10.08.2023: Inequality results in negative reactions. This sense of justice is not limited to humans and has also been demonstrated in experiments in many non-human animal species. Domestic dogs are particularly well suited to study this inequality aversion. A study just published by the University of Veterinary Medicine Vienna adds a new aspect to previous knowledge: the research team found that the perceived attainability of a reward could underlie the observed reaction of dogs.

Domestic dogs show a basic form of aversion to inequality - they refuse to continue participating in a task with an experimenter if they are not rewarded in the presence of another, rewarded dog. This was first demonstrated by a team led by Friederike Range, deputy director of the Konrad Lorenz Institute for Comparative Behavioural Research (KLIVV) at Vetmeduni, using the so-called "paw task". This study, published in 2009, caused an international sensation.

In the underlying experiment, dogs were asked to repeatedly give the paw and received identical or different rewards in each case. If, in contrast to the other dog, they did not receive any treats, they refused to give the paw. However, if no rewarded conspecifics were present, the dogs more often gave their paw to an experimenter without a reward - evidence of the aversion to inequality, which was confirmed in several other studies.

Sense of justice ...

In the current study, the researchers tested a new hypothesis: instead of rejecting inequality, dogs might give their paw more often when a partner is absent for a different reason. This is because of the experimenter's procedure of moving treats in front of the experimental animal to mimic feeding a conspecific. This action could increase the experimental animals' perception of the accessibility of a reward.

... or expectancies as triggers?

"We tested this hypothesis by introducing an enhanced control condition in the 'paw task': Dogs were not rewarded when they gave their paw in the presence of a box with a reward*," explains Jim McGetrick, former research associate at Vetmeduni's KLIVV. Result: The duration of paw-giving did not differ - regardless of whether the partner was another dog or a box. In addition, dogs gave the paw more often when no other dog was present and the experimenter imitated feeding a dog than when treats were placed in the box. McGetrick continued: "In summary, our results suggest that the longer duration of paw-giving in the no-reward control condition in dog disparity  studies is driven by the perceived attainability of rewards. This may create the illusion that dogs stop paw-giving in the inequality condition due to genuine inequality aversion."
 

*The experimenter placed the reward into the box on each trial and the dog witnessed this.
 

The article „Perceived reward attainability may underlie dogs’ responses in inequity paradigms“ by Jim McGetrick, Hugo Peters, Anna D. J. Korath, Romana Feitsch, Susanne Siegmann and Friederike Range was published in „Scientific Reports“.
 

Scientific article