Prosopagnosia is the medical term for being face blind. This is an extremely interesting condition in which a person lacks the ability to recognize faces. I heard about this today in a Radio Lab episode. It seems all of the really interesting things that I learn about are from that show.
Anyway, being "face blind" means that you can't recognize people's faces. In extreme cases this can be as severe as not recognizing your own mother. If you were face blind, you could only recognize people by their hair style, or their voice, or if they always wore some identifying mark. It kind of reminds me of the boy from the Life of Pi not being able to recognize that plain faced man.
One of the most interesting thing about prosopagnosia is how pervasive it is. Some people estimate that as much as 3% of the population suffers from this condition. Three percent! That's so many! I think people are just embarrassed by not being able to recognize others that they try to hide this problem.
If you want to test yourself to see if you actually are face blind, you can take this free test. I took the first test which tested your ability to recognize unfamiliar faces. It was pretty difficult, but I am happy to report that I got 71 of the 72 questions right.
How did you do?