Excerpted from Rational and Irrational Thought:
The Thinking That IQ Tests Miss.
1. Jack is looking at Anne, but Anne is looking at George. Jack is married, but George is not. Is a married person looking at an unmarried person?
C) Cannot be determined
More than 80 percent of people choose C.
But the correct answer is A.
Here is how to think it through logically: Anne is the only person whose marital status is unknown. You need to consider both possibilities, either married or unmarried, to determine whether you have enough information to draw a conclusion. If Anne is married, the answer is A: she would be the married person who is looking at an unmarried person (George). If Anne is not married, the answer is still A: in this case, Jack is the married person, and he is looking at Anne, the unmarried person.
This thought process is called fully disjunctive reasoning—reasoning that considers all possibilities. The fact that the problem does not reveal whether Anne is or is not married suggests to people that they do not have enough information, and they make the easiest inference (C) without thinking through all the possibilities.
Note: Interestingly, if option C were not included, fully disjunctive reasoning would be forced to kick in to solve the problem between choices A and B only. The presence of C allows for an easy out, and most people take it — which is a lot like life.