The new, invasive passenger screenings at airports are more than just expensive and unsettling. They decrease the chances of finding a problem and averting an attack!

The transportation security (TSA) government agents are tasked with detecting a very uncommon event--the one among millions of passengers who intends to do harm and who has the capacity to succeed. A notable new line of defense the TSA is rolling out, which is also its most expensive and ineffective one, is the physical examination of passengers at airport security checkpoints.

It is well known in psychology, but evidently unknown in government, that rare events--that is, very low frequency events like a passenger with explosive underwear--are very hard to accurately perceive. Watching passengers day after day after day, human screeners are very likely to miss seeing objects that very rarely appear. So the best way to make TSA agents ever less likely to succeed in finding such a threat is to make them screen huge numbers of passengers.

There's more. With such massive screening, there will also be many "false positives"--that is, cases in which a potential threat is identified but turns out to be not really a problem. The TSA agents will find harmless prosthetics, medical implants, deformities, illnesses, personal idiosyncrasies, and a whole host of similar variations. After extensive follow-up, the people thus singled out will be allowed through to the airplane. But the TSA agents will see so many cases of these false alarms, these false positives, that it will interfere further with their ability to recognize the true threats.

I would be perfectly happy to fly on a plane sitting next to Janet Napolitano (the US Secretary of Homeland Security) even if she had not been x-rayed and patted down. I would also be fine flying unprotected with Brad Pitt, Lady Gaga (though I probably wouldn't recognize her if she were not in costume), Sarah Palin and even my congressman. Come to think of it, I would be perfectly happy to get on an airplane in the USA and fly alongside millions of fellow Americans whom the TSA knows are not on any "watch lists" and have nothing at all in their pre-flight reservation screening to suggest that they pose much of a risk. I know, someday I might wind up sitting next to a wealthy grandmother, a church elder who has decided to join al qaeda and has secreted explosives up her derriere. But I'd rather go that way than being run off the highway by a drunken 18-year-old in a Chevy Camaro who could run rampant after the highway patrol police were down-sized due to budget cuts.

So we now have a perfect TSA storm: a vanishingly small chance of encountering a passenger with exploding underwear, a high rate of false alarms, and a significant misallocation of resources.

One of the secrets of longevity is rational application of resources to risk. Wearing a seatbelt is a rational response to risks of riding in a car, but trading your sedan for a tank to drive down the highway is going too far. Screening all passenger airplane luggage for bombs, using chemical sensors, bomb-sniffing dogs, and x-rays is a cost-effective means of protection, but x-raying so many passengers with body scanners and then squeezing their underwear is worse than ineffective. It means that the TSA agents will be more likely to miss real threats to security. If you know people in government, please explain this to them. And tell your friends.