What Lightman shows is what I first saw in news footage of Hinckley’s face ten seconds before he shot former President Reagan.
We consider it a warning, not a certainty that someone who shows this expression will soon attack. We have no way to find out how many times that expression is shown and no attack occurs (perhaps because a police officer pays noticeable attention to the potential attacker). We also cannot know how often attacks occur not preceded by this expression.
My research in five countries, two of them non-Western, strongly suggests that this expression is at least sometimes shown prior to a physical assault. We currently train security and law enforcement to be alert to it. We are continuing research to discover any other dangerous expressions.
Lightman tells Loker and Torres to look for hot spots. I use this phrase for signs of concealed emotions, or emotions that are not being concealed but don’t fit the words, or signs of thinking beyond what would be expected at that moment. Hot spots aren’t proof of lying or dangerous intent. Instead they mark the need to obtain more information to clarify why they were shown.
I discovered gestural slips in my very first study of nonverbal behavior, many years ago. In an experiment I arranged one of my fellow graduate students was being given a hard time by the head of the department. She gave him the finger, just as the best man, Obama, and Rumsfield did. Not out in the open, but much less noticeable. The person showing such a gestural slip is aware of the anger or disdain that he or she feels, but is unaware that the message has leaked out.