In 2010 I wrote a blog post called "Why Aren't Terrorists Bombing the Queues?" - but I chickened out of publishing it.
Bombing a plane is hard, you have to buy a ticket, get past airport security, detonate it at just the right time, etc. By comparison, anyone can walk into a busy airport - say during the school holidays - wait for the shear number of people to build up, and then...
But it doesn't happen. Why not?
My conclusion, such as it was, is that either terrorists are particularly stupid, or bombs are too hard to build, or that the Government frequently foils such attacks, or - perhaps - there just aren't that many people interested in causing indiscriminate carnage.
There are so many large gatherings of people every day, and explosives are relatively easy to build, and there are so few (public) convictions for terror related offences, that the only real conclusion must be that people - in the main - are good.
Wanton destruction, untargeted menace, and psychopathic levels of violence are so uncommon that, when they do happen, their significance is artificially raised within our society to a level which is unwarranted.
As Bruce Schneier says in his book Liars and Outliers
The Virginia Tech massacre is precisely the sort of event we humans tend to overreact to. Our brains aren't very good at probability and risk analysis, especially when it comes to rare occurrences. We tend to exaggerate spectacular, strange and rare events, and downplay ordinary, familiar and common ones. There's a lot of research in the psychological community about how the brain responds to risk -- some of it I have already written about -- but the gist is this: Our brains are much better at processing the simple risks we've had to deal with throughout most of our species' existence, and much poorer at evaluating the complex risks society forces us to face today.
Novelty plus dread equals overreaction.
If you want to do something that makes security sense, figure out what's common among a bunch of rare events, and concentrate your countermeasures there. Focus on the general risk of terrorism, and not the specific threat of airplane bombings using liquid explosives. Focus on the general risk of troubled young adults, and not the specific threat of a lone gunman wandering around a college campus. Ignore the movie-plot threats, and concentrate on the real risks.
We don't know yet the cause of the Boston Marathon bombings, but we owe it to the victims and to ourselves not to over-react. A disproportionate response to rare events could end up being far worse than the terror it is trying to prevent.