What This project addresses a central question in the humanities and social sciences: why do some ideas spread easily, whereas others fail? Recent approaches argue that this can be explained as the combined effect of the difficulty of an idea and universal features of human cognitive architecture. Some ideas are more easily comprehended, remembered, and deemed more relevant. This project explores a complementary, culturally informed factor: that the differential spread of ideas depends on their degree of fit with an already existing ecology of ideas that forms a cultural immune system. Ideas do not exist in a vacuum, but take part in larger constructs, such as cultural models and cultural systems. This project explores how such models constrain the formation and spread of new ideas. Why Understanding ecologies of human ideas points us in the direction of an even more important question: why do human cultures stabilize at certain levels of complexity? By reinvestigation culture as a number of immunological systems, I will explore the factors that allow certain ideas to penetrate the system, while others are rejected. Finding the basic mechanisms in recent accounts of human cognitive processing, Predicitive Processing, I will present a novel and innovative model of; What cultural immunological systems are and what they are good for How they attain resilience and stability with a special focus on religion How they interact with other cultural systems and with social institutions. How The project is predominantly conceptual. By integrating recent models of immunology from theoretical biology with state-of-the-art neurocognitive theories of Predicitive Processing, I will construct a theoretical model of the dynamic interaction between relatively stable, cultural systems, and their conceptual and behavioral environment. This will lead to an examination of their adaptive functions, why they seem to stabilize at certain levels of complexity, and what happens when they interact. Finally, I analyze a number of distinct cases, from small-scale societies, over the emergence of ‘axial-age’ or world religions, to the formation of modern nation-states in order to explore if religions function as the primary immune-response that instils resilience to cultural systems. SSR Humanity is in an unprecedented crisis. Despite globalization, understood as the ever more in-depth integration of economic, social and, to some extends cultural systems, we experience a national and ethnic revival, a local ‘immune-reaction,’ that could hamper solving urgent problems, most pertinently the climate crisis and biological mass extinction. Better models of how ideas spread and how they connect to social and cultural systems are called for, if we are to affect the necessary, major changes in behavioral patterns required by these crises. In the longer perspective, such models might explain the necessary conditions for creating new, global-level cultural systems able to conceptually match equally global crises.