Terrorism is a subject matter, not a discipline. It has been approached by scholars from various academic perspectives with political scientists in the lead.

In an effort to get a firm hold on a slippery subject, those studying the phenomenon of terrorism were obliged to define it more precisely. Terrorism could be described simply as the use or threat of violence to create an atmosphere of fear and alarm and thus bring about a political result. But making this definition operative in political debate, rules of war, or criminal codes was anything but easy. Is all politically motivated violence terrorism? How does terrorism differ from ordinary crime? Should terrorism be considered a crime at all, or should it be seen as simply another form of armed conflict that is no less legitimate than any other form of war? Is the term properly reserved for those trying to overthrow governments, or can governments also be terrorists?

Definition was crucial because it ultimately determined the way in which terrorism has been studied. A major problem was that terrorism almost always has a pejorative connotation and thus falls in the same category of words as “tyranny” and “genocide,” unlike such relatively neutral terms such as “war” and “revolution.” One can aspire to objective and dispassionate research, but one cannot be neutral about terrorism any more than one can be neutral about torture. Thus, defining terrorism became an effort not only to delineate a subject area but also to maintain its illegitimacy. Even the most clinical inquiry was laden with values and therefore political issues. The very study of terrorism implied to some a political decision.

Terrorism can be defined objectively by the quality of the act, not by the identity of the perpetrators or the nature of their cause. All terrorist acts are crimes, and many also would be war crimes or “grave breaches” of the rules of war if one accepted the terrorists’ assertion that they wage war. All terrorist acts involve violence or the threat of violence, sometimes coupled with explicit demands. The violence is directed against noncombatants (Lesser, 1999). The purposes are political. The actions often are carried out in a way that will achieve maximum publicity, and the perpetrators are usually members of an organized group.

Terrorist organizations are by necessity clandestine, but unlike other criminals, terrorists often but not always claim credit for their acts. Finally – the hallmark of terrorism – the acts are intended to produce psychological effects. This introduces a distinction between the actual victims of terrorist violence and the target audience. The connection between the victim and the target of terrorism can be remote. The identity of the victims may be secondary or even irrelevant to the terrorist cause. “Pure terrorism” is entirely indiscriminate violence. Terrorism differs from ordinary crime in its political purpose and its primary objective. However, not all politically motivated violence is terrorism, nor is terrorism synonymous with guerilla war or any other kind of war.

Terrorist techniques can be used by governments or those fighting against governments: however, scholars generally use the term “terror” when discussing fear-producing tactics employed by governments and ‘terrorism’ when referring to tactics used by those fighting against governments. The distinction is primarily semantic. Both groups may use threats, assassinations, or abductions, but government terror also may include arbitrary imprisonment, concentration camps, torture, mind-affecting techniques, and the use of drugs for political purposes. Antigovernment terrorists generally lack the infrastructure for such tactics. Government terror produces more victims than terrorism does. Terrorists tend to seek more publicity than do governments (Lesser, 1999).

Although a prerequisite to empirical research, the attempt to define terrorism inevitably lent greater coherence to disparate acts of violence than did any analysis offered by the terrorists themselves, few of whom thought of assassinations, bombings, kidnappings, and airline hijackings as elements of a unified tactical repertoire, let alone the basis of a strategy. Ironically, in an effort to understand a phenomenon, researchers ran the risk of attributing to terrorists a level of strategic thinking they may not have possessed.

Qualitative, case-study research method has dominated the terrorism topic for many years. Since the number of observations in the greater part of this work is really small, researchers have been cautious to delineate terrorism to fit the cases under examination. The undersized quantity of observations, regrettably, often disallows unreliable dubious parts of the definition. In one country, for instance, hostility against the military might take place, but in the second country it might not. In an assessment of the first country, one could diverge the definition beyond civilian targets to military targets. In an assessment of the second country, one could not diverge the definition to investigate the implications of unreliable degrees from minimal to maximal definitions (Lesser, 1999).

In current research on terrorism in the science writing, there is plenty of room to diverge the definition of terrorism to identify with its consequences. Specifically, there is no need to decide on one particular definition of terrorism; multiple definitions can be allowed and then the effects can be empirically sorted out. Empirical analysis might generate two measures of terrorism: one with civilians as the target and the second with both civilians and the military at peace time as the target. Moreover, empirical analysis may demonstrate whether results are alike or diverse dependent on the measure. And any results would have implications for future theoretical and empirical research (American Association for the Advancement of Science, 2004).

