# The Two Envelope Paradox and Using Variables Within the Expectation Formula

Sorites:135-140 (2008)

 Authors Eric Schwitzgebel University of California, Riverside Abstract You are presented with a choice between two envelopes. You know one envelope contains twice as much money as the other, but you don't know which contains more. You arbitrarily choose one envelope -- call it Envelope A -- but don't open it. Call the amount of money in that envelope X. Since your choice was arbitrary, the other envelope (Envelope B) is 50% likely to be the envelope with more and 50% likely to be the envelope with less. But, strangely, that very fact might make Envelope B seem attractive: Wouldn't switching to Envelope B give you a 50% chance of doubling your money and a 50% chance of halving it? Since double or nothing is a fair bet, double or half is more than fair. Applying the standard expectation formula, you might calculate the expected value of switching to Envelope B as (.50)½X [50% chance it has less] + (.50)2X [50% chance it has more] = (1.25)X. So, it seems, you ought to switch to Envelope B: Your expected return -- your return on average, over the long run, if you did this many times -- would seem to be 25% more. But obviously that's absurd: A symmetrical calculation could persuade you to switch back to Envelope A. Hence the paradox Keywords No keywords specified (fix it) Categories (categorize this paper) Options Mark as duplicate Export citation Request removal from index

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## Citations of this work BETA

Opening Two Envelopes.Paul Syverson - 2010 - Acta Analytica 25 (4):479-498.

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