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Research in behavioural ethics repeatedly emphasizes the importance of others for people’s decisions to break ethical rules. Yet, in most lab experiments participants faced ethical dilemmas in full privacy settings. We conducted three experiments in which we compare such private set-ups to situations in which a second person is co-present in the lab. Study 1 manipulated whether that second person was a mere observer or co-benefitted from the participants’ unethical behaviour. Study 2 investigated social proximity between participant and observer–being a friend versus a stranger. Study 3 tested whether the mere presence of another person who cannot observe the participant’s behaviour suffices to decrease unethical behaviour. By using different behavioural paradigms of unethical behaviour, we obtain three main results: first, the presence of an observing other curbs unethical behaviour. Second, neither the payoff structure (Study 1) nor the social proximity towards the observing other (Study 2) qualifies this effect. Third, the mere presence of others does not reduce unethical behaviour if they do not observe the participant (Study 3). Implications, limitations and avenues for future research are discussed.