Fair is Fine, but More is Better: Limits to Inequity Aversion in the Domestic Dog

Research with domestic dogs provides a unique approach for exploring the evolution of fairness and justice. Not only are dogs descended from highly social canids; they have also been bred for cooperative tasks with humans. Dogs act cooperatively in social play and are skilled on other social cognitive tasks. It is reasonable to ask whether dogs behave in ways similar to primates in other social contexts. In particular, do dogs perceive and respond to unfairness or injustice, a skill potentially borne of long-term affiliation with and selection by humans? Using a revised test of inequity aversion which looks at advantageous and disadvantageous inequity, the current research investigated the behavior of 38 domestic dogs. Subject dogs and a control dog approached two trainers in turn: one who rewarded them equally for sitting on command and one who rewarded them unequally-either over-rewarding or under-rewarding the control dog. After familiarization with the trainers, subjects chose which trainer to approach by themselves. Subjects preferred the over-rewarding trainer over the fair trainer; they had no preference between the under-rewarding and the fair trainer. Further analyses found that length of ownership, subjects' age, and cooperative work experience reversed the approach preference, predicting preference for the fair trainer-though breed did not. These results suggest that the precursory sensitivity, which dogs showed to iniquitous outcomes in prior research, does not extend to both advantageous and disadvantageous inequity and does not hold when the subject is continually rewarded. Dogs selected a trainer who had treated them unfairly, yet who presented a potentially greater opportunity for future rewards. When the stakes were high, dogs showed a greater sensitivity to the quantity of a reward than to the fairness of a reward.