Avoidance of pain of unpredictable intensity.

Unpredictability of the intensity of an aversive event might be an important factor in producing negative effects of the event, especially if the UCS becomes stronger than could have been expected. The present experiment tested the hypothesis that unpredictability of intensity of a painful stimulus contributes to avoidance behaviour. The experiment was concealed in a shock working-up procedure, which was done to assess the pain level subjects were willing to tolerate in a subsequent experiment. The experimental subjects, who received an unannounced sudden increase of the pain stimulus during the working-up procedure, tolerated less pain on a subjective as well as on an objective level (avoidance of high levels of pain) than the control subjects, who received the stimuli in a predictable pattern. The results support the hypothesis that unpredictability of intensity of an aversive event contributes to avoidance behaviour.