Here's an awesome quiz brought to our attention by Rick Bookstaber: Linda is 31 years old, single, outspoken, and very bright. She majored in philosophy. As a student she was deeply concerned with issues of discrimination and social justice and also participated in anti-nuclear demonstrations.
Now which of two alternatives is more probable?
A) Linda is a bank teller
B) Linda is a bank teller who is active in the feminist movement
The vast majority of U.S. college students who were posed this question picked B, thus showing themselves to be illogical. ("Banker" is a large category that contains both "feminist banker" and "not-feminist banker".)
Bookstaber says B is the natural answer based on contextual thinking:
People are told in detail about Linda, and everything points to her being a feminist. In the real world, this provides the context for any follow up. We don't suddenly shift gears, going from normal discourse based on our day-to-day experience into the parsing of logic problems. Unless you are a logician or have Asperger's, the term “probable” is going to be taken as “given what I just described, what is your best guess of the sort of person Linda is”. Given the course of the question, the bank teller is extraneous information, and in the real world where we have a context to know what is extraneous, we filter that information out.