Recent reporting on the weakness of behavioral profiling in airports has overlooked a key dimension of the problem with it.

According to this story in USA Today, interviewing or patting down 160,000 people with (unreported) indicia of suspicion at airports has resulted in 1,266 arrests. It has failed to find wrongdoing 99.3% of the time. Occasionally, investigations based on behavioral profiling have turned up such things as drug possession and the use of fake identification.

Behavioral profiling has never turned up someone planning harm to aviation security. It has never turned up a person with weapons, guns, bombs, or any other implement that would cause a flight to be delayed, much less brought down.

A 0.7% success rate in finding crime is not relevant. Behavioral profiling has a 0% success rate in finding threats to aviation. Behavioral profiling does not have a proximate relationship to securing against harm coming to commercial aviation.

The Fourth Amendment requires searches and seizures to be reasonable. Courts give law enforcement considerable leeway and often use the stamp “experienced officer” to grant the police broad authority to follow hunches. What we have here, though, is a basis for suspicion that has a 100% failure rate. It never finds what it is looking for.

It may be argued that the consequence of an aviation security breach is so great that behavioral profiling, despite its failings, is “reasonable.” But this argument proves too much.

If national security authorities developed a theory that vans with dented doors are likely to carry nuclear materials, this reasoning would allow the search of any van with dented doors. The consequence of a nuclear blast, of course, is thousands of times higher than an attack on aviation. But a wrong theory is still a wrong theory. The fact that searching vans for nuclear weapons turns up stolen goods 0.7% of the time would not save it. Arguing for the leeway to use a false basis for suspicion because of the size of the potential danger is simply a cleverly cloaked argument for a general warrant, which the Fourth Amendment prohibits.

In the future, there are likely to be more cases where statistical probabilities replace such things as the hunches of “experienced officers” in Fourth Amendment jurisprudence. It is important to remember that suspicion properly arises from observing behaviors that are both consistent with unlawful behavior (the part people remember) and inconsistent with lawful behavior (the part people often forget).

Exhibiting stress in airports — a likely part of behavioral profiling — is consistent with terrorism planning, yes, but it is also consistent with: arriving late, disagreeing with a travel companion, missing a flight, feeling sick, missing loved ones, being disorganized, fearing the security bureaucracy, and so on, and so on, and so on. There is not a rational relationship between exhibiting stress in airports and threats to aviation security. (A discussion of these concepts in the data mining context appears on page 9 of my paper with Jeff Jonas, “Effective Counterterrorism and the Limited Role of Predictive Data Mining.”)

Behavioral profiling is an unreasonable basis for search and seizure. Any arrest based on it is in violation of travelers’ constitutional rights.