Mark A.R. “15″ Kleiman makes a great point regarding my is-this-an-intelligence-failure post:

Ackerman’s analysis misses a key point: you’re not limited to one level of screen. A false positive on a mammogram is a bad thing, but its immediate result is an unnecessary biopsy, not an unnecessary mastectomy. In general, you want the first screen to be cheap to administer and very tight (”highly sensitive” in the technical jargon), accepting that it will produce a big crop of false positives, because the result of triggering that alarm is a follow-up test that can be much more expensive but is designed to be much less prone to false positives (”highly specific”).

There was no need to decide, just based on the information in hand, whether to let Mr. Abdulmutallab board the flight. All you needed to figure out was that he needed to have a body scan and a careful hand-luggage check before boarding. You might not want to do that to every passenger, but you’d be willing to do it to tens of millions of innocents to prevent one explosion.* Thought of that way, I’d say that the warning from Abdulmutallab père should have been enough, all by itself, to justify asking Abdulmutallab fils to step out of line and see the nice man in the booth.

I could quibble with this, but really, this is basically the balance that I’ve been vaguely pointing to. I’ll buy it. The only thing I’d say is that there may be a point at which we’re talking mastectomy (no-fly) passing itself off as biopsy. Because you can imagine some circumstance whereby a terrorist beats even a full-body scan. Let’s say he’s a bodybuilder and grabs a fellow passenger, threatens to snap his neck, and parlays that into control of the aircraft. (I know this is baroque, but so is underpants-bombing.) At that point, the congressional inquiry to follow asks — as the media has been asking in Abdulmutallab’s case — why was Jihadi The-Body Ventura allowed on the plane in the first place?

This isn’t to say I don’t buy Mark’s point. I do. But to some degree, homeland security fixes are answers that defer a fundamentally political problem: how much risk of terrorist attack are we willing to accept as a society? I don’t pretend to know how to quantify the answer, but it’s, uh, somewhere between Zero and One Hundred. There is a lot to be said for not facing up to the question. No-fly lists and airport scans and cargo-hold checks and so forth have the virtue of helping keep people alive at relatively low cost. I like the idea of technological ingenuity creating ever-better prophylactics against attack instead of saying, “Well, we just have to live with the occasional thing blowing up…” or even “that sucked; but as long as we have Hydrocarbon Man, we’re going to be backstopping some odious fucking dictatorships in the Middle East and people are going to be pissed off; a shame that conservatives are standing in the way of ending the era of Hydrocarbon Man…”

But at some point we reach the liberty-tradeoff point of absurdity, and it wouldn’t be such a bad thing to try and think through the point at which we want Security and Liberty to converge.