Human societies are characterized by complex and varied social systems that change through time due to communication and negotiation. Jérôme Rousseau makes cognitive complexity his starting point in an innovative study of how and why human societies evolve. The focus of Rousseau's enquiry is "middle-range" societies - a vast category between hunter-gatherers and states. Breaking away from traditional analyses of social evolution as a response to ecological constraints, he shows that social systems are maintained and transformed through self-interest and suggests that conflicts about sharing generate social transformations that result in inequality and increasingly encompassing socio-political structures. Rethinking Social Evolution is a wide-ranging exploration of how language and increased cognitive abilities constitute the motor of social evolution. Drawing on a wide range of ethnographic case studies, Rousseau offers a better understanding of how modern societies are the result of choices by people who both collaborate and compete.