Almost 3000 people died in the 9/11 terrorist attacks. That includes the victims in or near the World Trade Center, the Pentagon, and all the passengers in the four commandeered jets, including the flight that went down in rural Pennsylvania. But it does not include the many hidden victims of lingering terror—an additional 1500 whose dread of another attack led, indirectly and much later, to their deaths.
This is the gist of the so-called “dread risk effect”—first hypothesized in 2004. The idea is that terrorist acts indeed create terror. After witnessing such a mass murder, people begin to dread—and avoid—situations that are vulnerable to similar mass attacks; and their actions lead to more, but less dramatic deaths, over time. To be specific, scientists believe that terrorized Americans switched from (relatively safe) air travel to (relatively risky) driving in the aftermath of 9/11, a decision that increased road traffic, traffic accidents and fatalities.
The dread risk hypothesis seems logical, even if the behavior doesn’t make sense. And the scientists who proposed it provided evidence to support it. But the problem is that the effect has not been discernible following other terrorist attacks. In 2004, for example, terrorists bombed four trains in Spain, killing 191 people and injuring more than 1700. Rail travel did in fact decline after the attacks, but not dramatically and only briefly. More notably, there was no corresponding rise in road traffic—that is, no secondary dread risk effect. Indeed, road traffic actually fell along with rail traffic, reducing mortality.
Why would that be? Why were the reactions to the attacks on civilians so different in these two instances? Psychological scientist Peter Ayton, of City University in London, has been exploring this very question. He did another analysis, looking for the hypothesized effect following the 2005 bomb attacks in London, to see if he could clarify the conflicting evidence from the U.S. and Spain.
The London attacks targeted public transportation, killing 52 and injuring 700 on three subway trains and one bus. Ayton analyzed automobile accident records, but could detect no jump in auto accidents. He believes this may be because the UK, like Spain, lacks the car culture of the U.S. Londoners, like Spaniards, may be less willing and able to simply switch over to their cars—even if they do dread another attack.
But Ayton found another way to test this effect in the London attacks. He looked at cycling, on the possibility that Londoners might have switched to bicycles to avoid the underground and bus travel. Since bicycling in the city is riskier than public transportation, such a switch could conceivably lead to the fatalities that follow secondary dread. He examined records of subway travel, bicycle use, and cyclist casualties, and he reported the results this week at the meeting of the Association for Psychological Science in Chicago. Subway traffic did drop as predicted, and bicycling did increase—as did cycling casualties. There were 214 more casualties in the months right after the attacks than in the comparable time period before.
The jump in casualties was fairly small—and not all fatal—so nothing like the 1500 secondary deaths documented after 9/11. But the effect was real—unlike in the Spanish study. So again: Why the variability?
Ayton has some thoughts on this question. It’s possible that familiarity with such terrorist attacks might dampen the feelings of dread that lead to risky behavior later on. The UK, like Spain, has experienced decades of bomb attacks, which may inure citizens to the threat. Such a familiarity effect has been seen in Israel where, even with a prolonged wave of attacks, Israelis learn over time that public outings are not necessarily perilous.
It’s also possible, Ayton argues, that the drop in U.S. air travel after 9/11 was really not a result of dread at all—but instead an aversion to the inconvenience of extra security measures. There is also some evidence that the jump in traffic fatalities occurred only in the northeast region of the country, and coincided with an increase in alcohol- and drug-related citations. Perhaps what really happened is that the trauma of 9/11 lead to heightened stress, which led to more substance abuse and impaired driving—and finally to traffic fatalities.
This interpretation is provocative. If true, it would mean that the U.S. attacks led to an involuntary state of stress, not to willful but unhealthy choices. Interventions to reduce secondary deaths would be much different for stress than for distorted thinking about risk. But the dread risk effect remains the best explanation for London’s cyclists, and it raises the possibility that terrorism leads to other discretionary but harmful choices that are as yet unknown.
Wray Herbert’s book, On Second Thought, is about irrational thinking and judgment. Excerpts from his two blogs—“We’re Only Human” and “Full Frontal Psychology”—appear regularly in The Huffington Post and in Scientific American Mind.