What's the difference between causal and evidential decision theory?

1two schools

Decision theory asks: how should a rational agent pick actions? Two schools disagree about what 'best outcome' even means.

2causal decision theory

CDT says: pick the action that physically causes the best result. Trace the arrows of cause and effect forward from your choice.

3evidential decision theory

EDT says something different: pick the action whose news would be the best evidence about how things already are — even without causing it.

4Newcomb's problem

Newcomb's problem shows the split. A predictor filled two boxes based on a forecast of your choice, made before you decide.

5the split

CDT says: take both boxes — your choice now can't reach back and change what's already inside. EDT says: take only B — that choice is the best news you'll ever get about being a million-dollar winner.

6a simpler case

Let's try something everyday instead of boxes. Notice: people who wake up early tend to be more successful. Should you force yourself to wake up early?

7hidden cause

Here's the twist: it's not waking early that causes success. A hidden trait — ambition — causes both. Ambitious people happen to wake early AND happen to succeed.

8CDT's answer

CDT looks at real causation: forcing yourself to wake early, with no ambition, causes nothing extra — so CDT says don't bother just for that reason.

9EDT's answer

EDT looks at correlation as evidence: if you find yourself waking up early, that's good news — it's the kind of thing successful, ambitious people do — so EDT treats it as a favorable sign.

10the real split

So the two theories split exactly on whether an action's link to a good outcome must be causal, or just a reliable clue — that's the whole disagreement.

11back to the boxes

Same logic, just dressed as boxes. The predictor is like ambition — a hidden cause that already set things in motion before you act.

12already decided

The predictor already scanned your mind and filled Box B days ago — just like ambition already shaped whether you're an early riser, long before today.

13CDT: take both

CDT reasons the same way as before: taking both boxes can't causally change what's already sitting inside Box B, so grab both — free extra money.

14EDT: take B only

EDT reasons like the waking-early case: if you're the type of person who one-boxes, that's strong evidence the predictor foresaw it and loaded Box B with the million.

15same structure

So the predictor plays exactly the role ambition played — a hidden cause behind both your action and the outcome — and that's why the two examples teach the same lesson.

16the setup

Let's slow way down and lay out the setup itself, step by step, before any decision theory touches it.

17the predictor

There's a Predictor — imagine a superintelligent being who has studied you closely and is almost always right about what you'll choose.

18box A: guaranteed

Two boxes sit in front of you. Box A is clear — you can always see one thousand dollars inside it, guaranteed.

19box B: sealed

Box B is opaque. It either has one million dollars, or it's empty — and here's the key: it was filled BEFORE you make your choice.

20the choice

You have exactly two options: take only Box B, or take both boxes. The catch — the predictor already decided what to put in B, based on predicting which option you'd pick.

21the rule filled in

The rule: if the predictor foresaw you'd take only B, it put a million inside. If it foresaw you'd take both, it left B empty — so you'd only get the thousand.

22the puzzle itself

So the puzzle is: knowing that rule, and that the box is already sealed one way or the other, which do you pick — that's where CDT and EDT gave different answers.

23the setup

This is the classic case built to attack EDT. Imagine a genetic lesion that causes both a craving to smoke AND lung cancer — smoking itself is harmless.

24no direct link

Notice: there's no arrow from smoking to cancer. The lesion is the hidden common cause of both — just like ambition caused both waking early and success.

25EDT gets it wrong

EDT looks only at correlation: smokers get cancer more often, so smoking is 'bad news' — EDT wrongly tells you not to smoke, even though smoking causes nothing.

26CDT gets it right

CDT looks at actual causation: smoking doesn't cause cancer here, only the lesion does — so CDT correctly says smoke freely if you enjoy it, it changes nothing.

27the reversal

So this problem flips the earlier score: in Newcomb's box EDT looked smarter, but here CDT clearly wins — which is exactly why philosophers still argue over which theory is right overall.