Nonlinear Deterrence: Why Sensors Would Work In Preventing Terrorism


Abstract: Critics contend that investments in sensors for biological weapons are a “dead-end” [Brown, 2004]. However, over two decades of incidents involving explosives offer compelling evidence that terrorists are deterred if they perceive sufficient risk of failure. Therefore even imperfect WMD sensor systems can form the foundation of a highly effective deterrent.


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Sensors deployed strategically throughout an area, fixed or mobile, can be used as an early warning system to remotely detect the presence of biological agents, chemicals, explosives, and nuclear radiation. As recently discussed in Science [Brown, 2004], some critics question the wisdom of funding research in sensor systems that detect biological weapons of mass destruction (WMD),

1.       Can sensors systems ever be accurate enough to encompass the wide range of possible threats?

2.       Can sensor systems ever be cost effective?


These questions about biological sensors apply as well to sensors for conventional explosives, nuclear, and chemical materials. Even a single attack that evades detection might seem to render investment in a sensor system moot. Acknowledging that sensor systems can never be perfect, a more relevant question is whether the deployment of these systems increases terrorists’ risk of failure. In its report, the 9/11 Commission points out that,

“Just increasing the attacker’s odds of failure may make the difference between a plan attempted, or a plan discarded. The enemy also may have to develop more elaborate plans, thereby increasing the danger of exposure or defeat.” [p. 383, Kean, 2004].


Sensors, which are imperfect detectors, can serve as a disproportionately stronger deterrent to the would-be attacker, i.e. as a nonlinear deterrent.  On one hand, it is hard to predict exactly how effective a WMD sensor system would be in deterring WMD terrorism [see U.S. Congress, Office of Technology Assessment, 1993] simply because attacks using them have been historically rare, calling into question the value of investments in WMD sensor research. However, historical data involving use of conventional explosives in terrorism can be used to gauge the expected deterrent effect of sensors: we examine the role that screening of passengers and luggage for conventional explosives has played in deterring terrorist use of explosives within airports and airplanes. Critiques of airport screening measures have also focused primarily on less than perfect detection probability, i.e. whether or not a weapon or explosive will pass through the screening process at the airport. For instance, [p. 52, Szyliowicz, 2004] remarks on the “porousness” of airport screening by citing the United States GAO finding (July 2002) that fake weapons and explosives had passed through airport screeners a quarter of the time at 32 major airports [p. 2, Dillingham, 2002]. As we shall see below, the deterrence effect of screening has turned out to be far greater than would have been predicted simply by a linear extrapolation of the detection probability.


Here, we employ data from the RAND-MIPT database ([MIPT, 2002], [RAND, 2003]) of worldwide terrorism incidents from January 1, 1968 until March 12, 2003. In Figure 1, we chart the time series of cumulative fatalities and incidents across four decreasingly inclusive categories,

1.       ALL: all terrorist incidents

2.       EXPLOSIVES: explosives were used or detonated

3.       AIRPORT/PLANE: explosives in an airport or onboard an airplane

4.       AIRPLANE: explosives onboard an airplane.


From 1989 onwards, ALL fatalities and fatalities from EXPLOSIVES not only increase, but so does their rate as shown by the increasing steepness of the slope (the apparent discontinuity on the ALL graph represents the deaths of nearly 3000 victims on September 11, 2001). In contrast, we observe that fatalities for AIRPORT/PLANE and AIRPLANE remain nearly constant after 1989, with AIRPLANE fatalities rising by only 14 over the course of 14 years.



Figure 1


Upon a closer examination of the data for categories AIRPORT/PLANE and AIRPLANE, we find two extended periods of comparatively low fatalities preceded by periods of much greater intensity. In Table 2 below, we show these periods along with the fatalities for each category. Periods II and IV have much smaller fatalities for AIRPORT/PLANE and AIRPLANE when compared to the corresponding statistics during periods I and III. However, during all periods, including I and II, the number of fatalities in EXPLOSIVES or ALL showed no signs of slowing down. We conclude that during periods II and IV, there was a “displacement[1]” from airplanes as terrorist bombing targets onto other environments.










09/02/1974 to








03/28/1977 to








08/07/1984 to




















Table 1: Fatalities for each category and time period

Despite continuous increase in fatalities for ALL and EXPLOSIVES, the presence of two periods of accelerated fatalities followed by extremely low fatalities for AIRPORT/PLANE and AIRPORT is telling.  We believe that the underlying reason for this trend is that during these periods airport screening programs were greatly expanded:

·          First, following frequent hijackings beginning in the late 1960s, a variety of security measures including metal-detectors and X-Ray machines to screen carry-on baggage were introduced in the United States as part of the Air Transportation Security Act of 1974 [Malotky, 1998]. Indeed, a reduction in skyjackings in the United States could be attributed to the introduction of metal detectors in 1973, while also increasing incidents not protected by the detectors such as those involving hostages [Enders, 1993]. In light of the trends in Figure 1 and Table 2, it is likely that introduction of airport security measures worldwide contributed primarily to the reduced number of fatalities in period II.

