The emotional life of animals

How can emotional reactions be measured in creatures that cannot verbally describe their feelings, such as animals? Many answers have been given to this question in the history of psychology and neuroscience, and Paul and Mendl offer a thoughtful review and analysis in the theory section of this issue. In my commentary, the merits of various definitions of emotion based on different criteria are compared. These definitions use measures ranging from verbal reports to stimulus-elicited affective reactions, instrumental working for rewards or avoidance of punishments, and triangulation approaches.