In his Cold War classic, Strategies of Containment, John Gaddis observes that human behaviors can be analyzed either by splitting them into small conceptual units or by lumping them together into larger ones. Whereas splitters seek analytic leverage by highlighting fundamental behavioral differences, lumpers seek leverage by underscoring their essential commonalities. Within the field of terrorism studies, these divergent approaches yield markedly dissimilar counterterrorism implications.
Terrorist splitters carefully distinguish terrorism from other types of substate violence, especially guerrilla warfare and civil war. They define terrorism narrowly, as the select use of violence against civilians to strike fear in the broader public for putative political ends. Terrorist lumpers, by contrast, employ an expansive definition of terrorism, brooking no distinction between this tactic and either guerrilla warfare or civil war. Particularly outside of North America, lumpers even count as terrorism acts of violence from government officials or militaries.
In the lead essay of Cato Unbound, IRA expert Paddy Hillyard makes two main recommendations. The first is pedagogical — for us to retire the word “terrorism” for the more general term “political violence,” which lumps together a wide range of violent behaviors perpetrated by substate groups (e.g., the IRA), paramilitary agents of the state (e.g., Loyalists), and governments themselves (e.g., Her Majesty’s). The second is policy prescriptive — for us to combat terrorism by demonstrating its political ineffectiveness relative to alternative forms of protest. The first recommendation conflicts with the second, however, illustrating the perils of terrorist lumping.
Numerous scholars have hinted at conceptual problems resulting from what I call terrorist lumping. For example, Alex Schmid and Albert Jongman warn: “Without isolating terrorism from other forms of political violence, there can be no uniform data collection and no responsible theory building.” Similarly, RAND cautions: “Because context matters so greatly, data analysis needs to distinguish better among classes of political violence,” for example, between “terrorism versus rebellion, ethnic conflict, social movements, and civil war.” Indeed, recent works detail some of the conceptual pitfalls of lumping terrorism with these other kinds of violence, which are admittedly sometimes interrelated.
Nowhere are such pitfalls clearer than in the ongoing debate over terrorism’s political effectiveness. Lumpers such as Robert Pape invariably believe that terrorism is a winning tactic for coercing major government concessions. As evidence, they point to substate campaigns directed against military personnel that have indeed pressured concessions. Salient examples include the Soviet withdrawal from Afghanistan in 1989, the U.S. withdrawal from Lebanon in 1984, and the French withdrawal from Algeria in 1962. Significantly, terrorist splitters do not regard these substate campaigns as evidence of terrorism’s political effectiveness. Rather, they contend that disaggregating substate campaigns directed against civilian targets versus military ones is critical for appreciating terrorism’s abysmal political record. In my 2006 article in International Security entitled “Why Terrorism Does Not Work,” I demonstrate that the international groups of greatest concern to the United States have achieved their policy demands only in campaigns directed against military targets, not civilian ones. Building off of this research, fellow splitter Audrey Cronin develops in her new book several case studies illustrating how attacking civilians only hardens governments from making concessions. As Hillyard suggests with respect to the IRA, the terrorism led to government crackdowns, not compliance.
The good news is that because a strict definition of terrorism reveals that it does not pay politically, governments are armed with a powerful, ready-made message to deter aggrieved groups from targeting their populations. This message can and should be disseminated widely by the international community. In fact, there is strong empirical evidence that militant leaders embrace terrorism because they overestimate its political effectiveness by conflating this tactic with more effective types of substate violence, just as academic lumpers do. As the strategic rationale behind the 9/11 terrorist attacks, for example, Osama Bin Laden regularly invokes prior substate campaigns that succeeded against military — not civilian — targets, namely the Soviet withdrawal from Afghanistan and the U.S. withdrawals from Somalia and Lebanon in the 1980s and 1990s. Similarly, during the second Intifada, Hamas leaders routinely invoked the Israeli retreat from Lebanon in May 2000 as their main strategic rationale for blowing up Egged buses in Jerusalem — apparently unaware that the former triumphed by targeting the Israel Defense Forces rather than the local population, which only shifts electorates to the right and thereby rewards governments for digging in their political heels. Through splitting, we can help to deter terrorism by disabusing its leading practitioners of the common misperception that it pays politically.
The bad news is that if terrorism does not induce concrete political gains, then its practitioners presumably derive other utility from their actions, rendering counterterrorism fixes more difficult than simply addressing structural inequalities. Terrorism’s poor political track record suggests that adopting this tactic is an equifinal phenomenon in which there are apolitical motives for using it, such as the selective benefits of participating in a thrilling, tight-knit social unit. The so-called Underwear Bomber, Umar Farouk Abdulmutallab, says that he elected to affiliate with al-Qaida not to achieve its political platform, but because “I do not have a friend, I have no one to speak to, no one to consult, no one to support me, and I feel depressed and lonely.” This attempted mass casualty terrorist attack on the United States was apparently less a case of political violence than social violence. Indeed, recent research suggests that social motives may have greater explanatory power than political ones in accounting for the behavioral tendencies of terrorist leaders and foot soldiers alike. This finding implies that far from a counterterrorism panacea, any political solutions must be complemented with social ones to reduce the varied incentives of participating in terrorist groups.
As terrorism scholars increasingly rely on large-n analyses, establishing a universally agreed upon unit of analysis will become ever more important. Conflating conceptually unlike units may occasionally have normative appeal, but risks yielding ambiguous, misleading results that obscure rather than inform counterterrorism strategy. For this reason, splitting — not lumping — is the future of terrorism studies.
 John Lewis Gaddis, Strategies of Containment: A Critical Appraisal of Postwar American National Security Policy (Oxford: Oxford University Press, 1982), preface.
 Alex Schmid and Albert Jongman, Political Terrorism: A New Guide to Actors, Authors, Concepts, Data Bases, Theories, and Literature (New York: Transaction, 2005), 3.
 Paul K. Davis and Kim Cragin, eds., Social Science for Counterterrorism: Putting the Pieces Together (Santa Monica, Calif: RAND, 2009), xxii.
 See, for example, Erica Chenoweth, Nicholas Miller, and Elizabeth McClellan, Hillel Frisch, Paul Staniland, and Max Abrahms, “Correspondence: What Makes Terrorists Tick,” International Security 33:4 (Spring 2009), 180-202; and Audrey Cronin, How Terrorism Ends: Understanding the Decline and Demise of Terrorist Campaigns (Princeton, NJ: Princeton University Press, 2009).
 Max Abrahms, “Why Terrorism Does Not Work,” International Security 31:2 (Fall 2006), 42-78.
 Audrey Cronin, How Terrorism Ends: Understanding the Decline and Demise of Terrorist Campaigns (Princeton, NJ: Princeton University Press, 2009).
 The Daily Mail, “How a Middle-Class Nigerian Boy Was Seduced by Al Qaeda into Trying to Blow Up a Transatlantic Jet,” 2 January 2010.
 Max Abrahms, “What Terrorists Really Want: Terrorist Motives and Counterterrorism Strategy,” International Security, 32:4 (Spring 2008), 78-105.