People often mimic others more if the other is liked, a member of an ingroup, or in a cooperative relationship with the observer; we call this the interpersonal attitude effect. This study examines the degree to which this attitude effect on mimicry is an automatic or an effortful process. While under cognitive load or no load, participants observed positive, negative, and neutral others making emotional expressions. Electromyography measured corrugator supercilii (knits brow) and zygomaticus major (raises corners of mouth) activity. Under load, participants mimicked smiles of positive individuals but not neutral or negative individuals. During no-load trials, participants did not mimic negative individuals, but did mimic smiles of neutral and positive individuals. Participants enhanced their smiles in response to the smiles of liked others without effort, but smiling at neutral others' smiles required greater cognitive resources.