The Crespi effect is a behavioural contrast phenomenon observed in classical conditioning in which a conditioned response changes disproportionately to a suddenly changed reinforcement.
[1][2] It was first observed in rats by American psychologist Leo P. Crespi in 1942.
[2] He found that in a repeatedly carried out task such as finding food in a maze, the running speed of the rat is proportional to the size of the reward it obtained on the previous trial.
The effect also works in reverse: when rats were shifted from a larger to a smaller reward, they ran more slowly than the control rats that had always received the small reward.
Scholars have been only partially able to replicate Crespi's studies, which remains controversial.