As science research on terrorism is turning out to be more and more quantitative, enough deviation is present within most databases on terrorism to investigate variations on definitions. Researchers should make lucid conceptual and conjectural arguments, but on main points of contention, empirical analysis can clarify what significant role the contention plays when implemented in a large quantity of cases. Research on terrorism has been very prolific in some fields, but carries on to spin its wheels in other ones. In general, the primary requirement is that units of scrutiny should be chosen to fit the theoretical argument, which often does not take place. Carrying attention away from definitional subject might result in great progress in the research of terrorism (Lesser, 1999).

Among the latest applications of agent-based modeling to terrorism is Leweling and Nissen (2007). They apply ABMs to assess how various counterterrorism choices influence horizontally versus vertically organized terrorist groups. Here goes one impending frontier for terrorism research: applying dynamic agent-based models to create predictions about which policies by governments can efficiently trim down terrorism. Akin to many of the formal models in the records of terrorism articles, there is no empirical analysis of the predictions of this exacting model. Combining event data on terrorist attacks with agent-based models is a would-be promising way to the fore.

Much research on terrorism has focused more narrowly on the topic. In part, this reflects the desire of researchers to avoid the murky, politically loaded area of underlying causes, where any discussion might be seen as condemnation or rationalization of terrorist violence. Nonetheless, there have been excellent case studies of individual groups and their tactics.

Defining terrorism in terms of the act has enabled researchers to maintain a theoretically objective approach and conduct at least some primitive quantitative analysis. Event-based analysis has enabled them to discern broad patterns and trends and chart the growth of terrorism and its diffusion around the globe. They have been able to demonstrate statistically that as terrorism has increased in volume, it has also become bloodier. Researchers were able to illustrate a clear trend toward incidents of large-scale indiscriminate violence in the 1980s and infer that terrorists tend to be more imitative than innovative in their tactics. Event-based analysis also has permitted researchers to distinguish the operational profiles of specific terrorist groups, and these profiles have been useful in identifying changes in a group’s modus operandi.

At the same time, event-based analysis has led the analysts into some methodological traps. An exclusive focus on terrorist actions, for example, resulted in terrorists being viewed first as if they were all part of a single entity and second as if they were almost extraterrestrial. While there are connections and alliances among some terrorist groups, the only thing the terrorists of the world have in common is a propensity for violence and certain tactics. Moreover, each group is rooted in its own social, political, and cultural soil, and cross-national comparisons are difficult. This has led to the question of whether there is such a thing as a terrorist-prone society.

It is, however, dangerous to attribute the actions of a few to perceived political defects or cultural flaws of a society as a whole, and researchers’ attempts to discern deeper causes or conditions that lead to high levels of terrorism in certain societies have produced meager results. Terrorism is not demonstrably a response to poverty or political oppression. The liberal democracies of Western Europe have suffered high levels of terrorist violence, while totalitarian states are virtually free of terrorism. Overall, countries with perceived terrorist problems tend to be comparatively advanced politically and economically. They are more highly urbanized and have higher per capita incomes, larger middle classes, more university students, and higher rates of literacy. One may ask whether political and economic advancement simply brings a more modern form of political violence.

One obstacle to linking high levels of terrorism with environmental factors is the problem of measuring terrorism. For the most part, this has been done by counting terrorist incidents, but international terrorism was narrowly and, more important, artificially defined to include only incidents that cause international concern, a distinction that has meant very little to the terrorists. Counting all terrorist incidents, both local and international, is better but still inadequate. Terrorist tactics, narrowly defined, represent most of what some groups, particularly those in Western Europe, do but for other groups, terrorism represents only one facet of a broader armed conflict. In civil war situations, such as that in Lebanon in the 1970s, separating incidents of terrorism from the background of violence and bloodshed was futile and meaningless. And what about the extensive unquantified political and communal violence in the rural backlands of numerous third world countries? Broad statements about terrorist-prone or violence-prone societies simply cannot be made by measuring only a thin terrorist crust of that violence, if at all. The problem, however, is not merely one of counting. Although terrorists arise from the peculiarities of local situations, they may become isolated in a tiny universe of beliefs and discourse that is alien to the surrounding society. German terrorists were German, but were they Germany? In the final analysis, one is forced to dismiss the notion of a terrorist-prone society.

If terrorism cannot be explained by environmental factors, one must look into the mind of the individual terrorist for an explanation. Are there individuals who are prone to becoming terrorists – a preterrorist personality? Encouraged by superficial similarities in the demographic profiles of terrorists – many of them have been urban middle and upper class (not economically deprived) males in their early twenties with university or at least secondary school educations – researchers searched for common psychological features.

