Why do humans congregate in groups defined by distinct cultural styles? This book seeks to answer this question through a simple yet ambitious hypothesis. It explains the emergence and persistence of social groups as dependent upon the existence of cultural immune systems that regulate both internal and external flows of information. Based on a model of the human mind as a host of cognitive systems aimed to predict the world through probabilistic models, Sørensen argues that as humans congregate in groups, we gradually align these models forming distinct social and cognitive niches. This not only enhances our ability to predict the mind and behavior of others thereby easing in-group coordination; as shared cultural models stabilize internal cognitive processing, we become self-predictive thereby paving the way for the emergence of a more stable sense of self. Further, prompted by our predictive minds, cultural models form more coherent cultural systems that regulate which new ideas are accepted and which are rejected. Identifying particular cultural immune systems, notably ritual, law, and education, Sørensen argues that these actively align participants with the predominant cultural style of a social group, and regulate interaction at different levels of social complexity.