Avoidance learning is the behavioral product of an instrumental (operant) training procedure in which a predictable aversive event, typically electric shock, does not occur contingent upon the occurrence or nonoccurrence of a specified response by the learning organism. Avoidance training occurs in two forms: active and passive. In the active form, the avoidance contingency depends on the occurrence of a specified response on the part of the organism; in the passive form, the avoidance contingency depends on the nonoccurrence (i.e., the suppression) of some specified response. The response to be suppressed may be either spontaneous or learned by virtue of prior reward training. In both forms, however, the avoidance contingency consists of the prevention or omission of a predictable noxious event. Noxious events are defined in terms of the preference relation in which the absence...