Behavioral analysts painted an unappealing portrait: The composite terrorist appeared to be a person who was narcissistic, emotionally flat, easily disillusioned, incapable of enjoyment, rigid, and a true believer who was action-oriented and risk seeking. Psychiatrists could label terrorists as neurotic and possibly sociopathic, but they found that most of them were not clinically insane. Some behavioral analysts looked for deeper connections between terrorists’ attitude toward parents and their attitudes toward authority. A few went further in claiming a physiological explanation for terrorism based on inner ear disorders, but these assertions were not given wide credence in the scientific community. The growing number of terrorists apprehended and imprisoned in the 1980s permitted more thorough studies, but while these studies occasionally unearthed tantalizing similarities, they also showed terrorists to be a diverse lot.

Much research on terrorism has been government-sponsored and therefore oriented toward the practical goal of understanding terrorism in order to defeat it. While social scientists looked for environmental or behavioral explanations for terrorism, other researchers attempted to identify terrorist vulnerabilities and successful countermeasures. They achieved a measure of success in several areas. Studies of the human dynamics of hostage situations led to the development of psychological tactics that increased the hostages’ chances of survival and a better understanding (and therefore more effective treatment) of those who had been held hostage. In some cases, specific psychological vulnerabilities were identified and exploited. With somewhat less success, researchers also examined the effects of broader policies, such as not making concessions to terrorists holding hostages and using military retaliation. The conclusion in this area were less clear-cut.

Another area of research concerned the effects of terrorism on society. Here, researchers viewed terrorism as consisting of not only the sum of terrorist actions but also the fear and alarm produced by those actions. Public opinion polls, along with measurable decisions such as not flying and avoiding certain countries, provided the measure of effect.

Some critics who are skeptical of the entire field of terrorism analysis assert that the state and its accomplice scholars have “invented” terrorism as a political issue to further state agendas through manipulation of fear, the setting of public discourse, preemptive constructions of “good” and “evil”, and the creation of deliberate distractions from more serious issues. “Terrorism”, a pejorative term that is useful in condemning foes, has generated a lot of fear mongering, and the issue of terrorism has been harnessed to serve other agendas, but one would have to set aside the reality of terrorist campaigns to see terrorism solely as an invention of the hegemonic state. While such deconstructions reveal the ideological prejudices of their authors, they nonetheless have value in reminding other analysts to be aware of the lenses through which they view terrorism.

Over the years, research on terrorism has become more sophisticated, but in the end, terrorism confronts people with fundamental philosophical questions: Do ends justify means? How far does one go on behalf of a cause? What is the value of an individual human life? What obligations do governments have toward their own citizens if, for example, they are held hostage? Should governments or corporations ever bargain for human life? What limits can be imposed on individual liberties to ensure public safety? Is the use of military force, as a matter of choice, ever appropriate? Can assassination ever be justified? These are not matters of research. They are issues that have been dictated through the ages.

The free creation and exchange of knowledge by scientists can present dangerous, unintended consequences for society. A paper by Ronald Jackson and other researchers found that the insertion of IL-4 genes into mousepox viruses resulted in near total immunosuppression (Jackson, Ramsay, Christensen, et al. 2001). This advanced valuable knowledge about immune system functioning, but it also evoked fears that terrorists could use such knowledge to engineer hyper-virulent viruses. Similarly, the journal Science published a paper in 2002 that showed how to assemble a poliovirus from readily available chemicals (Cello, Aniko, Eckerd 2002). The threat of terrorist acts has caused political leaders and members of the scientific community to question whether such knowledge should be created, and if so, how its publication and exchange should be regulated.

The twentieth century provided several examples of tradeoffs between security and openness in the pursuit of knowledge. The Manhattan Project that produced the first atomic bomb cultivated a culture of secrecy. A similar culture developed among researchers studying microwaves during World War II. During the Cold War, the U.S. government attempted to constrain information exchange in some areas of mathematics and the physical sciences that may have aided Soviet nuclear weapons development (Monastersky 2002).

In 1975, an international group of scientists held the Asilomar conference to debate the proper use and regulatory oversight of recombinant DNA research. During the late 1970s, the National Security Agency (NSA) regulated cryptographers developing new algorithms, but the two groups eventually agreed to a system of voluntary submission of papers for review. In 2002, the U.S. government began to withdraw from public release more than 6,600 technical documents dealing mainly with the production of germ and chemical weapons. In a controversial move, the U.S. national policy for the restriction of information that may threaten national security was altered in the wake of the September 11 attacks to include restrictions on publication of federally-financed research deemed to be “sensitive but not classified” (Greenberg 2002).

