Editor's note: Bruce Schneier is a security technologist, and author of "Liars and Outliers: Enabling the Trust Society Needs to Survive."
(CNN) -- Horrific events, such as the massacre in Aurora, can be catalysts for social and political change. Sometimes it seems that they're the only catalyst; recall how drastically our policies toward terrorism changed after 9/11 despite how moribund they were before.
The problem is that fear can cloud our reasoning, causing us to overreact and to overly focus on the specifics. And the key is to steer our desire for change in that time of 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. We fear them more than probability indicates we should.
There is a lot of psychological research that tries to explain this, but one of the key findings is this: People tend to base risk analysis more on stories than on data. Stories engage us at a much more visceral level, especially stories that are vivid, exciting or personally involving.
If a friend tells you about getting mugged in a foreign country, that story is more likely to affect how safe you feel traveling to that country than reading a page of abstract crime statistics will.
Novelty plus dread plus a good story equals overreaction.
And who are the major storytellers these days? Television and the Internet. So when news programs and sites endlessly repeat the story from Aurora, with interviews with those in the theater, interviews with the families and commentary by anyone who has a point to make, we start to think this is something to fear, rather than a rare event that almost never happens and isn't worth worrying about. In other words, reading five stories about the same event feels somewhat like five separate events, and that skews our perceptions.
We see the effects of this all the time.
It's strangers by whom we fear being murdered, kidnapped, raped and assaulted, when it's far more likely that any perpetrator of such offenses is a relative or a friend. We worry about airplane crashes and rampaging shooters instead of automobile crashes and domestic violence -- both of which are far more common and far, far more deadly.
Our greatest recent overreaction to a rare event was our response to the terrorist attacks of 9/11. I remember then-Attorney General John Ashcroft giving a speech in Minnesota -- where I live -- in 2003 in which he claimed that the fact there were no new terrorist attacks since 9/11 was proof that his policies were working. I remember thinking: "There were no terrorist attacks in the two years preceding 9/11, and you didn't have any policies. What does that prove?"
What it proves is that terrorist attacks are very rare, and perhaps our national response wasn't worth the enormous expense, loss of liberty, attacks on our Constitution and damage to our credibility on the world stage. Still, overreacting was the natural thing for us to do. Yes, it was security theater and not real security, but it made many of us feel safer.
The rarity of events such as the Aurora massacre doesn't mean we should ignore any lessons it might teach us. Because people overreact to rare events, they're useful catalysts for social introspection and policy change. The key here is to focus not on the details of the particular event but on the broader issues common to all similar events.
Installing metal detectors at movie theaters doesn't make sense -- there's no reason to think the next crazy gunman will choose a movie theater as his venue, and how effectively would a metal detector deter a lone gunman anyway? -- but understanding the reasons why the United States has so many gun deaths compared with other countries does. The particular motivations of alleged killer James Holmes aren't relevant -- the next gunman will have different motivations -- but the general state of mental health care in the United States is.
Even with this, the most important lesson of the Aurora massacre is how rare these events actually are. Our brains are primed to believe that movie theaters are more dangerous than they used to be, but they're not. The riskiest part of the evening is still the car ride to and from the movie theater, and even that's very safe.
But wear a seat belt all the same.
Follow @CNNOpinion on Twitter
Join us at Facebook/CNNOpinion
The opinions expressed in this commentary are solely those of Bruce Schneier