This Post Could Save Your Life

Since I like to blog about things that have practical value, this post is about how to survive the situation where there are two identical cups of something, possibly wine or a speckled pill, and one of the two cups contains a lethal poison. Now I know you may be thinking, “The odds of that happening to me are incredibly low.” I, however, have watched both The Princess Bride and the first episode of Sherlock, and I can tell you that based on what I’ve seen, the odds of this happening to you are actually 100%. So if you aren’t prepared, you will never achieve your lifelong dream of dying peacefully, at age 87, of natural causes. I am not making any assumptions or judgments here; if, instead of being the victim, you would like to try murdering someone in this fashion, this post will also be very useful.

It is obvious that the people behind the two films mentioned above never figured out the solution to this problem. They just thought it was an interesting problem. In The Princess Bride, as everyone knows, both cups are poisoned, and in Sherlock the answer is not revealed. Furthermore, the answer is weird and surprising. If you are the victim, you should always drink from your own cup, and if you are the murderer, you should (almost) always put the poison in your own cup.

Let me explain why. This way, you will see that I’ve read several books on game theory*, and probably deserve a MacArthur Grant. Honestly, if someone asked me right now why I continue to live in the United States, rather than moving to a stable developing country where the cost of living is lower, or else to a more sanely governed country in Europe, or else to Canada, I would say: “Because of the possibility of winning a MacArthur Grant. Also, my security deposit.”

This situation always involves either three or four moves, which are played out in the minds of the murderer and his victim. The first move is to put the poison in the victim’s cup. This is too obvious. The second move is to put the poison in your own cup, as a bluff. This is also pretty obvious — which is why the third move is to put the poison in the victim’s cup. The final move, because poisoning the victim’s cup is a childish double-bluff, is to put the poison in your own cup.

Of course, it’s tempting to imagine that the game continues from there, becoming an infinite series of levels, with the poison changing places each time. It does not. If you were to poison the victim’s cup as a “fifth” move, your reasoning would be exactly the same as it was on the third move, so the game is exhausted. If you could figure out exactly how many moves your opponent was thinking ahead, you could of course win by going one further, but that is always going to be impossible and is therefore not relevant.

The murderer has one of two temperaments: straightforward or sneaky. A straightforward murderer makes the third move, putting the poison in the victim’s cup. A sneaky murderer makes the fourth move, putting the poison in their own cup. This goes back to the fact that the position of the cups is not neutral: they are not lined up side by side and marked “A” and “B.” Instead, by custom, each player is in possession of one cup, from which, in a better world, they would both happily drink. In order to drink from the murderer’s cup, the victim has to reach across the table, which is psychologically difficult since (a) it violates custom, which is risky whenever pure savagery is being constrained, just barely, by rules, and (b) it means believing that the murderer has not made the “coolest” available move, despite choosing a method of killing that is clearly aesthetically motivated.

However, both Vizzini and Sherlock do exactly this. They reach across the table. In fact, Vizzini is so sure that the Man In Black will abandon the game if he reaches across the table that he falls back on the old “look over there” ploy. They fail to realize that the resistance anyone would feel to reaching across the table is precisely what makes reaching across the table, to a personality like theirs, irresistible. In fact, even if the victim has read this post, and the murderer knows they have read it, and they’ve discussed it previously over tea, a victim like Sherlock will still choose the murderer’s cup after rationalizing their way to the irrelevant “fifth” move, or to some other distant relative of the fifth move. The only time to poison the victim’s cup is when the victim is so psychologically broken, so resigned to death, that they will drink whatever is in front of them and hope for the best. (Most such victims live, as the murderers call off the game out of boredom.)

The explanation is even easier from the victim’s perspective. Is your murderer straightforward (Move 3) or sneaky (Move 4)? Sneaky. They are, after all, a murderer of an odd, analytical persuasion. Their sneakiness — their desire to trap you with the bait of reaching across the table — will win. Regardless of whether they’ve read this post, their dominant inclination will emerge victorious once Gladwellian “thin-slicing” sets in, as it does whenever deciding between two alternatives presents insuperable logical difficulties.

So here’s to surviving. Live long and prosper.


*Not true.