How does airport security choose who to search?

People have been asking me about how security personal at the airport picks the people to screen more thoroughly. While there is standard screening rules, such as screening everyone who changes their flight times. I have been flying just about every week for the last 20 years and I have not observed nonverbal assessment screening rules being used.

I have noticed that screeners tend to pick what I call high complaints. That is people who appear nonverbally they will not give the screener a hard time. In fact when I mentioned to a friend who was a fireman at the Atlanta airport that I get asked to do the extra search all the time he was said he talked with screeners every day and they all say they choose high compliants. It makes sense. Why choose the big grumpily or scary guy to check when you can choose a short smiling blonde women. I mention this screening practice in my speeches and my audiences are filled with women who say they fit the compliant profile and are always given the extra screening. It may be a coincidence, but a scary one.