Peer effects arise in situations where workers observe each others' work activity. In this paper, we disentangle the effect of observing a peer from that of being observed by a peer, by setting up a real effort experiment in which we manipulate the observability of performance. In particular, we randomize subjects into three groups: in the first one subjects are observed by another subject, but do not observe anybody; in the second one subjects observe somebody else's performance, but are not observed by anybody; in the last group subjects work in isolation, neither observing, nor being observed. To assess the importance of payoff externalities in the emergence of peer effects, we consider both a piece rate compensation scheme, where pay de...