Margaret Levi and I were awarded a seed grant from the Center for Innovation in Global Health to investigate the aggregate consequences of individuals making decisions with potentially existential consequences under structural uncertainty. This research will build on the work that Paul Smaldino and I have started in our recent Evolutionary Human Sciences paper. In particular, we will investigate the consequences for aggregate behavior of people employing social-learning heuristics such as conformist or prestige bias when faced with uncertainty. Potentially existential threats, such as catastrophic climate change or severe pandemics, are characterized by uncertainty. Information important for making decisions is unknown and, for all practical purposes, largely unknowable on the relavant time scale for many types of action. Starting with Boyd and Richerson's classic (1985) work, there is a broad belief that when faced with uncertainty, people employ conformist-biased learning to make their decisions. Paul and I show that these types of social heuristics have consequences for dynamics of coupled diffusion processes (e.g., infection) and we believe there is much richer system behavior waiting to be discovered. For this CIGH project, we will develop detailed simulation models, but we hope to extend this project with empirical research in subsequent work. This work has lots of implications for the dynamics of misinformation, epidemic control in heterogeneous populations, and facilitating collective action for solving societal problems.