Juergen Weichselgartner’s 2001 paper entitled “Disaster Mitigation: The Concept of Vulnerability Revisited” (Disaster Prevention and Management, Vol. 10, No. 2, pp. 85-94, doi:10.1108/09653560110388609) provided a nice summary of alternative definitions for the word “vulnerability” gleaned from a variety of academic publications (copied below; see original paper for citations).

• Gabor and Griffith (1980) Vulnerability is the threat (to hazardous materials) to which people are exposed (including chemical agents and the ecological situation of the communities and their level of emergency preparedness). Vulnerability is the risk context.
• Timmerman (1981) Vulnerability is the degree to which a system acts adversely to the occurrence of a hazardous event. The degree and quality of the adverse reaction are conditioned by a system’s resilience (a measure of the system’s capacity to absorb and recover from the event)
• UNDRO (1982) Vulnerability is the degree of the loss to a given element or set of elements at risk resulting from the occurrence of a natural phenomenon of a given magnitude
• Petak and Atkisson (1982) The vulnerability element of the risk analysis involved the development of a computer-based exposure model for each hazard and appropriate damage algorithms related to various types of buildings
• Susman et al. (1983) Vulnerability is the degree to which different classes of society are differentially at risk
• Kates (1985) Vulnerability is the “capacity to suffer harm and react adversely”
• Pijawka and Radwan (1985) Vulnerability is the threat or interaction between risk and preparedness. It is the degree to which hazardous materials threaten a particular population (risk) and the capacity of the community to reduce the risk or adverse consequences of hazardous materials releases
• Bogard (1989) Vulnerability is operationally defined as the inability to take effective measures to insure against losses. When applied to individuals, vulnerability is a consequence of the impossibility or improbability of effective mitigation and is a function of our ability to detect hazards
• Mitchell (1989) Vulnerability is the potential for loss
• Liverman (1990) Distinguishes between vulnerability as a biophysical condition and vulnerability as defined by political, social and economic conditions of society. She argues for vulnerability in geographic space (where vulnerable people and places are located) and vulnerability in social space (who in that place is vulnerable)
• Downing (1991) Vulnerability has three connotations: it refers to a consequence (e.g. famine) rather than a cause (e.g. drought); it implies an adverse consequence (e.g., maize yields are sensitive to drought; households are vulnerable to hunger); and it is a relative term that differentiates among socioeconomic groups or regions, rather than an absolute measure or deprivation
• UNDRO (1991) Vulnerability is the degree of the loss to a given element or set of elements at risk resulting from the occurrence of a natural phenomenon of a given magnitude and expressed on a scale from 0 (no damage) to 1 (total loss). In lay terms, it means the degree to which individual, family, community, class or region is at risk from suffering a sudden and serious misfortune
  following an extreme natural event
• Dow (1992) Vulnerability is the differential capacity of groups and individuals to deal with hazards, based on their positions within physical and social worlds
• Smith (1992) Human sensitivity to environmental hazards represents a combination of physical exposure and human vulnerability ± the breadth of social and economic tolerance available at the same site
• Alexander (1993) Human vulnerability is function of the costs and benefits of inhabiting areas at risk from natural disaster
• Cutter (1993) Vulnerability is the likelihood that an individual or group will be exposed to and adversely affected by a hazard. It is the interaction of the hazard of place (risk and mitigation) with the social profile of communities
• Watts and Bohle (1993) Vulnerability is defined in terms of exposure, capacity and potentiality. Accordingly, the prescriptive and normative response to vulnerability is to reduce exposure, enhance coping capacity, strengthen recovery potential and bolster damage control (i.e., minimize destructive consequences) via private and public means
• Blaikie et al. (1994) By vulnerability we mean the characteristics of a person or a group in terms of their capacity to anticipate, cope with, resist and recover from the impact of a natural hazard. It involves a combination of factors that determine the degree to which someone’s life and livelihood are put at risk by a discrete and identifiable event in nature or in society
• Green et al. (1994) Vulnerability to flood disruption is a product of dependence (the degree to which an activity requires a particular good as an input to function normally), transferability (the ability of an activity to respond to a disruptive threat by overcoming dependence either by deferring the activity in time, or by relocation, or by using substitutes), and susceptibility (the probability and extent
  to which the physical presence of flood water will affect inputs or outputs of an activity)
• Bohle et al. (1994) Vulnerability is best defined as an aggregate measure of human welfare that integrates environmental, social, economic and political exposure to a range of potential harmful perturbations. Vulnerability is a multilayered and multidimensional social space defined by the determinate, political, economic and institutional capabilities of people in specific places at specific times
• Dow and Downing (1995) Vulnerability is the differential susceptibility of circumstances contributing to vulnerability. Biophysical, demographic, economic, social and technological factors such as population ages, economic dependency, racism and age of infrastructure are some factors which have been examined in association with natural hazard
• Gilard and Givone (1997) Vulnerability represents the sensitivity of land use to the hazard phenomenon
• Comfort, L. et al. (1999) Vulnerability are those circumstances that place people at risk while reducing their means of response or denying them available protection
• Weichselgartner and Bertens (2000) By vulnerability we mean the condition of a given area with respect to hazard, exposure, preparedness, prevention, and response characteristics to cope with specific natural hazards. It is a measure of capability of this set of elements to withstand events of a certain physical character

