The following dilemma may be posed to you at the beginning of Baldur's Gate II:

Suppose that an evil wizard has captured you and someone else and locked you in separate rooms, each with a button. You can either press the button within the next hour or not. If you press the button and the other person doesn't, they will live and you will die. If they press the button and you don't, you will live and they will die. If you both press the button, or if neither of you presses it, you will both die. You do not have a chance to discuss your choice with each other beforehand. What do you do?

In Baldur's Gate II, the other person was your sibling, but it's slightly easier to analyse if you assume that they are a random stranger and you are indifferent to whether they live or die. Then you might think that you should not press the button, because if you do, then you will die no matter what the other person does. But if they think that way as well, you will both refrain from pressing your buttons and you will both be killed. On the other hand, if you both decide to be heroic and lay down your life for the other one, you will still both be killed.

We can look at this using the concept of super-rational thinking described in Metamagical Themas. Super-rational thinking means that you reason:

"There is a right thing to do here (as in maximising the expected value of the result, not as in ethical). I don't know what it is yet, but because the other person and I are both reasoning beings, and the situation is symmetrical (nothing changes if you swap our roles), I know that we'll come to the same conclusion about what it is."

That doesn't seem to work here, because if you both do the same thing you get the worst possible result. That's because we've ignored the possibility that rather than choosing to press the button or not, you might use a randomising device and elect to press it with probability p. Then, given that you both use this strategy, your chance of survival is p(1-p), which is maximised when p=0.5.

Therefore if you are both focused on your own survival, you would do best to flip a coin. This is also the best strategy if you each believe the other person's survival to be infinitely more important than your own. It even works if you both think something like "I value her life more than mine, but I am indifferent between a 90% chance of survival for her and a 100% chance of survival for me", so long as it's the same something and you both know it's the same something. (If the other person is your enemy, you want her to die and you know she feels the same way about you, you can still apply super-rational thinking but the results will be different.)

Unfortunately, this strategy breaks down if you value her life more than she values yours or vice versa, because then the situation is no longer symmetrical. You might also wonder whether she knows about super-rational thinking - it doesn't work unless you both do it.

Obviously what Maestrom says is correct. If your goal is to live, then the only correct thing to do is not to press the button. This seals the other guy's fate, and your own life is in his unknowing hands.

If you care about the other person, then it gets more complicated. Without knowing anything about the other person's reasoning or values, all you can know is that

1. If you push the button, you will surely die.
2. If you flip an unbiased coin to decide, there is precisely a 50% chance of one of you surviving.

There is a very practical problem with 2. Once you flip the coin, if it says to push the button, you know that you are dead, and you still have no idea what the probablity of the other person's survival is. It could be 0, it could be 1, or anything in between. It is undefined. But your own survival probability is now 0. The information you now have is more disconcerting than when you started. So your inclination would be to renege on the coin, which negates the effect of flipping an unbiased coin and invalidates proposition 2.

So what is the correct thing to do? That depends what your number one priority is. If it is for yourself to live, absolutely don't press the button. If it is for the other guy to live, then absolutely press the button. If it is for one of you to live, then it depends on whether or not you can second-guess the other person. Use a coin if you are afraid you will guess wrong with a probability of greater than one half.

To me the problem is of a different nature. To maximize the greatest good for the greatest number, neither you nor your sibling (or total stranger) should press the button.

As was stated in the original dilemma, the wizard who is holding you is evil. Assuming you are good, or more importantly in Baldur's Gate (I or II), if you are lawful good you cannot press the button. By your devotion to law and the good, you realize that the very proposition that you would participate in this evil act would trigger your higher brain functions to think about the proper course you should choose. Your first thought may be that sacrificeing yourself would be the greatest good, but it is not. If you and your sibling both refuse to press the button, neither of you has done anything wrong. In fact, you've kept both of you alive for the moment. The responsibility for your act lies with the evil wizard, and not with you or your sibling. You are simply lawful good, you are not neccessarily a hero. And what would you gain by being a hero, other than death? You would gain a lifetime of guilt for your sibling, and she might go on to exact revenge (which is not considered good in our world, and its inherent goodness in D&D is open for discussion). The point is, if you are both concerned with the greatest good, you would not submit to an evil act, and would remain alive until the end of the hour, when the wizard returns to kill you both. But his act is his evil, whereas the evilness of suicide is debatable.

So if you are good, you cannot press the button. You must allow the wizard to be evil, while you and your sibling uphold the good.