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The official death toll for the September 11 attacks stands at 2,996, including the 19 hijackers, but research suggests that there is a further, indirect toll as a result of behavioural changes induced by fear.
In the months after the 2001 terror attacks, passenger miles on the main US airlines fell by between 12% and 20%, while road use jumped.
The change is widely believed to have been caused by concerned passengers opting to drive rather than fly. Travelling long distances by car is more dangerous than travelling the same distance by plane.
Measuring the exact effect is complex because there is no way of knowing for sure what the trends in road travel would have been had 9/11 not happened. However, Professor Gerd Gigerenzer, a German academic specialising in risk, has estimated that an extra 1,595 Americans died in car accidents in the year after the attacks – indirect victims of the tragedy.
He used trends in road and air use to suggest that, for a period of about 12 months, there was a temporary increase in road use before citizens again became more willing to fly at similar rates to before the attacks.
Gigerenzer ascribed the extra deaths to people's poor understanding of danger. "People jump from the frying pan into the fire," he said.
"We have an evolutionary tendency to fear situations in which many people die at one time. This is likely a hold-over from when we lived in small groups, where the death of a small part of the group could place the lives of everyone else in jeopardy.
"That's no longer the case, but it is very difficult to elicit the same fear for the same number of deaths spaced over a year."
As an example of a major killer that elicits little fear in the population, Gigerenzer cited a US study by the Institute of Medicine which found that between 44,000 and 98,000 people a year in the US die of documented, preventable, medical accidents in hospital. Smoking and driving were similar factors, he said.
Michael Rothschild, then emeritus professor at the University of Wisconsin, argued in the immediate aftermath of the September 11 attacks that the risks of terrorism were severely overstated by governments.
Rothschild calculated the risk of a traveller who took four flights every month dying as a result of terror attacks. If hijackers managed to destroy one plane a month, he argued, this frequent traveller had a one in 540,000 chance of being killed. At one plane a year, the risk was less than one in 6,000,000.
In contrast, the risk of being killed in a US car accident in any given year is one in 7,000, dying of cancer one in 600 and dying of heart disease one in 400.
Gigerenzer said governments shared the blame for excess deaths in the wake of terrorism or natural disasters, due to efforts to reduce their culpability for any misfortune. Citing last year's grounding of planes in European airspace due to a volcanic ashcloud, he argued: "When British Airways and Air France did test flights through the ash, they encountered no problems.
"But decision-making is defensive. Politicians would be held responsible if a plane had crashed when flying through the ash cloud. If people are killed because they are forced to take their car instead, they are not blamed."
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