Capuchin monkeys are sensitive to others' welfare

proposer’s payoffs constant across the two planks but varied the size of the payoffs to the receiver’s chamber to investigate if and when capuchins were willing to provide the receiver’s chamber with a large reward (a marshmallow) instead of a small reward (a celery piece). This led to two kinds of trials: ones in which the proposer delivered marshmallow to his own chamber and chose either marshmallow or celery for the receiver’s chamber, and ones in which the proposer delivered celery to his own chamber and chose either marshmallow or celery for the receiver’s chamber. We then compared monkeys’ likelihood of delivering the larger reward in a condition in which a receiver monkey was present (test condition) to one in which no monkey was present (empty control condition) and another in Capuchin monkeys are sensitive to others’ welfare