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Security is both a feeling and a reality, and they're different. You can feel secure even though you're not, and you can be secure even though you don't feel it. There are two different concepts mapped onto the same word — the English language isn't working very well for us here — and it can be hard to know which one we're talking about when we use the word.

There is considerable value in separating out the two concepts: in explaining how the two are different, and understanding when we're referring to one and when the other. There is value as well in recognizing when the two converge, understanding why they diverge, and knowing how they can be made to converge again.

Some fundamentals first. Viewed from the perspective of economics, security is a trade-off. There's no such thing as absolute security, and any security you get has some cost: in money, in convenience, in capabilities, in insecurities somewhere else, whatever. Every time someone makes a decision about security — computer security, community security, national security — he makes a trade-off.

People make these trade-offs as individuals. We all get to decide, individually, if the expense and inconvenience of having a home burglar alarm is worth the security. We all get to decide if wearing a bulletproof vest is worth the cost and tacky appearance. We all get to decide if we're getting our money's worth from the billions of dollars we're spending combating terrorism, and if invading Iraq was the best use of our counterterrorism resources. We might not have the power to implement our opinion, but we get to decide if we think it's worth it.

Now we may or may not have the expertise to make those trade-offs intelligently, but we make them anyway. All of us. People have a natural intuition about security trade-offs, and we make them, large and small, dozens of times throughout the day. We can't help it: It's part of being alive.

Imagine a rabbit, sitting in a field eating grass. And he sees a fox. He's going to make a security trade-off: Should he stay or should he flee? Over time, the rabbits that are good at making that trade-off will tend to reproduce, while the rabbits that are bad at it will tend to get eaten or starve.

So, as a successful species on the planet, you'd expect that human beings would be really good at making security trade-offs. Yet, at the same time, we can be hopelessly bad at it. We spend more money on terrorism than the data warrants. We fear flying and choose to drive instead. Why?

The short answer is that people make most trade-offs based on the feeling of security and not the reality.

I've written a lot about how people get security trade-offs wrong, and the cognitive biases that cause us to make mistakes. Humans have developed these biases because they make evolutionary sense. And most of the time, they work.