The opposite of habituation: a process by which repeated exposure to a stimulus produces an increased rather than a decreased response.

An animal who has been overwhelmed by a particular stimulus may sensitise to it, becoming more reactive each time rather than less. The horse who is sensitised to the float startles more on each loading attempt, not less. The dog who is sensitised to children avoids them more strenuously each time, not less. The cat who is sensitised to vacuum cleaners hides faster each time, not eventually accepting them.

Sensitisation is the welfare risk of getting habituation wrong, and is one of the reasons exposure work needs to be carefully managed across species. The general rule is that the animal’s arousal needs to stay below threshold during exposure, and the exposure needs to be controlled so the animal does not have repeated experiences of being overwhelmed. When these conditions are not met, sensitisation often results.

Sensitisation typically reflects an animal whose nervous system has classified the stimulus as a genuine threat, often through a small number of intense experiences. Once sensitised, the animal’s response is not under voluntary control; the response is automatic, like ducking when a ball is thrown at the face. Working with sensitised animals requires counterconditioning and systematic desensitisation rather than further exposure, because additional exposure tends to deepen the sensitisation rather than resolve it.

Sensitisation can also be a normal, adaptive response. An animal who has had a single bad experience with a particular stimulus may sensitise to it as a protective mechanism, and this can be appropriate when the stimulus genuinely represents a risk. The welfare concern arises when sensitisation persists toward stimuli that do not pose an actual threat, and when the animal’s quality of life is significantly degraded as a result.