Variant two, “Inter-Group Violence,” is more interesting. Now agents are divided into two ethnicities, blue and green. “Legitimacy becomes each group’s appraisal of the other group’s right to exist,” Epstein explained. In this context, an agent’s going activist means that it kills a member of the opposing ethnic group. The cops are peacekeepers, and if the model is run without them and L among all agents is reduced by as little as 20 percent, ethnic cleansing quickly begins. When cops are introduced, safe havens emerge. Nonetheless, interethnic hostility continues. Ultimately, as figure 2 shows and Epstein told me, “when you drop legitimacy in this variant, it always ends with one side wiping the other out.” Cop density can be set at any level. “At low cop densities, you get rapid genocide. At high cop densities, you likewise can sometimes get rapid genocide, but also a highly variable outcome. On average, more cops makes it take longer.” Enough longer to justify the expense of extra policing? It’s all just highly uncertain, Epstein says; merely to have a surge of cops would not guarantee a good outcome.

Altogether, in fact, Epstein stressed that his models were mostly aimed at achieving explanatory power. “To explain something doesn’t mean that you can predict it,” he said. He pointed out that though we can explain lightning and earthquakes, we can’t forecast either. If we’re hoping, like Asimov, to predict the future, Epstein’s models will disappoint. In fact, because his models give widely divergent results even when their agents are programmed with very simple rules, they indicate that predicting the future will never be possible. Still, Epstein’s artificial societies do more to make plain the hidden mechanisms underlying social shifts–and their unexpected consequences–than any tool that social scientists have hitherto possessed. In the future, they and others like them could suggest how policymakers can engineer the sorts of small, cheap interventions that have large, beneficial results.

Mark Williams is a Technology Review contributing editor.

Generative Social Science: Studies in Agent-Based Computational Modeling
By Joshua M. Epstein
Princeton Studies in Complexity series
Princeton University Press, 2006, $49.50