Like many other Bostonians on Friday, April 19th, I was stuck inside all day, obsessively following twitter, cable news and police scanners. As I sat voluntarily confined to my home, several miles away from any apparent danger, I didn't see anything wrong with what was being asked of me and my city. A city wide-lockdown was clearly extraordinary, but we were living through extraordinary times; it seemed justified.

But was it?

No one denies the tragedy of the Boston Marathon bombing, which killed three people and severely injured hundreds of others, but there are many other tragedies that go seemingly unnoticed. In the first three months of 2013, there were 10 homicides in Boston, and in 2012, there were 51 homicides, including a triple homicide on August 12th. As far as I could determine, there hasn't been an arrest in that latter case - why no citywide manhunt to find those three victims' killer? What is it that makes three dead and 282 wounded more emotionally salient than the 2.3 million people killed or injured in car accidents every year (that's over six thousand per day)? Why does it make sense to shut down all the economic activity of a major American city to catch two murderers, but politicians are ridiculed for attempting to cut back on the tens of thousands of deaths attributable to diabetes each year?

The fact is, our ape brains are profoundly bad at assessing risk in the modern world. Evolved for anticipating danger on the savannahs of Africa, in relatively small groups where the spectrum of potential dangers are relatively small, our brains use a number of assumptions or "heuristics" in order to understand the world - and these assumptions are often wrong. Research over the past several decades has begun to shine a light on why I'm more afraid of terrorists than of car crashes, even though the latter is far more likely to kill me.

The Availability Heuristic

Are there more words in the English language that begin with the letter "R," or that have "R" as the third letter? If you're like most people, it's much easier to call to mind words that begin with "R," and you therefore assume that such words are more numerous. In fact, there are three times as many words that have "R" in the third position.

In general, it makes sense to think that the easier it is to recall an example of a thing, the more common that thing is. But ease of recall can be easily skewed by a number of factors. Try this: try to recall five terrorist attacks. Right off the top of my head I can recall 9/11, 7/7, Oklahoma city, Mumbai and Benghazi - personally, that took me about ten seconds. You might have a different list, but I'm willing to bet it was pretty easy. Now, try to recall five fatal car accidents. I thought of Princess Diana, and a friend from highschool that was killed by a drunk driver. That's it. I might be able to rack my brain for some other examples, but those are the only ones that spring immediately to mind. In terms of people killed, it's terrorists: thousands, car accidents: 3. Based on ease of recall, I might be forgiven for thinking that terrorism is a far greater threat than car accidents.

In reality, less than ten thousand people have died in the US as the result of terrorist attacks in the last 20 years, while at least that many die behind the wheel every year.

The Illusion of Control and Illusory Superiority Bias

Rational people might scoff at the sports fan that wears his lucky hat to the game (or Robert De Niro's character in Silver Linings Playbook), but we all suffer from the erroneous belief that our actions have more impact than they actually do. In a 1980 study, subjects were given a button to press, and shown lights labeled "Score" or "No Score" that would flash at pre-set intervals. Despite the fact that there was no connection between pressing the button and flashing lights, subjects still believed they had some measure of control.

We also believe we are better than other people. When asked, clear majorities of people believe themselves to be better than average in terms of IQ, memory, and job performance. In one of the earliest studies of this sort, 25% of respondents believed themselves to be in the top 1% in terms of ability to get along well with others. Mathematically, it's clear that these people can't all be correct.

Because of our illusory superiority, we feel safer when we are in control of a situation, and our illusions of control lead us to believe that we are more in control of some situations than we actually are. Respondents consistently rate the likelihood of a driving accident as significantly lower when they are driving than when someone else is driving. This is also why people tend to be more afraid of commercial flying than of driving - we have no control over the what goes on in the cockpit. Terrorism is terrifying precisely because it is out of our control.

Zero-Risk Bias

The federal budget for the Transportation Security Administration (the folks that check your bags at airports) in 2012 was $7.8 billion. By contrast, the Centers for Disease Control and Prevention (CDC) received just $6.1 billion. The CDC's mandate covers just about every risk to human health, including infectious disease, non-communicable diseases like cancer, and even accidental death.

Imagine that if you doubled the CDC's budget, it could reduce the incidence of infectious and non-communicable diseases by 2%, but that all of the money would come from the TSA, leaving it with only $1.7 billion to protect airlines and railways. Now imagine that the TSA claimed that this reduction would allow a 9/11-scale terrorist attack to occur every year - would you take that deal? I certainly wouldn't and I'd venture to guess that any politician proposing such a scheme would be run out of office. But if you go by the numbers, the CDC would be a better bet.

The top five causes of death in the US are (in order) heart disease, cancer, respiratory diseases (like pneumonia), stroke and accidents. The infectious and non-communicable diseases on this list alone account for about 1.5 million deaths in 2010, meaning a 2% reduction would save thirty thousand people per year, almost ten times the number that would die from terrorism in this scenario. Knowing this, would you be willing to take the deal now?

The reason that the math seems so counter-intuitive is something called the zero-risk bias - we are far more inclined to reduce unlikely threats to zero than to make small reductions in much more probable threats, even if the latter have a much greater overall benefit.

Just World Hypothesis

Perhaps the most pernicious bias is our brain's tendency to interpret the world in ways that suggest that good people are rewarded and bad people are punished. This bias is responsible for the assumption that if someone is rich, they must have worked hard to be successful, and if someone is poor, they must be lazy. It also allows people to ignore or downplay many risks. If someone is in a car wreck for instance, they must have been a bad driver. Yes, a lot of people die from gun violence every year, but it's only the poor (lazy) people that joined gangs.

Events like terrorist attacks are all the more frightening because, we can't rationalize them away as

Safety, Security and Fear

Events like terrorist attacks are all the more frightening because we can't rationalize them away as just consequences of the victim's actions. What does all of this mean? Was shutting down Boston the right thing to do? None of the people I've talked to that live here were against the idea, and most of the city diligently followed orders. But that one day could cost hundreds of millions of dollars in lost productivity, not to mention the cost in overtime pay for the police, lost revenue for the public transit system etc. We should ask ourselves if it was worth it.

In hindsight, it’s unlikely that Dzokhar Tsarnaev was much of a threat in his condition, but knowing what we knew early on that Friday morning, was the decision the right one? I’m not in a position to evaluate the risk, let alone measure the cost of a life or lives that might have been lost. I’m grateful to the public officials and law enforcement agencies that acted heroically in the face of a profoundly difficult situation. But knowing what we know about how fear can negatively influence our decisions, it’s worth understanding where that fear comes from. And when we’re making decision in the midst of that fear, understanding might help us confront our biases, and make decisions based on reality, rather than our imperfect perception of it.

Images: Image 1 by author, Image 2 from CBS News, Image 3, by Deviant Art user ADDattack with permission, Image 4 by Flickr user jurvetson.