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In game theory, "guess ⁠2/3⁠ of the average", otherwise known as the Keynesian beauty contest, is a game that explores how a player’s strategic reasoning process takes into account the mental process of others in the game.

In this game, players simultaneously select a real number between 0 and 100, inclusive. The winner of the game is the player(s) who select a number closest to ⁠2/3⁠ of the average of numbers chosen by all players.

History

Alain Ledoux is the founding father of the guess ⁠2/3⁠ of the average-game. In 1981, Ledoux used this game as a tie breaker in his French magazine Jeux et Stratégie. He asked about 4,000 readers, who reached the same number of points in previous puzzles, to state an integer between 1 and 1,000,000,000. The winner was the one who guessed closest to ⁠2/3⁠ of the average guess. Rosemarie Nagel (1995) revealed the potential of guessing games of that kind: They are able to disclose participants' "depth of reasoning."

In his influential book, Keynes compared the determination of prices in a stock market to that of a beauty contest. The competitors had to pick out the 6 prettiest faces from 100 photos, and the winner is the competitor whose choices best matches the average preferences of all the competitors. Keynes observed that “It is not a case of choosing those that, to the best of one’s judgment, are really the prettiest, nor even those that average opinion genuinely thinks the prettiest. We have reached the third degree where we devote our intelligences to anticipating what average opinion expects the average opinion to be. And there are some, I believe, who practice the fourth, fifth and higher degrees.”

Due to the analogy to Keynes's comparison of newspaper beauty contests and stock market investments the guessing game is also known as the Keynesian beauty contest. Rosemarie Nagel's experimental beauty contest became a famous game in experimental economics. The forgotten inventor of this game was unearthed in 2009 during an online beauty contest experiment with chess players provided by the University of Kassel: Alain Ledoux, together with over 6,000 other chess players, participated in that experiment which looked familiar to him.

Equilibrium analysis

In this game, there is no strictly dominant strategy, but there are strongly dominated strategies. There is a unique pure strategy Nash equilibrium. This equilibrium can be found by iterated elimination of weakly dominated strategies.

Intuitively, guessing any number higher than 2/3 of what you expect others to guess on average cannot be part of a Nash equilibrium. The highest possible average that would occur if everyone guessed 100 is 66+2/3. Therefore, choosing a number that lies above ⁠66+2/3⁠ is strictly dominated for every player. These guesses can thus be eliminated. Once these strategies are eliminated for every player, 66+2/3 becomes the new highest possible average (that is, if everyone chooses 66+2/3). Therefore, any guess above ⁠44+4/9⁠ is weakly dominated for every player since no player will guess above ⁠66+2/3⁠, and ⁠2/3⁠ of ⁠66+2/3⁠ is ⁠44+4/9⁠. This process will continue as this logic is continually applied. With each step, the highest possible logical answer keeps getting smaller, until all numbers above 0 have been eliminated. If all players understand this logic and select 0, the game reaches its Nash equilibrium, which also happens to be the Pareto optimal solution.

However, this degeneration does not occur in quite the same way if choices are restricted to, for example, the integers between 0 and 100. In this case, all integers except 0 and 1 vanish; it becomes advantageous to select 0 if you expect that at least ⁠1/4⁠ of all players will do so, and select 1 otherwise. (In this way, it is a lopsided version of the so-called "consensus game", where one wins by being in the majority.)

Experimental results

This game is a common demonstration in game theory classes, where even economics graduate students do not guess 0. When performed among ordinary people it is usually found that the winner's guess is much higher than 0: the winning value was found to be 21.6 in a large online competition organized by the Danish newspaper Politiken. 19,196 people participated and the prize was 5000 Danish kroner.

Rationality versus common knowledge of rationality

This game illustrates the difference between perfect rationality of an actor and the common knowledge of rationality of all players. Even perfectly rational players playing in such a game should not guess 0 unless they know that the other players are rational as well and that all players' rationality is common knowledge. If a rational player reasonably believes that other players will not follow the chain of elimination described above, it would be rational for him/her to guess a number above 0.

