The cultural evolution of large-scale human cooperation

Abstract

Large-scale cooperation remains one of the most compelling puzzles in the social sciences. Despite being a defining feature of human sociality and crucial to our species' success, it emerged only recently in our evolutionary history and remains absent throughout the rest of the animal kingdom. I employ cultural evolutionary theory to develop three mathematical models that illuminate distinct aspects of how humans achieve cooperation at unprecedented scales. First, I model how cooperation can expand in scale from increasing cooperative returns and endogenous population dynamics. So long as groups are able to take advantage of population growth to reach new cooperative goals, they can climb a proverbial “cooperation ladder” to continue to increase their scale of cooperation. Second, I model how scales of cooperation interact. In this model, individuals are placed in both a large cooperative group and an embedded small cooperative group, and I find that the smaller group is more stable and can undermine cooperation in the larger group, explaining tensions across cooperative scales. Third, I model cooperative and economic behaviour under different socio-ecological conditions. I find that productive and destructive competition are two sides of the same phenomena. Modelling property rights and social competition, I uncover how competition can lead to cooperative collapse. Altogether, my work reveals how large-scale cooperation is not a natural progression of the human condition but a fragile achievement requiring specific conditions. By studying cooperation as an ecosystem of interacting processes, we can better understand how large-scale cooperation evolved and also how it can fail. These findings have implications for understanding historical transitions, modern challenges like climate change, and the design of institutions that align incentives across social scales. This thesis contributes to a unified theory of human cooperation that explains both its remarkable successes and persistent failures.