We present a model of contagion that unifies and generalizes existing models of the spread of social influences and microorganismal infections. Our model incorporates individual memory of exposure to a contagious entity (e.g. a rumor or disease), variable magnitudes of exposure (dose sizes), and heterogeneity in the susceptibility of individuals. Through analysis and simulation, we examine in detail the case where individuals may recover from an infection and then immediately become susceptible again (analogous to the so-called SIS model). We identify three basic classes of contagion models which we call epidemic threshold, vanishing critical mass, and critical mass classes, where each class of models corresponds to different strategies for prevention or facilitation. We find that the conditions for a particular contagion model to belong to one of the these three classes depend only on memory length and the probabilities of being infected by one and two exposures, respectively. These parameters are in principle measurable for real contagious influences or entities, thus yielding empirical implications for our model. We also study the case where individuals attain permanent immunity once recovered, finding that epidemics inevitably die out but may be surprisingly persistent when individuals possess memory.

This article seeks to explain why some countries experience recurring civil war while others do not. It argues that renewed war has less to do with the attributes of a previous war, as many people have argued, and more to do with the incentives individual citizens have to join rebel groups at any given point in time. Civil wars will have little chance to get off the ground unless individual farmers, shopkeepers, and workers choose to enlist in the rebel armies that are necessary to pursue a war, and enlistment is likely to be attractive when two conditions hold: the status quo for the average citizen is perceived to be worse than the possibility of death in combat, and there is no non-violent outlet for political change. An analysis of all civil wars ending between 1945 and 1996 suggests that improvements in basic living conditions and in the average person's access to political participation have a significant negative effect on the likelihood of renewed war. Countries that are able to increase the economic well-being of their citizenry, and create a more open political system are less likely to experience multiple civil wars regardless of what happened in a previous conflict. Civil wars create what has been called a conflict trap.

In this paper I show that an equilibrium exists to the sequential voting game in which a bandwagon begins with probability one. These bandwagons are driven by a combination of beliefs and the desire of voters to vote for the winning candidate. Signi…cantly, in this equilibrium the pivot probability for each voter is nonzero, even in an in…nite population. Consequently the bandwagons do not always start after one (or at most two) favorable decisions (as do economic cascades) and varying levels of informative voting are observed, consistent with observations from sequential voting in U.S. presidential primaries. Further, voters are exposed to counterintuitive incentives, referred to as “buyers ’ remorse, ” that have been attributed to real primary voters. From the play of this equilibrium an explanation of momentum arises that is consistent with empirical regularities. This interpretation provides a formal distinction between the often ambiguous concepts of momentum and bandwagons, and permits a separation of their e¤ects on the sequential voting mechanism.

This paper investigates between-voter interactions in a social network model of turnout. It shows that if 1) there is a small probability that voters imitate the behavior of one of their acquaintances, and 2) individuals are closely connected to others in a population (the “smallworld” effect), then a single voting decision may affect dozens of other voters in a “turnout cascade. ” If people tend to be ideologically similar to other people they are connected to, then these turnout cascades will produce net favorable results for their favorite candidate. By changing more than one vote with one’s own turnout decision, the turnout incentive is thus substantially larger than previously thought. We analyze conditions that are favorable to turnout cascades and show that the effect is consistent with real social network data from Huckfeldt and Sprague’s South Bend and Indianapolis-St. Louis election surveys. We also suggest that turnout cascades may help explain over-reporting of turnout and the ubiquitous belief in a duty to vote. I thank Robert Bates, Lars-Erik Cederman, Eric Dickson, Paul E. Johnson, Orit Kedar, Gary King, Ferran Martinez I Coma, and Ken Shepsle for valuable feedback on earlier drafts. How does the turnout decision of a single person affect an election? Decision-theoretic models of voting show that the probability of one vote being “pivotal ” in a large electorate is extremely small (Tullock

We present a model of sequential choice which explains the emergence and persistence of unpopular, inefficient behavioral norms in society. We model individuals as naı¨ve Bayesian norm followers, rational agents whose subjective expected utility is increased by adherence to an established norm. Agents use Bayesian reasoning to combine their private preferences and prior beliefs with empirical observations of others’ decisions. When agents must infer the preferences of others from observation, this can result in negative cascades, causing the majority of agents to choose a dispreferred action (because they believe, incorrectly, that they are following the majority preference). We demonstrate that negative cascades can result even when the degree of conformity is relatively low, and under a wide range of conditions (including heterogeneity in preferences, priors, and impact of public opinion). This allows us to present a general model of how rational norm-following behavior can occur, and how unpopular norms might emerge, in real populations with heterogeneous preferences and beliefs.

as failed states, but as state-building failures. The list could easily be extended. In each instance, an international trustee, typically the United Nations or a “coalition of the willing ” sanctioned by the United Nations, sought to rehabilitate a state that had fallen into anarchy and chaos. In only a handful of cases, arguably Cambodia and East Timor, did the international trustee succeed in creating a legitimate government, but even in these examples success was short-lived. The record of state-building by international trustees since the end of the Cold War is grim, and offers few reasons for optimism. The current model of state-building employed by international trustees has been roundly criticized. This model emphasizes national reconstruction through the writing of a new, more democratic constitution and the prompt election of a more representative government. Greater democracy is assumed to be the path to peace, not only between but also within countries (Paris 2004). Some critics, while supporting democratization, see state-building as yet another form of social engineering that no government can do well

This paper characterizes behavior with “noisy” decision making for models of political interaction characterized by simultaneous binary decisions. Applications include: voting participation games, candidate entry, the volunteer's dilemma, and collective action problems with a contribution threshold. A simple graphical device is used to derive comparative statics and other theoretical properties of a “quantal response” equilibrium, and the resulting predictions are compared with Nash equilibria that arise in the limiting case of no noise. Many anomalous data patterns in laboratory experiments based on these games can be explained in this manner.

political opinion, social simulation. Abstract: Our aim is to review the network concept and its relevance on theories of public opinion formation. For this purpose, after discussing social and policy networks, we are reviewing certain network theories of (i) collective action and (ii) voting choices and preferred modes of political participation. Finally, we are presenting a network simulation of public opinion formation that generalizes Axelrod’s adaptive culture model and it is based on both convergent and divergent communicative processes. 1

Rational choice and the dynamics of collective political action:

: A guessing game very popular in some European countries involves several players hiding in their hands a number of coins (or pebbles) between zero and three, then attempting to guess in turn the total number of coins in the hands of everyone, with the restriction that no player can repeat the guess issued by any predecessor. After a full round, the player, if any, who guesses correctly wins. Of course, rounds without a winner are also possible, in which case a new round is started afresh. The purpose of the present article is to present an analysis of this game (called Chinos in Spain, as a perturbation of \chinas", i.e. pebbles), and some of its possible variants. Our primary aim is to show its potential to shed light on some issues of strategic behavior and information transmission that seem very germane to some social and economic problems. KEYWORDS : Information transmission, herding, Guessing game, Chinos game. 1. Introduction As explained, Chinos is a simple non-c...