In a recent essay, called “In Defense of Profiling,” Sam Harris defends profiling. The basic idea is sound, even though he’s gotten some opposition. You look at some person and figure “Oh, that person is very unlikely to be a terrorist” based on some model or another, and ignore them. Then you look at another person and you go “Oh, that person is much more likely to be a terrorist … better check ’em out” and so on. If your concept of what makes a person more likely a terrorist is correct, then you will have a better chance of catching a terrorist, and it will take fewer resources to do so.