A substantial body of research in social psychology suggests that egocentric biases inhibit individuals’ ability to discern norms. This chapter identifies and explains two of those biases - the False Consensus Effect and the False Uniqueness Effect - and then explores their relevance and implications for efforts to identify customary international law (CIL). Among other things, the analysis offers insights on the persistent objector rule, the origins and evolution of CIL, and the merits of rational choice models of state action.