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Pigeons were conditioned to peck a response key under a procedure that alternated periods of food reinforcement with periods of extinction. The pigeons attacked a nearby pigeon at the onset of extinction. Some also attacked a stuffed model of a pigeon. The duration of attack was an inverse function of the time since the last food reinforcement and a direct function of the number of reinforcements. The pigeons attacked after the last food delivery whether or not the conditioned pecking response was required and whether or not the extinction period was signaled. The food had to be eaten; the mere sight and sound of food being delivered did not produce attack. Prior satiation reduced attack. The phenomenon was not attributable to a past history of competition between pigeons since socially deprived pigeons also attacked. Superstitious reinforcement of attack was not found to be a factor. The results indicated that the transition from food reinforcement to extinction was an aversive event that produced aggression.When shock is delivered to the feet of an animal, attack results (Ulrich and Azrin, 1962 (Skinner, 1938), attacking of the response lever (Mowrer and Jones, 1943), increased vocalization between children (Azrin and Lindsley, 1956), and increased running speed after omission of a food reinforcement

Reflexive fighting was elicited between paired rats as a reflex reaction to electric shock prior to any specific conditioning. Such fighting was fairly stereotyped and easily differentiated from the rats' usual behavior. The strength of this reflex was not attributable to any apparent operant reinforcement. Elicitation of fighting was a direct function of the enclosed floor area and a nonmonotonic function of the shock intensity. Failure to scramble the polarity of the electrified grid produced inconsistent fighting. Under optimal conditions fighting was consistently elicited by shock regardless of the rat's sex, strain, previous familiarity with each other, or the number present during shock. Repeated shock presentations did not produce an appreciable decrease in fighting until signs of physical debility appeared. Although shock did not cause a rat to attack inanimate objects, it did produce attack movements toward other small animals. Failure of guinea pigs to defend themselves revealed that the elicitation of fighting from the rat does not require reciprocal attack. Paired hamsters showed fighting reactions similar to those of the rats, whereas guinea pigs failed to fight. Electrode shock and a heated floor elicited fighting between the rats, but intense noise and a cooled floor did not.

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When an aversive stimulus, such as electric shock or noise, is explicitly arranged to follow a response, the procedure can be described as punishment. In studying the punishment process as defined above, three major types of procedure have been used extensively. The first of these procedures (Muenzinger, 1934) distinguishes between two classes of response such as correct vs. incorrect and left vs. right. The punishment is typically delivered following one of these responses, but not the other, and the relative frequency is compared. No attempt is usually made in this type of study to determine changes in the absolute strength of either response.A second type of procedure used in studying punishment does measure the absolute strength of a response, but limits the punishment to the period of experimental extinction (Estes, 1944). Since the extinction process in itself produces a rapid reduction of responding, any reduction due to the punishment can be determined for only short periods of time. Long-term effects of punishment cannot be studied with this method. Of more serious consequence is the possibility, and likelihood, that this simultaneous use of punishment and extinction will give rise to discriminative properties of the punishment. This would be expected to occur inasmuch as the punishment is present only during extinction and not during reinforcement.The third method of studying punishment involves the simultaneous use of punishment and reinforcement (Skinner, 1938;Dinsmoor, 1952). A substantial level of responding may be established with a food reinforcement, and the punishment is added while this food reinforcement is still maintained. The level of responding during the punishment procedure can then be compared with that in the absence of punishment. Long-term studies are thereby possible in which relative, as well as absolute, measures of responding can be obtained without allowing the punishment to assume discriminative properties with respect to the underlying food reinforcement. Various schedules of food reinforcement may be used to maintain the selected response. In a previous study (Azrin, 1959b), a fixed-ratio schedule of reinforcement was used; and it was found that once responding began, the initial onset, or increase, in punishment produced a large degree of suppression with subsequent recovery in time. The present study is an attempt to study these progressive changes in the effects of punishment. In order to eliminate the cyclic changes in behavior associated with fixed-ratio reinforcement, a variable-interval schedule was used, since this schedule produces a fairly uniform rate of responding. METHODS AND APPARATUSThe subjects were six male White Carnaux pigeons, each of which was maintained at about 80% of its free-feeding weight.The experimental chamber measured 16 by 16 by 16 inches. These dimensions provided ample space for the gross movements which were often produced by punishment.