We spend far more effort defending our countries against specific movie-plot threats, rather than the real, broad threats. In the US during the months after the 9/11 attacks, we feared terrorists with scuba gear, terrorists with crop dusters and terrorists contaminating our milk supply. Both the UK and the US fear terrorists with small bottles of liquid. Our imaginations run wild with vivid specific threats. Before long, we're envisioning an entire movie plot, without Bruce Willis saving the day. And we're scared.

It's not just terrorism; it's any rare risk in the news. The big fear in Canada right now, following a particularly gruesome incident, is random decapitations on intercity buses. In the US, fears of school shootings are much greater than the actual risks. In the UK, it's child predators. And people all over the world mistakenly fear flying more than driving. But the very definition of news is something that hardly ever happens. If an incident is in the news, we shouldn't worry about it. It's when something is so common that its no longer news - car crashes, domestic violence - that we should worry. But that's not the way people think.

Psychologically, this makes sense. We are a species of storytellers. We have good imaginations and we respond more emotionally to stories than to data. We also judge the probability of something by how easy it is to imagine, so stories that are in the news feel more probable - and ominous - than stories that are not. As a result, we overreact to the rare risks we hear stories about, and fear specific plots more than general threats.

The problem with building security around specific targets and tactics is that its only effective if we happen to guess the plot correctly. If we spend billions defending the Underground and terrorists bomb a school instead, we've wasted our money. If we focus on the World Cup and terrorists attack Wimbledon, we've wasted our money.

It's this fetish-like focus on tactics that results in the security follies at airports. We ban guns and knives, and terrorists use box-cutters. We take away box-cutters and corkscrews, so they put explosives in their shoes. We screen shoes, so they use liquids. We take away liquids, and they're going to do something else. Or they'll ignore airplanes entirely and attack a school, church, theatre, stadium, shopping mall, airport terminal outside the security area, or any of the other places where people pack together tightly.

These are stupid games, so let's stop playing. Some high-profile targets deserve special attention and some tactics are worse than others. Airplanes are particularly important targets because they are national symbols and because a small bomb can kill everyone aboard. Seats of government are also symbolic, and therefore attractive, targets. But targets and tactics are interchangeable.

The following three things are true about terrorism. One, the number of potential terrorist targets is infinite. Two, the odds of the terrorists going after any one target is zero. And three, the cost to the terrorist of switching targets is zero.

We need to defend against the broad threat of terrorism, not against specific movie plots. Security is most effective when it doesn't require us to guess. We need to focus resources on intelligence and investigation: identifying terrorists, cutting off their funding and stopping them regardless of what their plans are. We need to focus resources on emergency response: lessening the impact of a terrorist attack, regardless of what it is. And we need to face the geopolitical consequences of our foreign policy.

In 2006, UK police arrested the liquid bombers not through diligent airport security, but through intelligence and investigation. It didn't matter what the bombers' target was. It didn't matter what their tactic was. They would have been arrested regardless. That's smart security. Now we confiscate liquids at airports, just in case another group happens to attack the exact same target in exactly the same way. That's just illogical.

· Bruce Schneier is a security technologist and author: schneier.com/blog