An Epidemic Model

An Epidemic Model of Mass Killing and Public Support in Civil War
Yuri M. Zhukov∗ May 2, 2011

Abstract If civil war is a contest for popular support, why would a government ever embark on a policy of disproportionate force and mass killing? The logic of civilian defection expects such an approach to easily backfire, as civilians respond to massive losses by opposing the side that inflictedthem. Yet even if civilians balance against the side they believe most likely to kill them, massive violence can still occur and – when it does – is sometimes seen as a key source of success. Using an epidemic model of popular support dynamics, this paper derives a set of conditions under which mass killing can occur in civil war. Such conditions emerge when combatants have imperfect information aboutcivilian defection, when learning is slow, or when one side initially enjoys an asymmetric advantage in levels of active support or flows of recruits.

Civil war is a competition for the support of the population. To the side able to secure it, popular support facilitates the extraction of necessary provisions, tributes and taxes, and generates a supply of military recruits, administrativepersonnel and informants. The smaller this pool of resources, the more difficult it becomes to sustain military operations and build the institutions of a sovereign state. If popular support is the center of gravity, the utility of violence – particularly violence against civilians – depends on how a population responds to the use of force against it. Conventional wisdom expects mass killing andoverwhelming force to easily backfire, as security-minded civilians will balance against the side they believe most likely to kill them. Yet disproportionate violence nevertheless occurs and – in a range of cases from Ukraine to Jordan to Kenya – it often becomes a defining feature of counterinsurgency strategy.1 If civil war is a contest for popular support, why would combatants ever
Department ofGovernment, Harvard University. Very preliminary. Please do not cite or quote without permission of the author. I am grateful to William Bossert, Jeff Friedman, Andrew Radin and seminar participants at MIT, Yale and the Association for the Study of Nationalities for helpful conversations and feedback on earlier versions of this model. All comments welcome: zhukov@fas.harvard.edu 1 Ralph Peters, citingseveral dozen such cases, argues that ignoring this experience while designing counterinsurgency doctrine is tantamount to “cheating on the history exam.” See Peters (2007).
∗

1

Electronic copy available at: http://ssrn.com/abstract=1829655

embark on policies of disproportionate force and mass killing? The targeting of civilians in war has been the subject of a growing volume oftheoretical and empirical research, although deep divisions remain over whether such violence suppresses insurgent support or inflames it. The conventional wisdom holds that the killing of civilians is usually counterproductive.2 Civilian targeting can compel an insecure public to withdraw its support and side with the opposition, thus “balancing” against the biggest threat to civilian survival. A classicexample of this phenomenon can be found in Nazi-occupied Yugoslavia, where German reprisals against civilians alienated the local population and facilitated partisan recruitment.3 If balancing is the dominant response to mass killing, combatants keen on preventing a wave of civilian defection should, as a rule, avoid escalating violence to a disproportionate level. If mass killing does take place,it will generally be a result of some miscalculation or erroneous assumption about civilian choice. This view has been disputed by a parallel body of research on cases where repression and mass killing contributed to military success.4 In this view, civilians can be effectively terrorized into supporting the biggest killer, “bandwagoning” with the side that shows itself willing and capable of...