Q&A

# Why should the near-zero prior probability, of 9/11/2001 attacks on the World Trade Centre, overwhelm any subsequent response that may have lowered the objective risk?

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Can you please expound the embold phrase below? Can this be calculated?

It is notoriously difficult, however, to assess what one’s prior assessment of the risk would have been had one thought of the event before it actually occurred. Most Americans had never contemplated the possibility of a terrorist attack in this country at the level of 9/11. After it occurred, an attack of that magnitude—or much worse—was not beyond belief, and indeed was thought to have a positive probability of happening again. Thus, most Americans’ subjective assessment of terrorism risk should have been far higher after 9/11 than before, despite measures taken to lower such risks after 9/11, since the near-zero prior probability should overwhelm any subsequent response that may have lowered the objective risk [Emphasis mine]. A survey of law school students published in 2003 by economists Kip Viscusi and Richard Zeckhauser in the Journal of Risk and Uncertainty, however, found that around 40 percent of respondents believed their personal risk assessment was higher before the attacks than currently.$^2$ In another study of professional-school students and undergraduate business students in 2005, they showed that over two-thirds of respondents exhibited the same phenomenon.$^3$ These respondents experienced a recollection bias, whereby after the occurrence of a low-probability event, one thinks that one’s prior risk assessment was much higher than it actually was. This could be due to an attempt to reduce cognitive dissonance, for self-justification, or simply to misremembering.

Paul Slovic, The Irrational Economist (2010), p 102.

$^2$ Kip Viscusi and Richard Zeckauser, Journal of Risk and Uncertainty_ (2003).

$^3$ Kip Viscusi and Richard Zeckauser, “Recollection Bias and the Combat of Terrorism,” Journal of Legal Studies 34 (January 2006): 27-55.

Op. cit. p 286.

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