How do norms evolve when people have no choice to opt-out of social interactions? One example of such a setting is prison. Past research usually relies on ethnographic work to understand the emergence and maintenance of norms among prisoners. In their new paper, Chad W. Seagren and NOUS member David Skarbek instead use this rich qualitative literature to inform an agent-based model to demonstrate how norms evolve in response to demographic changes in prison. In the model, agents play a one-shot, though possibly repeated, prisoner’s dilemma with other agents. Agents lack the ability to decline to play with their selected opponent. They consider tag-mediated play and norm enforcement as mechanisms to facilitate prisoner cooperation and to examine the effects of increasing prison populations and increasing ethnic heterogeneity on the maintenance of cooperative norms. They also calibrate the model with empirical data from the California prison system. Parameters of the model correspond to demographic changes between 1951 and 2016, where the size of the prison population increased 14-fold and ethnic heterogeneity by 30%. Simulation results show that such changes dramatically decrease levels of cooperation and compliance. These results are consistent with the actual observed breakdown of the cooperative norms in California prisons.