The Two Envelope Problem a Mystifying Probability Paradox

There are two envelopes in front of you, and you know that one of them has ten times more money than the other. You pick randomly one envelope, but before taking it home, you are given the option to switch, and actually take the other envelope. Should you switch That was (one version of) the infamous twoenvelope paradox. It s a paradox, because there is a seemingly convincing argument for why switching is pointless (the two envelopes are symmetrical), but also a seemingly convincing mathematical argument for why switching increases your expected winning. This paradox has baffled some of the greatest minds, including the king of recreational mathematics, Martin Gardner. Unlike most other famous probability paradoxes the Monty Hall problem, the boy or girl paradox, and Simpson s paradox), the twoenvelope problem has a relatively intricate solution. At its heart, the solution is related to the limitations of the notion of expected value, and to an often overlooked caveat regarding conditioning