We can suppose that all the players are rational, but they do not have common knowledge of each other's rationality. Even in this case, it is not required that every player guess 0, since they may expect each other to behave irrationally.

See also

• Keynesian beauty contest
• Unique bid auction
• Unexpected hanging paradox

Notes

1. ^ Coricelli, Giorgio; Nagel, Rosemarie (2009-06-09). "Neural correlates of depth of strategic reasoning in medial prefrontal cortex". Proceedings of the National Academy of Sciences. 106 (23): 9163–9168. doi:10.1073/pnas.0807721106. ISSN 0027-8424.
2. Duffy, John; Nagel, Rosemarie (1997-11-01). "On the Robustness of Behaviour in Experimental 'Beauty Contest' Games". The Economic Journal. 107 (445): 1684–1700. doi:10.1111/j.1468-0297.1997.tb00075.x. ISSN 0013-0133.
3. Ledoux, Alain (1981). "Concours résultats complets. Les victimes se sont plu à jouer le 14 d'atout" [Competition results complete. The victims were pleased to play the trump 14]. Jeux & Stratégie (in French). 2 (10): 10–11.
4. ^ Nagel, Rosemarie (1995). "Unraveling in Guessing Games: An Experimental Study". American Economic Review. 85 (5): 1313–26. JSTOR 2950991.
5. Maynard., Keynes, John (2018). The General Theory of Employment, Interest, and Money. Springer International Publishing. ISBN 978-3-319-70344-2. OCLC 1055269540.{{cite book}}: CS1 maint: multiple names: authors list (link)
6. Keynes, John M. (1936). The General Theory of Interest, Employment and Money. London: Macmillan. p. 156.
7. Duffy, John; Nagel, Rosemarie (1997). "On the Robustness of Behaviour in Experimental 'Beauty Contest' Games". The Economic Journal. 107 (445): 1684. doi:10.1111/j.1468-0297.1997.tb00075.x. JSTOR 2957901. S2CID 153447786.
8. Bühren, Christoph; Frank, Björn (2010). "Chess Players Performance Beyond 64 Squares: A Case Study on the Limitations of Cognitive Abilities Transfer" (PDF). MAGKS Joint Discussion Paper Series in Economics. 19–2010.
9. Bühren, Christoph; Frank, Björn; Nagel, Rosemarie (2012). "A Historical Note on the Beauty Contest" (PDF). MAGKS Joint Discussion Paper Series in Economics. 11–2012.
10. Nagel, Rosemarie; Bühren, Christoph; Frank, Björn (2016). "Inspired and inspiring: Hervé Moulin and the discovery of the beauty contest game" (PDF). Mathematical Social Sciences. 90: 191–207. doi:10.1016/j.mathsocsci.2016.09.001.
11. Nagel, Bosch-Domènech, Satorra, and Garcia-Montalvo, Rosemarie, Antoni, Albert and José (5 December 2002). "One, Two, (Three), Infinity, ...: Newspaper and Lab Beauty-Contest Experiments". AMERICAN ECONOMIC REVIEW. 92(5): 1687–1702 – via JSTOR.{{cite journal}}: CS1 maint: multiple names: authors list (link)
12. Schou, Astrid (22 September 2005). "Gæt-et-tal konkurrence afslører at vi er irrationelle". Politiken (in Danish). Retrieved 29 August 2017. Includes a histogram of the guesses. Note that some of the players guessed close to 100. A large number of players guessed 33.3 (i.e. ⁠2/3⁠ of 50), indicating an assumption that players would guess randomly. A smaller but significant number of players guessed 22.2 (i.e. ⁠2/3⁠ of 33.3), indicating a second iteration of this theory based on an assumption that players would guess 33.3. The final number of 21.6 was slightly below this peak, implying that on average each player iterated their assumption 1.07 times.

External links

• Short video explanation of the game by TED-Ed

• Non-cooperative games