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The models are fascinating. In both of the variants described in “Generating Patterns of Spontaneous Civil Violence” (see figures 1 and 2), there are regular agents as well as agents called cops, representing a central political authority. The left screen depicts regular agents’ overt behavior (blue if quiescent, red if active) and the right the underlying “emotionscape,” where agents are colored according to their level of political grievance (the darker the red, the higher the grievance). Grievance has two components: legiti­macy (L) of the state, as perceived by the agents, and hardship (H), which is physical or economic privation and varies between agents. Furthermore, agents can deceive: on the left screen, aggrieved agents can turn blue (appearing nonrebellious) when cops (always black) are near, then turn red (actively rebellious) when cops move away. Epstein also assigned varying levels of risk aversion (R) to the agents: some are more inclined to rebel than others. Agents assess their likelihood of arrest by cops before joining a rebellion, and their assessments depend on their vision (v) of what’s around them–that is, how many grid positions (north, south, east, and west) they can see. Finally, agents arrested by cops receive jail sentences (J). “Arrested agents go to jail for a random duration and emerge as aggrieved as they went in,” Epstein told me. “I always joke that those are the only two realistic assumptions in the whole model.”

Though this model may seem overly simple, it generates realistic enough patterns once the human operator sets the parameters of L and J, the agents’ and cops’ vision, and their initial densities and then lets both groups move around and interact. In variant one, “Generalized Rebellion against Central Authority” (see figure 1), high concentrations of activist, aggrieved agents can arise in zones with low cop densities. When that happens, even mildly aggrieved agents find it rational to risk rebellion. It’s for just this reason that freedom of assembly is generally the first thing curtailed under repressive regimes. Furthermore, the model displays the hallmark of a complex system: punctuated equilibrium, with long periods of relative stability broken by rebellious outbursts. In some runs, the right-hand “emotionscape” screen may be bright red with the agents’ grievance, while the left screen is entirely blue because of their public quiescence. Which would be more likely to trigger revolution: a large absolute reduction of L (legitimacy) in small increments or a smaller reduction carried out in one large step? The latter, it turns out. In the case of the large but incremental reduction, cops can pick off activist agents one by one and jail them. Conversely, a sudden, sharp reduction in legitimacy spurs multiple aggrieved agents into active rebellion at once. As Epstein noted, “Once there are 50 people rebelling, it’s a lot less risky to be the 51st.”

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