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Persistence of individual differences in animal behavior in reactions to various environmental challenges could reflect basic divergences in temperament, which might be used to predict details of adaptive response. Although studies have been carried out on fear and anxiety in various species, including laboratory, domestic and wild animals, no consistent definition of fearfulness as a basic trait of temperament has emerged. After a classification of the events that may produce a state of fear, this article describes the great variability in behavior and in physiological patterns generally associated with emotional reactivity. The difficulties of proposing fearfulness--the general capacity to react to a variety of potentially threatening situations--as a valid basic internal variable are then discussed. Although there are many studies showing covariation among the psychobiological responses to different environmental challenges, other studies find no such correlations and raise doubts about the interpretation of fearfulness as a basic personality trait. After a critical assessment of methodologies used in fear and anxiety studies, it is suggested that discrepancies among results are mainly due to the modulation of emotional responses in animals, which depend on numerous genetic and epigenetic factors. It is difficult to compare results obtained by different methods from animals reared under various conditions and with different genetic origins. The concept of fearfulness as an inner trait is best supported by two kinds of investigations. First, an experimental approach combining ethology and experimental psychology produces undeniable indicators of emotional reactivity. Second, genetic lines selected for psychobiological traits prove useful in establishing relationships between behavioral and neuroendocrine aspects of emotional reactivity. It is suggested that fearfulness could be considered a basic feature of the temperament of each individual, one that predisposes it to respond similarly to a variety of potentially alarming challenges, but is nevertheless continually modulated during development by the interaction of genetic traits of reactivity with environmental factors, particularly in the juvenile period. Such interaction may explain much of the interindividual variability observed in adaptive responses.