As these examples illustrate, limitations on research and the availability of technical knowledge can come in the form of self-imposed screening mechanisms by the scientific community or government regulation. The Asilomar conference, for example, led to a suite of self-policing mechanisms within the scientific community, including the decentralized system of Institutional Biosafety Committees (IBCs). This same mechanism has been proposed by the National Science Advisory Board for Biosecurity (NSABB) as a way to prevent the misuse of biological research by terrorists. The NSABB also works to develop codes of conduct for researchers and laboratory workers, which underscores the importance of ethical conduct by individuals, especially where no rules exist or where the precise meaning of rules is unclear. Some professional associations and journals, including Science and Nature, have instituted procedures to give special scrutiny to papers that raise security concerns (Malakoff 2003). Putting such control in the hands of journal editors has caused some to argue that an advisory group like the Recombinant DNA Advisory Committee (RAC) would be a better mechanism.

Mitchel Wallerstein (2002) points out that the dangers posed by terrorists acquiring sensitive science and technology information differ from the state-related threats that were of primary concern during World War II and the Cold War. Terrorists generally do not seek out and would not be able to use the results of most basic research, but states may possess the intellectual and financial capital necessary to turn basic research into weapons. Daniel Greenberg (2002) contends that terrorists do not rely on new science. Rather, readily accessible information that has long been available suffices to fulfill most of the goals of terrorist organizations.

Restricting the publication of information deemed sensitive and controlling access to technologies and research materials can help achieve security goals, but not without costs (Knezo 2002a). Some impacts are relatively minor, such as new standards for the construction and management of laboratories. Other impacts are more severe, including the impact of national security policy measures on the research process. Tightened laboratory access policies, publication rules, and visa restrictions may reduce the number of applications by foreign students to U.S. universities and colleges. This could hamper cross-cultural understanding. According to State Department rules, consular officials may deny visas for study in the United States in sixteen categories specified on the Technology Alert List to students from countries listed as “state sponsors of terrorism”. Additional exemptions to the Freedom of Information Act (FOIA) and the withdrawal of information from federal agency websites have also sparked concerns about constraints on legitimate scientific work and academic freedoms.

Scientific research and technological innovations can improve performance of all phases, from threat analyses and vulnerability assessments to post-attack investigations and restoration of services. For example, the Bush administration established BioWatch, a nationwide system of sensors to detect the presence of certain pathogens, and a public-health surveillance system that monitors the databases of eight major cities for signs of disease outbreaks. Early warning systems can detect the presence of certain pathogens by utilizing computer chips and antibodies or pieces of DNA (Casagranda 2002). Explosives-detection technologies have also been spurred since September 11, 2001 in order to bolster airline security.

Other examples include the use of biometrics (e.g., fingerprints and retinal signatures) to develop national security identity cards. The shipping industry is slowly adopting new security measures such as sophisticated seals and chemical sensors. Other researchers are developing strategies for securing information systems. Military infrared countermeasures for surface-to-air missiles may be used on civilian aircraft. Technologies for decontamination, blast-resistant walls, and protective gear for first responders are other components of research programs. Increasing flexibility and innovating measures to isolate failing elements could increase security of more complex technical systems such as transportation and communication infrastructures. Researching and developing broader applications of renewable energy can harden the energy infrastructure. Social scientists and psychologists also provide research for understanding causes and motivations of terrorists as well as the dynamics of terrorist group formation. Some (e.g., Susser, Herman, Aaron 2002) have demonstrated that, because terrorists choose targets to maximize psychological impact, mental health must be considered a top response priority.

With all of these potential applications of science and technology, decision makers need to address questions about how to coordinate, organize, prioritize, and evaluate investments to serve the goals of security and public health. Genevieve Knezo (2002b) reported that prior to September 11, 2001, the Government Accountability Office (GAO) and other authorities had questioned whether the U.S. government was adequately prepared to conduct and use R&D to prevent and combat terrorism. Partially in response to the need to better coordinate counterterrorism efforts (including R&D), the cabinet-level Department of Homeland Security (DHS) was created by legislative act in 2002. This incorporated half of all homeland security funding within a single agency. In addition to legislative activity, new advisory bodies such as the NSABB have been formed to guide the creation of new rules and development of new institutions to maximize the benefits of science and technology while minimizing unintended negative impacts.

Increased scientific research on counterterror measures will create new knowledge and opportunities for terrorist exploitation, which will create new challenges for securing that knowledge. Given that security, health, and civil liberties are at stake in decisions about science and terrorism, it is important that measures be taken to involve and inform citizens. This entry has focused on actions by the U.S. government because it plays a leading role in matters of science and terrorism. But other countries and international coalitions face similar ethical dilemmas and policy choices. Private companies own many of the infrastructures that are targets for terrorist attacks, so regulations may be required to induce the private sector to invest in counterterrorism technologies that may not have commercial markets. Some scientific research, however, may have viable market applications, meaning that some of the R&D burden can be privatized, which raises other ethical issues that partially mirror those involved in the privatization of war.