Numerous studies indicate that observing or knowing about another's action automatically activates the same motor representations that are active when we perform the other's action by ourselves. We investigated how affect influences this mirror mechanism. Based upon findings that positive affect encourages and negative affect impairs spreading activation, we hypothesised that positive affect should increase and negative affect decrease the automatic co-representation of other individuals' actions during jointly performed tasks. Recent research has shown that joint-action effects in a go/no-go variant (...) of the Simon task provide a good index for co-representing another's action. Participants performed such a Simon task together with another person after the induction of neutral, positive, or negative affect. Consistent with our predictions, the joint Simon-like effect was strongest after positive affect induction and absent after negative affect induction. These results represent the first evidence of affect-induced differences in the mirroring of another's action and suggest that affect influences one of our essential abilities for successful social interactions. (shrink)