timtyler comments on Decision Theories: A Less Wrong Primer - Less Wrong

69 Post author: orthonormal 13 March 2012 11:31PM

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Comment author: timtyler 12 March 2012 06:42:31PM *  0 points [-]

But there is one key property that distinguishes CDT from the decision theories we'll talk about later: in its modeling of the world, X only allows event A to affect the probability of event B if A happens before B. (This is what causal means in Causal Decision Theory.)

This leads to the assumption that X's decision is independent from the simultaneous decisions of the Ys- that is, X could decide one way or another and everyone else's decisions would stay the same.

That doesn't seem to follow.

It is scientifically conventional to have the past causing the future.

However, decisions made by identical twins (and other systems with shared inner workings) aren't independent. Not because of some kind of spooky backwards-in-time-causation, but because both decisions depend on the genetic makeup of the twins - which was jointly determined by the mother long ago.

So: this "independence" property doesn't seem to follow from the "past causality" property.

So: where is the idea that CDT involves "independent decisions" coming from?

Comment author: orthonormal 13 March 2012 04:50:40AM 0 points [-]

You know, you're right. The independence assumption doesn't follow from time-causality; it's the main assumption itself. (X's programmer writing a CDT agent is a past cause of both the prediction and the action.) I'll fix the post.

Comment author: timtyler 13 March 2012 01:27:00PM *  2 points [-]

Thanks. I was interested in where the "independent decisions" idea comes from. This page on Causal Decision Theory suggests that it probably came from Robert Stalnaker in the 1970s - and was rolled into CDT in:

  • Gibbard, Allan and William Harper. [1978] 1981. “Counterfactuals and Two Kinds of Expected Utility.”
Comment author: [deleted] 12 March 2012 08:18:13PM 0 points [-]

However, decisions made by identical twins (and other systems with shared inner workings) aren't independent. Not because of some kind of spooky backwards-in-time-causation, but because both decisions depend on the genetic makeup of the twins - which was jointly determined by the mother long ago.

Then again, in the chewing-gum variant of the smoking lesion problem, your decision whether to chew gum and your genetic propensity to get throat abscesses aren't independent either. But everybody would agree that choosing to chew is still the right choice, wouldn't they?

Comment author: timtyler 12 March 2012 08:30:27PM *  0 points [-]

I don't think that affects my point (which was that considering decisions made by different agents to be "independent" of each other is not a consequence of common-sense scientific causality). The idea seems to be coming from somewhere else - but where?

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