“Better safe than sorry” is a popular English idiom that can be traced back to Irish novelist Samuel Lover’s Rory O’More (1837). Essentially, it means that it is better to take precautionary actions rather than be sorry that you didn’t if something bad happens. One example was when we were worried about the H1N1 (or swine flu) outbreak. Even if the chance of catching H1N1 (let alone dying from it) was relatively low, many people thought it was better to be vaccinated rather than risk catching swine flu. A more mundane example is having someone hold a ladder while you climb up – even though you believe you’re perfectly capable of maintaining your balance and don’t have any reason to think it will fall over.

In the post-9/11 world, “better safe than sorry” has become an article of faith guiding the actions we take in the name of preventing terrorism. But are we truly better safe than sorry?

To begin, the main reason so many people are willing to accept “better safe than sorry” is because they believe the consequences are too terrible to act otherwise. In other words, we should be willing to do almost anything to prevent another terrorist attack. Although another terrorist attack on the scale of 9/11 – which killed some 3,000 people – would be a great catastrophe and tragedy, it would not be an end-of-the-world event. As a nation, we survived 9/11, and we would (or at least we should) survive if there was another 9/11. That is not to trivialize or marginalize the people killed by the 9/11 attacks (or who would be killed in any future terrorist attacks), but it’s important to understand that terrorism is not an existential threat – otherwise, our responses are disproportionate (in magnitude or cost, or both) to the actual threat. It’s hard to be dispassionate because of the emotionalism surrounding 9/11, but here are some numbers worth considering to put “better safe than sorry” in context when it comes to terrorism. According to the Global Terrorism Database, from 1970 through 2007, there have been 1,347 terrorist incidents in the United States resulting in 3,340 fatalities (2,949 of which were on 9/11) and 2,234 injuries. That’s less than 100 fatalities per year on average (and more like 10 if you exclude 9/11 as an extraordinary event).

By way of comparison, consider these 2006 fatality statistics from the the Centers for Disease Control:

• Unintentional fall deaths: 20,853
• Motor vehicle traffic deaths: 43,646
• Unintentional poisoning deaths: 27,531
• Homicides: 18,573
• Firearms homicides: 12,791

Put another way, far more people die in a single year from other causes than have died as result of terrorism over a span of more than 35 years. Yet we have a Chicken Little attitude that the sky is falling when it comes to the potential threat of terrorism.

Another reason “better safe than sorry” is widely accepted is the belief that the costs are relatively minor. But the financial costs are hardly trivial. The Department of Homeland Security’s fiscal year 2010 budget is $55 billion, and the proposed fiscal year 2011 budget is $56 billion. And we are waging two wars in Iraq and Afghanistan in the name of fighting terrorism that have already cost more than $1 trillion to date and could end up costing four to six times that according to Joseph Stiglitz (chief economist at the World Bank and winner of the Nobel Memorial Prize for Economics in 2001) and Linda Bilmes (lecturer in public policy at the Kennedy School of Government at Harvard University).

There is also the cost of false positives. Most recently, an anonymous call from a Canadian phone booth forced a Pakistan-bound airliner to make a nine-hour stop in Stockholm after authorities were tipped off that a passenger was carrying explosives. It turned out to be a false alarm and likely a hoax. It’s not just that all the passengers were needlessly inconvenienced. A Canadian man had to endure being (wrongfully) arrested by a SWAT team – all on the scant evidence of an anonymous phone call. And this is just one of many false alarms in the more than nine years since 9/11.

Many people would argue that such episodes are simply a small price to pay for security. After all, no one was hurt, and the suspect was eventually released. But a more extreme situation is when we have a false positive identifying a terrorist threat against whom we employ deadly force – which has happened countless times in both Iraq and Afghanistan. In those cases, incorrectly identifying someone as a terrorist threat when, in fact, they aren’t results in killing an innocent person. As has happened all too many times in Iraq and Afghanistan, false positives result in collateral damage beyond the intended target – often women and children. (And for those who believe the use of such force is appropriate given the potential threat, I have to wonder how they would feel if similar action – with similar results – were taken here in the United States.)

It’s not surprising, but the public is either ignorant or indifferent about the implications of false positives. Here’s a simple example. Suppose instead of metal detectors or body scanners at airports there were terrorist scanners that were 90 percent accurate (which means they are also wrong 10 percent of the time). Let’s also suppose that 3,000 people need to pass through a scanner and that one of them is a would-be terrorist (like, for example, Umar Farouk Abdulmutallab who is the underwear bomber who managed to light his pants on fire aboard Northwest flight 254 last Christmas). Thirty people pass through the scanner without any problem and the next person fails. How sure are you that the person is actually a terrorist?

A. 90 percent

B. 10 percent

C. 0.3 percent

The answer is actually C, or less than one-half of 1 percent. How is that so? If 3,000 people are tested, and the test is 90 percent accurate, it is also 10 percent wrong. So it will probably identify 301 terrorists – 300 by mistake and 1 correctly. The problem is that you won’t know from the result which is the real terrorist. So the chance that passenger 31 walking through the detector is actually a terrorist is 1 in 301 or 0.3 percent.

The problem with “better safe than sorry” is that it focuses almost exclusively on the consequences of “what if” and largely ignores the costs and consequences of “what is.” Yet the latter may be more real than the former.

Read more by Charles V. Peña

• Stick a Fork in al-Qaeda – May 6th, 2012
• Coming to a Neighborhood Near You – March 30th, 2012
• WMDs Redux – February 26th, 2012
• The Sky Is Falling – February 9th, 2012
• Another Reason Not to Go to War So Often – January 19th, 2012