Chances are, you'll snag the wrong people, and when you do, how can you tell? How do you clear suspects of crimes that haven't happened?
Pre-crime prevention is a terrible idea.
Here is a quiz for you. Is predicting crime before it happens: (a) something out of Philip K. Dick's Minority Report; (b) the subject of of a Department of Homeland Security research project that has recently entered testing; (c) a terrible and dangerous idea which will inevitably be counter-productive and which will levy a high price in terms of civil liberties while providing little to no marginal security; or (d) all of the above.
If you picked (d) you are a winner!
The U.S. Department of Homeland security is working on a project called FAST, the Future Attribute Screening Technology, which is some crazy straight-out-of-sci-fi pre-crime detection and prevention software which may come to an airport security screening checkpoint near you someday soon. Yet again the threat of terrorism is being used to justify the introduction of super-creepy invasions of privacy, and lead us one step closer to a turn-key totalitarian state. This may sound alarmist, but in cases like this a little alarm is warranted. FAST will remotely monitor physiological and behavioral cues, like elevated heart rate, eye movement, body temperature, facial patterns, and body language, and analyze these cues algorithmically for statistical aberrance in an attempt to identify people with nefarious intentions. There are several major flaws with a program like this, any one of which should be enough to condemn attempts of this kind to the dustbin.
Thanks to Mano Singham.