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Probability
Rationality

Consider the following game that costs $2 to play: You roll a fair, six-sided die. You are awarded a $6 prize if, and only if, you roll a six; otherwise, you get nothing. Should you play the game? Well, considering the odds, the average payout - or "expected utility" - is (1/6)x($6)=$1, which is *less* than the $2 cost of playing. Therefore, since over many trials you would lose out, you should not play this game. That line of reasoning sounds OK. But let's say you are given a chance to play only once. What sort of bearing does this "average payout" argument have on this special "one shot" case? If you are in this for a single trial, it is not obviously irrelevant what the trend is "over many trials?"
Accepted:
January 24, 2008

Comments

David Papineau
January 25, 2008 (changed January 25, 2008) Permalink

Good question. My own view is that what happens in the long run is irrelevant to the rationality of betting (or in your case not betting) according to the odds in the single case. I think that it is a basic principle of practical rationality that your choices should be guided by the probabilities and that, surprisingly, there is no further justification for this.

A first point. You say 'over many trials you would lose out'. Well, if you are talking about a finite number of trials, that's not guaranteed. It is possible--indeed there will be a positive probability--that in a finite number of trials you will win even if you bet against the odds. All we can say it that the probability of winning over many trials is low. So now we are just back with the original problem. Why is it rational to avoid doing something just because the probability of success is low?

Does the situation change if we think about an infinite number of trials? Well, it's not even obvious that you are guaranteed to lose if you bet against the odds an infinite number of times. Of all the infinitely many sequences of results that might happen, there's a non-empty (indeed infinite) subset of sequences on which you win in the infinite long run. True, there is a 'zero probability' of any such sequence, even thought they are all possible (that's a nice puzzle in itself). But why take that 'zero probability' to be an argument for betting against these perfect possible sequences? Once more, this looks like the puzzle we started with.


Anyway, isn't there something odd from the start about long-run justifications of betting with the odds? Why is it an argument against betting on a 'six' now that something bad will happen if I do this lots of times (or even worse if I do it infinitely many times)? In the long run we will all be dead, as Keynes said.

Suppose I am a feckless fellow with no concern for tomorrow--I need some money now. Even so it is surely rational for me to bet with the odds. But you can't persuade me of this by saying that I need to bet with the odds in order to win in the long run. I don't care about the long run.

The more I think about it, the more it seems to me that betting with the odds is a basic principle of rationality, with no further justification.

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