So what I am saying is: you are betting against whether the Alien can in fact predict the future. When the bastard of an alien takes a hike, he either has the content in the boxes reflect the choice you are about to make (in which case it's best to pick box B alone) or you are the 1st out of 101 cases where his prediction does not reflect your choice, in which case it would be best to pick both.
The point is that once the alien has buggered off it doesn't
matter what your
actual decision is. The contents of box B are already determined.
Given that, there are a number of options.
Firstly, let's say that the alien has predicted you take box B. In that case, there will be one million dollars in B and one thousand dollars in A. If you take just box B you will win a million dollars. If you take box A and B you will win $1,001,000. Your actual action bears no consequence on what is in the boxes.
Let's say the alien has predicted you take both boxes. In this case there will be no money in box B and $1000 in box A. Consequently taking only box B will lead to you being zero dollars richer, whilst taking both box A and box B will lead to you being $1000 richer.
We can write this in matrix form.
*********Just B**********Both
Just B****1,000,000*********0*
***************************
***************************
Both*****1,001,00******1,000**
Where the horizontal axis represents the alien's prediction and the vertical represents your actual behaviour. As we can see, 'both'
strictly dominates 'just B'; the outcome is better every time.
It's perhaps clearer in the form of a sequential tree graph:
*****************/Just B (0)
****************/
****Both----------
**/*************\
*/***************\ Both (1,000)
*****************
(1)*************(2)
******************
*\****************/Just B (1,000,000)
**\**************/
***Just B-----------
*****************\
******************\Both (1,001,000)
The Alien is 'player one' (1). He plays either 'Both' or 'Just B'. You are player two (2) and play either both or just B. You do not know what the alien has played and nor can you affect the aliens play; your 'move' comes after that of the alien. That is, you don't know if you are playing with the top branch sub-game or the bottom branch sub-game. But again, we see that choosing Just B gives you an average pay-off of 500,000 whilst choosing Both gives you an average payoff of 501,000. And choosing Both is superior to choosing Just B in both games; you gain a higher payoff in both. Again Both strictly dominates just B because the game is sequential; your actions as player two do not affect those of player one, because his 'move' is before yours. The contents of box B are not decided by your actions.