Pessimism bias
The tendency for an animal in a negative affective state to interpret ambiguous cues as predicting negative outcomes.
A dog who treats a half-filled food bowl as if it has been emptied rather than as if it will be filled, a horse who treats an unfamiliar object as threatening rather than interesting, a cat who treats every approach as a potential threat, may all be showing pessimism bias. The interpretation of the ambiguous information leans toward the negative possibility.
Pessimism bias is the mirror of optimism bias and is used as a negative welfare indicator in cognitive bias testing across species. Animals showing pessimism bias may be experiencing chronic stress, sustained frustration, social isolation, or other long-term welfare concerns. The bias often appears before other welfare indicators show clear deterioration, making it potentially useful as an early warning signal.
The development of pessimism bias has been documented in animals subjected to a range of welfare-negative conditions: restricted housing, social isolation, chronic mild stressors, unpredictable aversive events. The bias often persists for some time after the welfare-negative condition is removed, suggesting that the underlying affective state takes time to recover.
Conversely, the resolution of pessimism bias has been documented when welfare conditions are improved. Animals moving from restricted to enriched environments, from social isolation to appropriate social contact, from chronic stressors to predictable safe conditions, typically show gradual shifts from pessimistic to neutral to optimistic interpretations of ambiguous information.
In clinical animal behaviour work, persistent pessimism bias in an individual animal is one of the indicators that welfare interventions are needed and that the interventions need to address underlying affective state rather than just specific behavioural problems. An animal in chronic pessimism is in a different welfare situation than an animal showing situational fear or one-off behavioural problems.
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