Just as humans, individual animals differ in their propensity to see if the glass is “half-full”, or “half-empty”. Animals cannot talk, but they can react to ambiguous situations and cues in a way that suggests expectations of a positive outcome (optimism) or expectations of a negative outcome (pessimism).
For example, when black bowl is associated with sweet food, and white bowl with bitter food, grey bowl would be an ambiguous cue. How would animals react to grey? A spatial version of this test would be placing identical bowls either on the left or right sides of a test chamber, and then checking whether an animal would approach a bowl at intermediate locations.
A “judgement bias test”, based on this phenomenon, is not only a window into animals’ inner lives, but also a potential tool for improving animal welfare and developing new psychoactive medicines. To validate this test’s potential promises, we have recently conducted two meta-analyses (both accepted in Neuroscience and Biobehavioral Reviews), collecting results of tests on animals ranging from insects to primates.
The first meta-analysis summarises studies using judgement bias assays as a measure of animal welfare (comparing animals in relatively better and relatively worse conditions/treatments), whereas the second meta-analysis synthesises studies measuring effects of various psychoactive medicines (comparing animals given the drugs to animals given placebo/no drug).
In our meta-analyses, we have shown that “judgement bias test” is valid and widely applicable measure of animal affect in both cases. However, our work has also revealed that the test is not always showing clear or large effect and it needs to be customised for specific species tested.