1 a 1 Indirect reciprocity with private, noisy, and incomplete information Christian Hilbe11, Laura Schmid', Josef Tkadleca, Krishnendu Chatterjeee, and Martin A. Nowak" 'Institute of Science and Technology Austria, 3400 Klostemeuburg, Austria; °Program for Evolutionary Dynamics, Harvard University, Cambridge, MA 02138; 'Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138; and °Department of Mathematics, Harvard University, Cambridge, MA 02138 Edited by Brian Skyrms, University of California, Irvine, CA, and approved October 16, 2018 (received for review June 19, 2018) Indirect reciprocity is a mechanism for cooperation based on shared moral systems and individual reputations. It assumes that members of a community routinely observe and assess each other and that they use this information to decide who is good or bad, and who deserves cooperation. When information is trans- mitted publicly, such that all community members agree on each others reputation, previous research has highlighted eight cru- cial moral systems. These "leading-eight" strategies can maintain cooperation and resist invasion by defectors. However, in real populations individuals often hold their own private views of oth- ers. Once two individuals disagree about their opinion of some third party, they may also see its subsequent actions in a different light Their opinions may further diverge over time. Herein, we explore indirect reciprocity when information transmission is pri- vate and noisy. We find that in the presence of perception errors, most leading-eight strategies cease to be stable. Even if a leading- eight strategy evolves, cooperation rates may drop considerably when errors are common. Our research highlights the role of reli- able information and synchronized reputations to maintain stable moral systems. cooperation I indirect reciprocity I social norms I evolutionary game theory H umans treat their reputations as a for