Squirrel monkeys trained to respond under a schedule in which each response postponed the delivery of electric shock developed a steady rate of responding. When a schedule in which an electric shock was presented following the first response occurring after 10 minutes (10-minute fixed-interval schedule) was programed concurrently, there was little effect on the pattern of responding. When the shock-postponement schedule was eliminated, the fixed-interval schedule of shock-presentation maintained a pattern of positively accelerated responding that is characteristic of fixed-interval schedules of reinforcement.

In squirrel monkeys responding under a schedule in which responding postponed the delivery of electric shock, the presentation of response-dependent shock under a fixed-interval (FI) schedule increased the rate of responding. When the schedule of shock-postponement was eliminated, so that the only shocks delivered were those produced by responses under the FI schedule, a pattern of positively accelerated responding developed and was maintained over an extended period. When responses did not produce shocks (extinction), responding decreased. When shocks were again presented under the FI schedule, the previous pattern of responding quickly redeveloped. In general, response rates were directly related to the intensity of the shock presented, and inversely related to the duration of the fixed-interval. These results raise fundamental questions about the traditional classification of stimuli as reinforcers or punishers. The basic similarities among FI schedules of food presentation, shock termination, and shock presentation strengthen the conclusion that the schedule under which an event is presented and the characteristics of the behavior at the time the event is presented, are of overriding importance in determining the effect of that event on behavior.

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In squirrel monkeys previously trained under a continuous avoidance schedule, characteristic patterns of responding were maintained under a 3-min variable-interval schedule of shock presentation (response-produced shock). Responding in the presence of a periodically presented stimulus, the termination of which coincided with the delivery of a response-independent electric shock (Estes-Skinner procedure), was not reliably affected. When shocks followed every response during certain signalled portions of the session, and were presented under the variable-interval schedule during the rest of the session (multiple 1-response fixed-ratio, 3-min variable-interval schedule of shock presentation), responding was suppressed during the fixed-ratio component and maintained during the variable-interval component. Environmental consequences do not have immutable properties, and may either support or suppress behavior, depending on the schedule of presentation.