The short answer is emotion trumps probability.
There is a longer answer.
80 Americans were killed in terrorist attacks from 2004 to 2013, including perpetrators and excluding deaths in Afghanistan and Iraq, the majority of which are combat related. Of those 80 Americans killed, 36 were killed in attacks that occurred in the United States. [emphasis added]
That’s about 1 death in the United States every 100 days for the past 10 years.
Reporting on a study published in the journal Psychological Science, Julie Sedivy writes:
From 2002 to 2015, the proportion of Americans worried that they or someone in their family would become a victim of terrorism increased from 35 to 49 percent—despite the fact that since 9/11, Americans were less likely to have been killed by a terrorist than by furniture falling on them.
Compare terrorism with traffic deaths.
The National Safety Council estimates that in 2015, “38,300 people were killed on U.S. roads, and 4.4 million were seriously injured, meaning 2015 likely was the deadliest driving year since 2008.”
That’s about 105 deaths and 12,000 injuries each day.
Imagine what life in the United States would be like if the deaths and injuries were the result of terrorism.
But as a nation we are resilient to vehicle carnage – “resilient” as defined by Presidential Policy Directive 21:
[The] ability to prepare for and adapt to changing conditions and withstand and recover rapidly from disruptions. Resilience includes the ability to withstand and recover from deliberate attacks, accidents, or naturally occurring threats or incidents.
Our national resilience allows us to absorb 105 vehicle deaths every day. Those deaths may be devastating for the families and friends involved. But the nation carries on. (One could make a similar national resilience argument for the 36 gun deaths every day, but that’s for another time.)
Mueller and Stewart, in Chasing Ghosts – their latest effort to bring economic rationality to the domestic terrorism wars – estimated “the yearly chance an American will be killed by a terrorist within the country is about one in 4 million….”
The yearly chance of dying in a motor vehicle accident are about 1 in 9,000.
What might explain why the odds of being killed by a terrorist (1 in 4 million) creates more fear than dying in a vehicle accident (1 in 9,000)?
Cass Sunstein, in an essays titled “Why People Stay Scared After Tragedies Like Boston Attack,” relies on behavioral economics ideas for a plausible explanation.
Often [the] feeling of fear is far greater than reality warrants. This is so because of two facts about how human beings respond to risk. The first is that we often assess probabilities not by looking at statistics, but by asking what events come readily to mind…. [P]eople use the “availability heuristic,” which means that we assess risks by asking whether a bad (or good) event is cognitively “available.” It is hardly unreasonable to use the availability heuristic, yet we can be misled by it, and far more frightened than we need to be….
The second problem is that for some risks, we tend to focus mostly on the possible outcome, and not so much on the likelihood that it will actually come to fruition. Much of the time … we really care about probability…. But when people’s emotions are running especially high, the outcome is the dominant consideration, and it can crowd out consideration of probability….
The lesson is straightforward. In situations that trigger strong negative emotions, people tend to focus on the very worst that might happen, and the question of probability turns out to be secondary…. When terrorists succeed in generating widespread fear, it is also because they get people to focus on terrible outcomes, and not on the likelihood that they will come about. Because strong emotions are produced by the prospect of a terrorist attack, people might well become more frightened than reality warrants.