The tendency for an animal in a positive affective state to interpret ambiguous cues as predicting positive outcomes.
A dog who treats a half-filled food bowl as if it will be filled rather than as if it has been emptied, a horse who approaches an unfamiliar object as if it is interesting rather than threatening, a parrot who tests new foods readily rather than avoiding them, may all be showing optimism bias. The interpretation of the ambiguous information leans toward the positive possibility.
Optimism bias is one of the two main outcomes measured in cognitive bias testing (the other being pessimism bias) and is used as a positive welfare indicator across species. An animal showing consistent optimism bias is in an affective state where ambiguous information is being processed in the welfare-positive direction.
The mechanism by which affective state produces optimism bias is partly attentional (the animal in a positive state attends to positive information more readily) and partly evaluative (the animal in a positive state weights positive interpretations more heavily). Both mechanisms contribute to the same overall pattern: ambiguous information is interpreted in a positive direction when the animal is in a positive affective state.
Optimism bias has been documented across species including humans, primates, dogs, horses, rats, pigs, sheep, and several bird species. The cross-species replication of the basic finding is part of what makes cognitive bias testing useful as a welfare assessment method.
In applied animal behaviour work, the development of optimism bias in a previously withdrawn or pessimistic animal is one of the indicators that welfare interventions are working. An animal moving from pessimistic to optimistic interpretations of ambiguous information is showing improvement in their underlying affective state.
The opposite is also informative. An animal who has been consistently optimistic and begins showing pessimistic interpretations may be experiencing a welfare deterioration that warrants investigation.
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