I. ABSTRACTThis chapter considers what factors influence the occurrence of attack and fear behavior (e.g., immobility, flight, distress signals). It is argued that, with the exception of conditioned attack and fear behavior, the same types of basic situations are capable of evoking either type of response.These situations are as follows: (l) Pain. (2) A moving object coming too close to an animal (individual distance intrusion). (3) A novel object or animal entering another's familiar area: this includes not only the typical territorial situation, but also a wide range of other situations. (4) Entrance into an unfamiliar area: most commonly, fear behavior occurs, but attack can occur if there is a suitable target. (5) A familiar object in an unfamiliar place. (6) Frustrative nonreward. (7) Thwarting (a physical object preventing completion of a learned response). (8) Low reinforcement schedules, which involve delayed reward. (9) Morphine withdrawal, which is similar to frustrative nonreward.It is suggested that all these situations can summate with one another, up to a certain limit, to increase the likelihood and intensity of attack, but that above this limit fear behavior occurs.It is then suggested that all the situations involve the common property of a discrepancy between observed and expected stimulation. Different types of "neuronal models" may represent the different types of expect a-