When truisms clash: Coping with a counterintuitive problem concerning the notorious two-child family

摘要

You know that a two-child family has a son. What is the probability that the family has two sons? And what is this probability if you know that the family has a son born on a Tuesday? The former question has been widely discussed previously. The latter adds a new puzzling twist to the situation. In both cases the answer should depend on the specifics of the assumed underlying procedure by which the given information has been obtained. Quantitative analysis, assuming one scenario, shows that the information on the son's day of birth changes the target probability. However, the relevance of being born on Tuesday to the question of the children's genders seems bizarre, since the same would be true for any other day. This apparent paradox is further probed in an attempt to alleviate the ensuing psychological difficulty.