Editor's note: Bruce Schneier is a security technologist and author of "Liars and Outliers: Enabling the Trust Society Needs to Survive."
(CNN) -- Terrorism causes fear, and we overreact to that fear. Our brains aren't very good at probability and risk analysis. We tend to exaggerate spectacular, strange and rare events, and downplay ordinary, familiar and common ones. We think rare risks are more common than they are, and we fear them more than probability indicates we should.
Our leaders are just as prone to this overreaction as we are. But aside from basic psychology, there are other reasons that it's smart politics to exaggerate terrorist threats, and security threats in general.
The first is that we respond to a strong leader. Bill Clinton famously said: "When people feel uncertain, they'd rather have somebody that's strong and wrong than somebody who's weak and right." He's right.
The second is that doing something -- anything -- is good politics. A politician wants to be seen as taking charge, demanding answers, fixing things. It just doesn't look as good to sit back and claim that there's nothing to do. The logic is along the lines of: "Something must be done. This is something. Therefore, we must do it."
The third is that the "fear preacher" wins, regardless of the outcome. Imagine two politicians today. One of them preaches fear and draconian security measures. The other is someone like me, who tells people that terrorism is a negligible risk, that risk is part of life, and that while some security is necessary, we should mostly just refuse to be terrorized and get on with our lives.
Fast-forward 10 years. If I'm right and there have been no more terrorist attacks, the fear preacher takes credit for keeping us safe. But if a terrorist attack has occurred, my government career is over. Even if the incidence of terrorism is as ridiculously low as it is today, there's no benefit for a politician to take my side of that gamble.
The fourth and final reason is money. Every new security technology, from surveillance cameras to high-tech fusion centers to airport full-body scanners, has a for-profit corporation lobbying for its purchase and use. Given the three other reasons above, it's easy -- and probably profitable -- for a politician to make them happy and say yes.
For any given politician, the implications of these four reasons are straightforward. Overestimating the threat is better than underestimating it. Doing something about the threat is better than doing nothing. Doing something that is explicitly reactive is better than being proactive. (If you're proactive and you're wrong, you've wasted money. If you're proactive and you're right but no longer in power, whoever is in power is going to get the credit for what you did.) Visible is better than invisible. Creating something new is better than fixing something old.
Those last two maxims are why it's better for a politician to fund a terrorist fusion center than to pay for more Arabic translators for the National Security Agency. No one's going to see the additional appropriation in the NSA's secret budget. On the other hand, a high-tech computerized fusion center is going to make front page news, even if it doesn't actually do anything useful.
This leads to another phenomenon about security and government. Once a security system is in place, it can be very hard to dislodge it. Imagine a politician who objects to some aspect of airport security: the liquid ban, the shoe removal, something. If he pushes to relax security, he gets the blame if something bad happens as a result. No one wants to roll back a police power and have the lack of that power cause a well-publicized death, even if it's a one-in-a-billion fluke.
We're seeing this force at work in the bloated terrorist no-fly and watch lists; agents have lots of incentive to put someone on the list, but absolutely no incentive to take anyone off. We're also seeing this in the Transportation Security Administration's attempt to reverse the ban on small blades on airplanes. Twice it tried to make the change, and twice fearful politicians prevented it from going through with it.
Lots of unneeded and ineffective security measures are perpetrated by a government bureaucracy that is primarily concerned about the security of its members' careers. They know the voters are more likely to punish them more if they fail to secure against a repetition of the last attack, and less if they fail to anticipate the next one.
What can we do? Well, the first step toward solving a problem is recognizing that you have one. These are not iron-clad rules; they're tendencies. If we can keep these tendencies and their causes in mind, we're more likely to end up with sensible security measures that are commensurate with the threat, instead of a lot of security theater and draconian police powers that are not.
Our leaders' job is to resist these tendencies. Our job is to support politicians who do resist.
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The opinions expressed in this commentary are solely those of Bruce Schneier.