Of course, this list is by no means complete; in fact, the definitions from obvious sources such as Webster’s dictionary, Department of Defense doctrine, and a host of other papers were not included.  I leave it to the readers of this blog to discover alternative definitions that are most suited for his or her particular application.  But if one was looking for a really short definition of vulnerability to sum up everything above, consider the following two (my preferences):

Vulnerability is the manifestation of the inherent states of a system that render is susceptible to harm or loss (a paraphrased definition of the notion of vulnerability offered by Prof. Yacov Haimes at the University of Virginia)

The vulnerability of an entity to realizing a specified adverse outcome following the occurrence of a particular triggering or initiating event is measured as the conditional probability of the outcome given the triggering event has occurred (an expanded version of the definition I offer in my SRA 311 class at Penn State)

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I recently came across a hard-copy of an email I sent to a colleague sometime back in 2004 while I was an ASME Federal Fellow to the Department of Homeland Security.  If my memory serves me correctly, I sent this email in January 2004, or about midway through my tenure in DHS’s Information Analysis and Infrastructure Protection (IAIP) directorate.  Basically, the question was not how to estimate the psychological impact associated with a terrorist attack, but rather what psychological impact means.  After all, definitions precede measurement.  As a reminder, this note was written over 4 years ago when I was less enlightened, or rather, just 6 months after leaving my job as an aerospace structural engineer focused solely on designing the structural subsystems for scientific, non-defense-related spacecraft.  Nonetheless, I felt that posting it here might inspire similar thoughts entertained by others or perhaps even prompt discussion.

A recent lecture in my Political Analysis course [PUAF 620 at the University of Maryland] inspired me to think about “psychological impact” as a form of consequences.  The lecture was on special interest groups, their causes, and the consequences of their existence.  One theory is that special interest groups are created to protect and preserve the rights of its constituency (either natural rights or rights/benefits bestowed on the group from previous legislation).  A special interest group will form (or mobilize) to protect its interests if it feels its rights are being threatened.

Based on what I learned during this lecture, I propose the following definition for psychological impact: “psychological impact is the degree to which an individual or group of individuals perceive they have been deprived of their rights.”

Let’s think about this - if several department stores scattered across the nation are targeted for a coordinated attack [a very common scenario that has provided the basis for numerous thought experiments], following an attack people will feel that their freedom to shop a department stores has been taken away from them.  Similarly, in the wake of September 11, many Americans felt deprived of their freedom to travel by air.  One can come up with a host of other examples.

It is also interesting to consider the collateral economic impacts.  Perhaps coordinated attacks on several department stores will prevent people from shopping anywhere such stores are located.  One might argue that the downstream impact of this behavior could propagate throughout the entire retail industry.  On the flip side, the inability (and unwillingness) to travel by air following 9/11 attacks did not impact the entire transportation industry.  Rather, people who would otherwise fly opted to travel by trains and automobiles.

So how does one assess psychological impact?  For any attack scenario, one must identify how a successful attack might threaten perceived rights and freedoms.  To do this, we must first understand what rights the public thinks it has.  In the two examples above, the focus was on either freedom to shop and freedom to travel.  To prioritize scenarios based on potential psychological impact, we must order all these freedoms according to their perceived importance to the affected public.  This can be done at the national, regional, state, local, sector, etc. level.  Doing so will facilitate cost-benefit tradeoffs (in the descriptive sense).  Proposed countermeasures must demonstrate a tradeoff between the freedoms such policies protect versus the rights they appear to take away.  In the case of the Patriot Act, freedom to live without terrorism is enhanced in exchange for a weakened right to privacy [was there a positive ROI here?].

It would also be interesting to explore how the economy might respond to any perceived loss of freedom.  For example, a perceived loss of freedom to shop will keep people from spending money.  [how does percieved deprivations of freedom correspond to economic impact?]

Now that I had four years to sit on this, I still think that the proposed definition for psychological impact has merit despite the fact that it essentially equates psychology to perceptions, and does not consider such things as PTSD.  But before I or anyone else accepts this definition, more thought is needed on whether it is complete and all encompassing, whether it precisely articulates what we care about, to what degree such a measure is redundant with respect to other measures of loss (economic impact is a function of societal behaviors), and how such impact can be assessed with confidence.   Even now, 5 years after DHS opened its doors, I am sure any answer to the question of measuring psychological impact would be of interest to DHS risk analysts.

I can finally throw the hard-copy of this email out now that it is posted to my blog.  Just another step toward a purely paperless life…

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