Re: Quantum Decision Theory

From: Wei Dai <>
Date: Tue, 14 Jan 2003 20:15:48 -0500

On Tue, Jan 14, 2003 at 11:20:26AM -0800, Tim May wrote:
> "The point here is that rational decision makers should choose actions
> on the basis of their _efficacy in bringing out desirable results_
> rather than their auspiciousness as harbingers of these results."
> (p. 150, "The Foundations of Causal Decision Theory," 1999.)
> Now in Hal's retelling of your model, Alice occasionally chooses
> non-optimal choices of rewards in a game of chance precisely because
> she hopes that other Alices in other worlds will do the same and that
> this will increase diversity in the Multiverse. But how can such
> behavior in other worlds affect _her_ world, the world where she made a
> sub-optimal choice? Isn't this the "good news" fallacy writ large?

You're misunderstanding Joyce, and Alice is not commiting the "good news"
fallacy. Alice is choosing "non-optimal" actions (I put that in quotes
because they ARE optimal in a global sense) and expects other Alices in
other branches to do the same, but she does not think that her choosing
"non-optimal" actions increases the probability that other Alices do the
same. Only the latter is a fallacy according to Joyce.

(I'm undecided on whether Joyce is correct on the issue of evidentiary vs.
causal decision theory, but that isn't really relevant here. BTW, I
recommended Joyce's book not because I think causal decision theory is
better than evidentiary decision theory, but because it's a good general
introduction to modern decision theory.)

> If Wei can find a way, no pun intended, to show that "quantum decision
> theory" produces different results OTHER THAN THOSE BASED ONLY ON THE
> BELIEF ITSELF, this would seem to contradict the "identical
> experimental predictions" and would be an important contribution.

You're missing the point that decision theory, as applied here, is a
normative theory. It tells you how you should reason and make decisions,
so it can't possibly produce different results other than "those based
only on the belief itself." (There is also positive decision theory, which
in contrast is the study of how people actually make decisions.)

Why is normative decision theory interesting? Because it's the only
satifactory way I know to explain what probabilities are and to justify
their use. Without probabilities, how can we say which positive theories
are more likely to be true than others?

The "fruit" thought experiment that Hal described a few posts ago is meant
to illustrate that decision theory needs to be updated because in its
current form (as related in Joyce's book), it's not compatible with
multiverse theories.
Received on Tue Jan 14 2003 - 20:16:39 PST

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