Indirect reciprocity is cooperation through reputation: third parties cooperate with those known to cooperate and defect against those known to defect. Defection, then, can have the unjust motive of greed or the just motive of retaliation. To establish cooperation, observers should distinguish both motives for defection and respond more cooperatively to the latter. We propose that the expression of emotions may facilitate this inferential process. Indeed, in two laboratory studies participants inferred that defection out of anger or disappointment was a just response to a defector and they responded more cooperatively than when no emotion was communicated. Moreover, participants inferred that defectors who evoked disappointment instead of anger had a relatively positive reputation. We conclude that emotions help establish cooperation through indirect reciprocity.