It is often suggested that people in large groups behave and act differently than when they are alone. More precisely, it is an often-repeated claim that they tend to act in a morally problematic or plainly reprehensible way. Still, a fully satisfying explanation has not yet been given for why this is the case. In this paper, I suggest that the phenomenon of shared moods may play a crucial role here. In order to explicate and support this thesis, first, I (...) will point out the problem in a more detailed way while also considering findings in social psychology. Second, I will give a sketch of how shared moods should be conceptualized as a special affective phenomenon by refering to the discussion of collective intentionality and emotions. Thirdly and finally, I will address concretely the question of what might make shared moods morally precarious by distinguishing also between collective and joint moods that both can change a person’s self-understanding as an actor. (shrink)