I like magician Penn Jillette mainly because he is an outspoken atheist. In a nation where many people equate atheism with evil, he is refreshing. He puts a positive face on my belief system (or lack thereof). One day a few years ago, Jillette was going back stage after finishing a performance and was greeted by his mother. She approached him and he didn't know who she was. This is because he has a neurological disorder called prosopagnosia (or more common called face blindness).
People with prosopagnosia cannot recognize faces so if they see someone out of context, like seeing your mom backstage and not at home, they don't recognize them. They might not even recognize you if your facial expression changed. The condition was originally diagnosed in people who had brain damage thought to be caused by head trauma. Very recently a congenital form has been diagnosed. Some estimates are as high as 2.5% of the population. In other terms, if 200 of your Facebook friends were all in the same room, those 5 people who haven't said "hi" may not be rude. They may just have prosopagnosia. Neurologist Oliver Sacks, anthropologist Jane Goodall and ironically, portrait painter Chuck Close all have prosopagnosia.
When I was a kid I watched a lot of television with my dad. Just about every black man he saw on television that he didn't know he would say "Is that the guy from Barney Miller?" and "I'd say, Ron Glass? No, that's not him, it doesn't even look like him." To my knowledge my dad doesn't have prosopagnosia, but he could just be displaying own-race bias. We are simply far more likely to recognize people of our own race over others. The old comment, "they all look a like to me," might just have some legs to it. This is something to ponder if you are ever on a jury and someone from another race is being identified.