Bertrand's box paradox


Bertrand's box paradox is a veridical paradox in elementary probability theory. It was first posed by Joseph Bertrand in his 1889 work .
There are three boxes:
  1. a box containing two gold coins,
  2. a box containing two silver coins,
  3. a box containing one gold coin and one silver coin.
A coin withdrawn at random from one of the three boxes happens to be a gold. What is the probability the other coin from the same box will also be a gold coin?
A veridical paradox is a paradox whose correct solution seems to be counterintuitive. It may seem intuitive that the probability that the remaining coin is gold should be, but the probability is actually. Bertrand showed that if were correct, it would result in a contradiction, so cannot be correct.
This simple but counterintuitive puzzle is used as a standard example in teaching probability theory. The solution illustrates some basic principles, including the Kolmogorov axioms.

Solution

The problem can be reframed by describing the boxes as each having one drawer on each of two sides. Each drawer contains a coin. One box has a gold coin on each side, one a silver coin on each side, and the other a gold coin on one side and a silver coin on the other. A box is chosen at random, a random drawer is opened, and a gold coin is found inside it. What is the chance of the coin on the other side being gold?
The following reasoning appears to give a probability of ⁠1/2⁠:
  • Originally, all three boxes were equally likely to be chosen.
  • The chosen box cannot be box SS.
  • So it must be box GG or GS.
  • The two remaining possibilities are equally likely. So the probability that the box is GG, and the other coin is also gold, is ⁠1/2⁠.
The reasoning for the 2/3 is as follows:
  • Originally, all six coins were equally likely to be chosen.
  • The chosen coin cannot be from drawer S of box GS, or from either drawer of box SS.
  • So it must come from the G drawer of box GS, or either drawer of box GG.
  • The three remaining possibilities are equally likely, so the probability that the drawer is from box GG is ⁠2/3⁠.
Bertrand's purpose for constructing this example was to show that merely counting cases is not always proper. Instead, one should sum the probabilities that the cases would produce the observed result.

Experimental data

A survey of psychology freshmen taking an introductory probability course was conducted to assess their solutions to the similar three-card problem. In the three-card problem, three cards are placed into a hat. One card is red on both sides, one is white on both sides, and one is white on one side and red on the other. If a card pulled from the hat is red on one side, the probability of the other side also being red is.
53 students participated and were asked what the probability of the other side being red were. 35 incorrectly responded with ; only 3 students correctly responded with.

Related problems

Other veridical paradoxes of probability include:
The Monty Hall and Three Prisoners problems are identical mathematically to Bertrand's Box paradox. The construction of the Boy or Girl paradox is similar, essentially adding a fourth box with a gold coin and a silver coin. Its answer is controversial, based on how one assumes the "drawer" was chosen.