The naked body scanners are taking over.

When we first checked in on them two years ago, the scanners, which see through clothing, were being deployed at a single airport. A few months later, they were upgraded to millimeter-wave technology, which delivered similar images with even less radiation—”10,000 times less than a cell phone transmission,” according to the Transportation Security Administration. At the time, TSA assured us that the scanners would be used only as a “voluntary alternative” to “a more invasive physical pat-down during secondary screening.” Only a few passengers, the ones selected for extra scrutiny, would face the scanners. The rest of us could walk through the metal detectors and board our planes.

Surprise! Two months ago, TSA revised its position. It began testing millimeter-wave scans “in the place of the walk-through metal detector at six airports.” At these airports, everyone—not just people selected for secondary screening—would face the see-through machines. Anyone who objected would “undergo metal detector screening and a pat-down.” You might even get the “enhanced pat-down,” which includes “sensitive areas of the body that are often used by professional testers and terrorists,” such as “the breast and groin areas of females and the groin area of males.” Show us your body, or we’ll feel you up.

Now the plan is going nationwide. Joe Sharkey of the New York Times reports that TSA “plans to replace the walk-through metal detectors at airport checkpoints with whole-body imaging machines—the kind that provide an image of the naked body.” All passengers will “go through the whole-body imager instead of the walk-through metal detector,” according to TSA’s chief technology officer, and the machines will begin operating soon after orders are placed this summer.

When the scanners first appeared, I endorsed them. When they were upgraded to millimeter-wave technology, I endorsed them again. I gave two reasons. One reason was that a scan was less invasive than a pat-down. The other reason was that TSA promised to blur your face and keep your scan private, so that nobody would ever connect your name to your revealed body. That, I argued, was a sufficient kind of privacy in the age of terrorism.

Now I’m having second thoughts. I still like the technology. It’s the people behind it who worry me. Yes, the scan is less invasive than the pat-down. But TSA has just demonstrated its ability and willingness to move the goalposts. When TSA offered pat-downs as the alternative to body scans in secondary screening, the scan sounded pretty good. Now TSA is offering pat-downs as the alternative to body scans in primary screening, and again, the scan sounds better. And if TSA announces tomorrow that pat-downs are the new alternative for all train or bus passengers, body scans will seem preferable there, too. Anywhere we’re threatened with pat-downs, we’ll settle for body scans. Where does it end?

And what about the content of the scans? Two years ago, I linked to a scan that seemed to expose every intimate body contour of TSA’s research lab director. TSA argued that the picture was moot because its machines (which at the time used backscatter technology) had been upgraded with a “privacy algorithm” to obscure such features. But you won’t find the phrase privacy algorithm on that page anymore; it’s been scrubbed. In fact, privacy algorithm has completely disappeared from TSA’s Web site. So have the images that used to show a frontal backscatter image of a male passenger. All you can find on TSA’s millimeter-wave page are four scans shrunk to a size so tiny you’d need a magnifying glass to make sense of them. Good luck figuring out how much they show—and why they look nothing like the image depicted in a video (WMV file) on the TSA site.

Why should I care what the government says or depicts about its latest scanner image or blurring technology, when the technology and the depictions keep changing? The lesson of the escalating body scans, like the escalating pat-downs, is that TSA will do whatever it thinks it needs to do. Last year, when the agency announced its “enhanced” pat-downs, it explained:

As the ongoing terror trial in London clearly illustrates, terrorists actively look for ways to manipulate security protocols. Intelligence has also shown for decades, terrorists’ manipulation of societal norms to evade detection or use social engineering techniques to their advantage. Terrorists have successfully hidden explosives in these areas. … TSA developed this pat down as a measure to close the gap on items hidden on sensitive areas of the body.

In other words, any detail omitted by airport screeners—a blurred crotch in the body scan, an untouched groin during the pat-down—becomes a “gap” exploited by terrorists or testers, which must then be closed.

“The enhanced pat-down will be used only after all other screening methods have been used and the alarm remains unresolved,” TSA promised last year. It added: “This new procedure will affect a very small percentage of travelers.”

Yeah, yeah, yeah. That’s what you said about the body scans. Just put on the gloves and get it over with.

(Now playing at the Human Nature blog: 1. Poverty, biology, and intelligence. 2. A black market in children. 3. Repossessing cars by remote control.)