Editor's note: Bruce Schneier is a security technologist, and author of "Beyond Fear: Thinking Sensibly About Security in an Uncertain World." You can read more of his writing at http://www.schneier.com//

(CNN) -- At a security conference recently, the moderator asked the panel of distinguished cybersecurity leaders what their nightmare scenario was. The answers were the predictable array of large-scale attacks: against our communications infrastructure, against the power grid, against the financial system, in combination with a physical attack.

I didn't get to give my answer until the afternoon, which was: "My nightmare scenario is that people keep talking about their nightmare scenarios."

There's a certain blindness that comes from worst-case thinking. An extension of the precautionary principle, it involves imagining the worst possible outcome and then acting as if it were a certainty. It substitutes imagination for thinking, speculation for risk analysis and fear for reason. It fosters powerlessness and vulnerability and magnifies social paralysis. And it makes us more vulnerable to the effects of terrorism.

Worst-case thinking means generally bad decision making for several reasons. First, it's only half of the cost-benefit equation. Every decision has costs and benefits, risks and rewards. By speculating about what can possibly go wrong, and then acting as if that is likely to happen, worst-case thinking focuses only on the extreme but improbable risks and does a poor job at assessing outcomes.

Second, it's based on flawed logic. It begs the question by assuming that a proponent of an action must prove that the nightmare scenario is impossible.

Third, it can be used to support any position or its opposite. If we build a nuclear power plant, it could melt down. If we don't build it, we will run short of power and society will collapse into anarchy. If we allow flights near Iceland's volcanic ash, planes will crash and people will die. If we don't, organs won't arrive in time for transplant operations and people will die. If we don't invade Iraq, Saddam Hussein might use the nuclear weapons he might have. If we do, we might destabilize the Middle East, leading to widespread violence and death.

Of course, not all fears are equal. Those that we tend to exaggerate are more easily justified by worst-case thinking. So terrorism fears trump privacy fears, and almost everything else; technology is hard to understand and therefore scary; nuclear weapons are worse than conventional weapons; our children need to be protected at all costs; and annihilating the planet is bad. Basically, any fear that would make a good movie plot is amenable to worst-case thinking.

Fourth and finally, worst-case thinking validates ignorance. Instead of focusing on what we know, it focuses on what we don't know -- and what we can imagine.

Remember Defense Secretary Donald Rumsfeld's quote? "Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns -- the ones we don't know we don't know." And this: "the absence of evidence is not evidence of absence." Ignorance isn't a cause for doubt; when you can fill that ignorance with imagination, it can be a call to action.

Even worse, it can lead to hasty and dangerous acts. You can't wait for a smoking gun, so you act as if the gun is about to go off. Rather than making us safer, worst-case thinking has the potential to cause dangerous escalation.

The new undercurrent in this is that our society no longer has the ability to calculate probabilities. Risk assessment is devalued. Probabilistic thinking is repudiated in favor of "possibilistic thinking": Since we can't know what's likely to go wrong, let's speculate about what can possibly go wrong.

Worst-case thinking leads to bad decisions, bad systems design, and bad security. And we all have direct experience with its effects: airline security and the TSA, which we make fun of when we're not appalled that they're harassing 93-year-old women or keeping first-graders off airplanes. You can't be too careful!

Actually, you can. You can refuse to fly because of the possibility of plane crashes. You can lock your children in the house because of the possibility of child predators. You can eschew all contact with people because of the possibility of hurt. Steven Hawking wants to avoid trying to communicate with aliens because they might be hostile; does he want to turn off all the planet's television broadcasts because they're radiating into space? It isn't hard to parody worst-case thinking, and at its extreme it's a psychological condition.

Frank Furedi, a sociology professor at the University of Kent, writes: "Worst-case thinking encourages society to adopt fear as one of the dominant principles around which the public, the government and institutions should organize their life. It institutionalizes insecurity and fosters a mood of confusion and powerlessness. Through popularizing the belief that worst cases are normal, it incites people to feel defenseless and vulnerable to a wide range of future threats."

Even worse, it plays directly into the hands of terrorists, creating a population that is easily terrorized -- even by failed terrorist attacks like the Christmas Day underwear bomber and the Times Square SUV bomber.

When someone is proposing a change, the onus should be on them to justify it over the status quo. But worst case thinking is a way of looking at the world that exaggerates the rare and unusual and gives the rare much more credence than it deserves.

It isn't really a principle; it's a cheap trick to justify what you already believe. It lets lazy or biased people make what seem to be cogent arguments without understanding the whole issue. And when people don't need to refute counterarguments, there's no point in listening to them.

The opinions expressed in this commentary are solely those of Bruce Schneier.