This document summarizes a study on religious extremism and the conditions under which extremist groups emerge and thrive. It presents a theoretical model and agent-based simulation showing that extremist groups are more sustainable when the substitutability between religious and secular activities is high. The model also shows that population income matters but is less influential than substitutability. Regression analysis of empirical data supports the theory, showing extremism is higher where religious and secular activities are more substitutable. Potential policy interventions discussed include reducing the appeal of extremism by providing public goods that compete with religious groups.