theory of which agents tend to leave with the most money, and from this theory, try to figure out what is "reasonable".

The problem, obviously, is that it's possible for irrational agents to receive externally-generated rewards for their dispositions, without this necessarily making their downstream actions any more 'reasonable'.  (At this point, you should notice the conflation of 'disposition' and 'choice' in the first quote from Eliezer.  Rachel does not envy Irene her choice at all.  What she wishes is to have the one-boxer's dispositions, so that the predictor puts a million in the first box, and then to confound all expectations by unpredictably choosing both boxes and reaping the most riches possible.)

To illustrate, consider (a variation on) Parfit's story of the threat-fulfiller and threat-ignorer.  Tom has a transparent disposition to fulfill his threats, no matter the cost to himself.  So he straps on a bomb, walks up to his neighbour Joe, and threatens to blow them both up unless Joe shines his shoes.  Seeing that Tom means business, Joe sensibly gets to work.  Not wanting to repeat the experience, Joe later goes and pops a pill to acquire a transparent disposition to ignore threats, no matter the cost to himself. The next day, Tom sees that Joe is now a threat-ignorer, and so leaves him alone.

So far, so good.  It seems this threat-ignoring disposition was a great one for Joe to acquire.  Until one day... Tom slips up.  Due to an unexpected mental glitch, he threatens Joe again.  Joe follows his disposition and ignores the threat.  BOOM.

Here Joe's final decision seems as disastrously foolish as Tom's slip up.  It was good to have the disposition to ignore threats, but that doesn't necessarily make it good idea to act on it.  We need to distinguish the desirability of a disposition to X from the rationality of choosing to do X.