We review empirical data from both ethology and psychology about generalization, that is how animals respond to sets of stimuli including familiar as well as novel stimuli. Our main conclusion is that many characteristics of generalization are universal behavioural phenomena, suggesting that they originate from general properties of nervous mechanisms and/or that evolutionary strategies to cope with novelty and variability in stimulation are limited. We also reach the following conclusions. First, patterns of generalization are largely independent of systematic group (evidence is available for insects, fish, amphibians, reptiles, birds and mammals, including humans), behavioural context (feeding, drinking, orientation, etc.), modality (light, sound, etc.) and of whether reaction to stimuli is learned or genetically inherited. Second, two major shapes of the generalization gradient can be identified, corresponding to two types of stimulus dimensions. When changes in stimulation involve a rearrangement of a constant amount of stimulation on the sense organs, the generalization gradient peaks close to familiar stimuli, and peak responding is not much higher than responding to familiar stimuli. When the dimension involves increase or decrease of the intensity of stimulation, the gradient is often monotonic, and responding to some novel stimuli is considerably stronger than responding to familiar stimuli, compared to non-intensity dimensions. Third, rearrangement gradients are better described by Gaussian curves, rather than exponentials as often suggested. Fourth, when several or many familiar stimuli are close to each other interferences occur, giving rise to predictable response biases such as peak shift or supernormality (interference between positive and negative stimuli), and stretched bell shaped curves (several positive stimuli). Lastly, no principle differences between biases referred to as peak shift and biases referred to as supernormal stimulation are found. The paper ends with a discussion of theoretical issues
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