There is empirical evidence against the MAD hypothesis.
During the Cuban Missile Crisis, a Russian submarine believed that nuclear war had broken out. Three officers on board the submarine were authorised to unanimously launch a nuclear torpedo. An argument broke out among the three, in which http://en.wikipedia.org/wiki/Vasili_Arkhipov was against the launch, preventing a nuclear missile from being launched (and presumably a nuclear retaliation would not be unlikely).
Let’s assign a probability that one of the officers would be in favour of launching a missile. Given that 2⁄3 in our sample were in favour of launching the missile, let’s assign the probability to be 2⁄3 for any single officer. Therefore, the chance that a missile would have been launched is (2/3)^3 − 27%. Even if the probability was less than 2⁄3 (and people’s opinions are interdependent—one person could convince another, or one person could take a contrarian stand), nevertheless it still could have happened. And this wasn’t the only time that there almost was a nuclear war. (Incidentally I believe these cases show that an individual effort can at certain points have a huge impact—proof of both the butterfly effect or the ‘great man theory’ - as long as you redefine ‘Great Man’ to be a person in the right time and place).
As to the question of whether MAD worked—well it can be certainly argued to have helped, but it demonstrably did not prevent circumstances that could lead to a nuclear war.
I meant it didn’t eliminate all circumstances.
I think you’ve pointed out a flaw in my argument. The statement “MAD made nuclear war impossible” is demonstrably false—nuclear war could still happen with MAD. The statement “MAD prevented nuclear war” could still be true—nuclear war may have taken place (and probably would have been more likely) in the absence of MAD, and therefore MAD did prevent a nuclear war.