Last night, 60 Minutes did a story on prosopagnosia–otherwise known as face blindness. People with this condition, to varying degrees, cannot distinguish different faces. Sometimes, they can't even recognize family members.

Or villains.

I learned about face blindness several years ago and thought it would make an interesting plot angle in The Gods of Greenwich. But I wish I had seen this 60 Minutes segment before writing my novel. The upside-down-head test makes prosopagnosia far more tangible. Watch the show, and you'll see what I mean.