# Decision theories as heuristics

Main claims:

1.  A lot of discussion of decision theories is really analysing them as decision-making heuristics for boundedly rational agents.
2.  Understanding decision-making heuristics is really useful.
3.  The quality of dialogue would be improved if it was recognised when they were being discussed as heuristics.

Epistemic status: I’ve had a “something smells” reaction to a lot of discussion of decision theory. This is my attempt to crystallise out what I was unhappy with. It seems correct to me at present, but I haven’t spent too much time trying to find problems with it, and it seems quite possible that I’ve missed something important. Also possible is that this just recapitulates material in a post somewhere I’ve not read.

### Existing discussion is often about heuristics

Newcomb’s problem traditionally contrasts the decisions made by Causal Decision Theory (CDT) and Evidential Decision Theory (EDT). The story goes that CDT reasons that there is no causal link between a decision made now and the contents of the boxes, and therefore two-boxes. Meanwhile EDT looks at the evidence of past participants and chooses to one-box in order to get a high probability of being rich.

I claim that both of these stories are applications of the rules as simple heuristics to the most salient features of the case. As such they are robust to variation in the fine specification of the case, so we can have a conversation about them. If we want to apply them with more sophistication then the answers do become sensitive to the exact specification of the scenario, and it’s not obvious that either has to give the same answer the simple version produces.

First consider CDT. It has a high belief that there is no causal link between choosing to one- or two- box and Omega’s previous decision. But in practice, how high is this belief? If it doesn’t understand exactly how Omega works, it might reserve some probability to the possibility of a causal link, and this could be enough to tip the decision towards one-boxing.

On the other hand EDT should properly be able to consider many sources of evidence besides the ones about past successes of Omega’s predictions. In particular it could assess all of the evidence that normally leads us to believe that there is no backwards-causation in our universe. According to how strong this evidence is, and how strong the evidence that Omega’s decision really is locked in, it could conceivably two-box.

Note that I’m not asking here for a more careful specification of the set-up. Rather I’m claiming that a more careful specification could matter -- and so to the extent that people are happy to discuss it without providing lots more details they’re discussing the virtues of CDT and EDT as heuristics for decision-making rather than as an ultimate normative matter (even if they’re not thinking of their discussion that way).

Similarly So8res had a recent post which discussed [Newcomblike problems faced by people](/r/lesswrong/lw/l1b/newcomblike_problems_are_the_norm/), and they are very clear examples when the decision theories are viewed as heuristics. If you allow the decision-maker to think carefully through all the unconscious signals sent by her decisions, it’s less clear that there’s anything Newcomblike.

### Understanding decision-making heuristics is valuable

In claiming that a lot of the discussion is about heuristics, I’m not making an attack. We are all boundedly rational agents, and this will very likely be true of any artificial intelligence as well. So our decisions must perforce be made by heuristics. While it can be useful to study what an idealised method would look like (in order to work out how to approximate it), it’s certainly useful to study heuristics and determine what their relative strengths and weaknesses are.

In some cases we have good enough understanding of everything in the scenario that our heuristics can essentially reproduce the idealised method. When the scenario contains other agents which are as complicated as ourselves or more so, it seems like this has to fail.

### We should acknowledge when we’re talking about heuristics

By separating discussion of the decision-theories-as-heuristics from decision-theories-as-idealised-decision-processes, we should improve the quality of dialogue in both parts. The discussion of the ideal would be less confused by examples of applications of the heuristics. The discussion of the heuristics could become more relevant by allowing people to talk about features which are only relevant for heuristics.

For example, it is relevant if one decision theory tends to need a more detailed description of the scenario to produce good answers. It’s relevant if one is less computationally tractable. And we can start to formulate and discuss hypotheses such as “CDT is the best decision-procedure when the scenario doesn’t involve other agents, or only other agents so simple that we can model them well. Updateless Decision Theory is the best decision-procedure when the scenario involves other agents too complex to model well”.

In addition, I suspect that it would help to reduce disagreements about the subject. Many disagreements in many domains are caused by people talking past each other. Discussion of heuristics without labelling it as such seems like it could generate lots of misunderstandings.