Epidemiological modelling of the 2005 French riots: a spreading wave and the role of contagion.

As a large-scale instance of dramatic collective behaviour, the 2005 French riots started in a poor suburb of Paris, then spread in all of France, lasting about three weeks. Remarkably, although there were no displacements of rioters, the riot activity did travel. Access to daily national police data has allowed us to explore the dynamics of riot propagation. Here we show that an epidemic-like model, with just a few parameters and a single sociological variable characterizing neighbourhood deprivation, accounts quantitatively for the full spatio-temporal dynamics of the riots. This is the first time that such data-driven modelling involving contagion both within and between cities (through geographic proximity or media) at the scale of a country, and on a daily basis, is performed. Moreover, we give a precise mathematical characterization to the expression "wave of riots", and provide a visualization of the propagation around Paris, exhibiting the wave in a way not described before. The remarkable agreement between model and data demonstrates that geographic proximity played a major role in the propagation, even though information was readily available everywhere through media. Finally, we argue that our approach gives a general framework for the modelling of the dynamics of spontaneous collective uprisings.

Democracy justifies the means: political group structure moderates the perceived legitimacy of intergroup aggression.

The present research tested the hypothesis that the political structure of conflicting groups moderates perceived legitimacy of intergroup aggression. In two experiments, participants read scenarios of fictitious summer camps in which members of one group aggressed members of another group. The political structure of both the perpetrator and the victim groups was described as either egalitarian (defined with democratic decision-making procedures) or hierarchical (authoritarian decision-making procedures). Results of both experiments showed that aggressions perpetrated by members of egalitarian groups at the expense of members of hierarchical groups were evaluated as less illegitimate than aggressions committed in the three remaining conditions. This effect is discussed as a function of the higher social value attributed to democratic groups.