·          Second, following an intense wave of fatalities during period III primarily due to use of explosives in airports/airplanes culminating in the Pan Am airliner explosion over Scotland[2], increased vigilance and better procedures to sniff and screen for explosives in airports appears to be directly responsible for the period of low fatalities beginning in 1989 and continuing on into the present era. The end of this period was also marked by the United States Aviation Security Improvement Act of 1990 [Bush, 1990].

·          Finally, over the 1990s, increasingly better detection techniques and a greater degree of automation to screen for explosives were introduced at airports worldwide [Malotky, 1998] thereby continuing to raise the bar for terrorists who sought to employ explosives aboard airplanes.


Also shown in Figure 1, during periods II and IV, there was also a significant reduction, but not elimination, in terrorist incidents (attempts) to deploy explosives in airports and airplanes, even though overall terrorist incidents using explosives continued to rise at an accelerated pace. These trends suggest two important conclusions: 1) airport screening of explosives, although imperfect, has successfully prevented fatalities despite a significant number of continued attempted attacks at airports. 2) The screening measures have been successful in deterring a much greater number of attempts that would have taken place had the screening not been present, serving as a nonlinear deterrent.


Detection systems for explosives employed in today’s airports could not be extended more broadly in populated areas since these systems are based on technology that requires physical contact (swabbing) to wipe off traces of explosive which can then be analyzed by techniques such as ion mobility spectrometry within specialized machines [Malotky, 1998 and p. 114 Committee on Science and Technology for Countering Terrorism, 2002]. But recent advances in remote sensor techniques open up the possibility of extending explosive screening to more spread-out areas without relying on checkpoints. They enable miniature trace-detection sensors for conventional explosives such as TNT [for instance, see Pinnaduwage, 2003] which sniff for explosive particulate matter in the air and recognize specific explosives from a distance, analogous to how a dog would. Similarly, the development of remote non-intrusive sensors for chemical, biological, nuclear, and radiological (dirty) bombs would be needed to defend against these threats.


Taking into account the experience in deterring terrorism in commercial aviation, research in sensors and systems to detect WMD is not a “dead-end” as critics contend, but would be a decision based on a successful proof of concept. The nonlinear deterrence that could be achieved through detection of WMD offers what may be the last line of defense for otherwise vulnerable metropolitan areas.



1.       Brown, K. 2004. “Up In The Air,” Science. 305: 1228-29, August 27, 2004.

2.       Kean, T. H., et. al. 2004. The 9/11 Commission Report, New York: W. W. Norton & Company.

3.       Bush, G. 1990. “Statement on Signing the Aviation Security Improvement Act of 1990,” 16 November. <>

4.       Committee on Science and Technology for Countering Terrorism. 2002. Making the Nation Safer, Washington. D.C.: The National Academies Press. < >

5.       Devabhaktuni, Srikrishna. 2004. “Could a citywide network of sensors prevent terrorist use of bombs?,” 14 September. < >

6.       Dillingham, G. L. 2002. “AVIATION SECURITY: Transportation Security Administration Faces Immediate and Long- Term Challenges,” United States General Accounting Office, GAO-02-971T, 25 July. <>

7.       Enders, W., T. Sandler. 1993. “The effectiveness of antiterrorism policies: a vector-autoregression-intervention analysis,” American Political Science Review, 87(4): 829-844.

8.       Malotky, L., S. Hyland. 1998. “Preventing Aircraft Bombings,” The Bridge, 28 (3), Fall, 1998. <>

9.       Medalia, J. 2003. “Terrorist Nuclear Attacks on Seaports: Threat and Response,” Congressional Research Service, Order Code RS21293, 13 August. < >

10.    MIPT. 2002. “MIPT Terrorism Database System,” 27 August. <>

11.    O’Hanlon, M. E., et. al. 2002. Protecting the American Homeland: A Preliminary Analysis, Washington D.C.: Brookings Institution Press, an updated version (March 2003) is available online at <>

12.    Szyliowicz, J. 2004. “Aviation Security: Promise or Reality?” Studies in Conflict and Terrorism, 27:47-63.

13.    Pinnaduwage, L. A., et. al. 2003. “Explosives: A microsensor for trinitrotoluene vapour,”  Nature. 425: 474, October 2, 2003.

14.    RAND. 2003. "The RAND-MIPT Terrorism Incident Database," 16 September. <>

15.    U.S. Congress, Office of Technology Assessment. 1993. Proliferation of Weapons of Mass Destruction: Assessing the Risks. OTA-ISC-559. Washington, DC: U.S. Government Printing Office. < >

[1] for more on the use of the term “displacement” in the terrorism context, see [p. 2, O’Hanlon, 2002]

[2] The bombing killed 259 people on board and 11 people on the ground [MIPT, 2002]