11-M_El_How many people died because of the September 11 attacks? The answer depends on what you are trying to measure. The official estimate is around 3,000 deaths as a direct result of hijacked aircraft and at the World Trade Center, Pentagon, and in Pennsylvania. Those attacks were tragic, but the effect was compounded by overreaction to terrorism. Specifically, enough Americans substituted driving for flying in the remaining months of 2001 to cause 350 additional deaths from accidents.

David Myers was the first to raise this possibility in a December, 2001, essay. In 2004, Gerd Gigerenzer collected data and estimated the 350 deaths figure, resulting from what he called "dread risk":

People tend to fear dread risks, that is, low-probability, high-consequence events, such as the terrorist attack on September 11, 2001. If Americans avoided the dread risk of flying after the attack and instead drove some of the unflown miles, one would expect an increase in traffic fatalities. This hypothesis was tested by analyzing data from the U.S. Department of Transportation for the 3 months following September 11. The analysis suggests that the number of Americans who lost their lives on the road by avoiding the risk of flying was higher than the total number of passengers killed on the four fatal flights. I conclude that informing the public about psychological research concerning dread risks could possibly save lives.

Does the same effect carry over to other countries and attacks? Alejandro López-Rousseau looked at how Spaniards responded to the March 11, 2004, train bombings in Madrid. He found that activity across all forms of transportation decreased--travelers did not substitute driving for riding the train.

What could explain these differences? One could be that Americans are less willing to forego travel than Spaniards. Perhaps more travel is for business reasons and cannot be delayed. Another possibility is that Spanish citizens are more accustomed to terrorist attacks and understand that substituting driving is more risky than continuing to take the train. There are many other differences that we have not considered here--the magnitude of the two attacks, feelings of being "in control" while driving, varying cultural attitudes.

This post is simply meant to make three points. First, reactions to terrorism can cause additional deaths if relative risks are not taken into account. Cultures also respond to terrorism in different ways, perhaps depending on their previous exposure to violent extremism. Finally, the task of explaining differences is far more difficult than establishing patterns of facts.

(For more on the final point check out Explaining Social Behavior: More Nuts and Bolts for the Social Sciences, which motivated this post.)