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Attempts to suppress traumatic material may be involved in the development and maintenance of post-traumatic stress disorder (PTSD). In order to investigate this possibility, analogue post-traumatic intrusions were induced in normal participants by means of a distressing film. For comparison, a second film was used to induce intrusions about polar bears. It was hypothesized that the suppression of these intrusions would produce an immediate decrease but a delayed increase (“rebound effect”) in their frequency. It was also predicted that the rebound effect would be larger for the analogue traumatic intrusions. Each film was followed by two consecutive time periods during which participants' thoughts were recorded. During the first period, the suppression group was instructed to suppress thoughts about the film whilst the control group merely recorded their thoughts. During the second period, both groups merely recorded their thoughts. The results supported the immediate decrease hypothesis for both types of intrusion. As predicted, there was a rebound effect for analogue traumatic intrusions although not for polar bear thoughts. Several methodological issues relating to the findings are highlighted. The possible implications of a rebound effect with trauma-relevant intrusions are discussed with reference to PTSD.

Post-traumatic stress disorder (PTSD) consists of a constellation of symptoms which develop after exposure to a traumatic event (American Psychiatric Association, 1994). The reexperiencing of the trauma has been regarded as the hallmark feature of PTSD (e.g. Jones and Barlow, 1990) and often takes the form of recurrent intrusive memories of the event. Initial PTSD is very common with recovery often taking place over successive months (e.g. Rothbaum et al., 1992) but unfortunately, the symptoms continue to persist in some individuals. A major question within the field is what determines the persistence of the disorder? One factor which has been implicated in the sometimes chronic course of PTSD is that of avoidance. This can take many forms including avoidance of situations and reminders but also cognitive avoidance. One particular type of cognitive avoidance is the conscious attempt to suppress intrusive cognitions which has been termed “thought suppression”. This was first experimentally investigated by Wegner et al. (1987)who found that attempts to suppress a neutral thought (a white bear) led to a subsequent increase in the frequency of this thought. Since then a number of studies have investigated whether a deliberate attempt “not to think about” particular thoughts produces an immediate increase and/or a delayed increase (“rebound effect”) in their frequency. Experiments have produced somewhat conflicting results but have generally shown either an immediate increase effect (e.g. Lavy and van den Hout, 1990; Salkovskis and Campbell, 1994; Salkovskis and Reynolds, 1994) or a rebound effect (e.g. Clark et al., 1991 and Clark et al., 1993; Wegner et al., 1991). Consistent with the idea that thought suppression may play a role in the persistence of PTSD, two correlational studies (Bryant and Harvey, 1995; Ehlers et al., in press) have found an association between self-reported thought suppression and symptom persistence. Recent theoretical accounts of the disorder have also suggested reasons why patients may be motivated to suppress memories of traumatic events (Ehlers and Steil, 1995). However, to date, no experimental studies of thought suppression have focused on trauma-related intrusions. Furthermore, experimental studies using other types of negative material have produced mixed results. Salkovskis and Campbell (1994)obtained an immediate increase effect with personally relevant negative intrusive thoughts. In contrast, Muris et al. (1992)failed to find an immediate increase effect with negative thoughts, but did obtain an immediate increase effect with neutral thoughts. Neither study obtained evidence of a rebound effect with negative material. The question of the influence of thought suppression on traumatic material was addressed using an analogue methodology which drew on the work of Horowitz (1975). This involved an experimental induction of “analogue post-traumatic intrusions” by showing volunteers a film of an actual disaster. Participants were then either asked to simply record their intrusions or to record them whilst trying to suppress them. Both the immediate and delayed effects of thought suppression were investigated in the experiment. It was predicted that whilst suppression would be immediately relatively successful (i.e. produce an immediate decrease in the frequency of intrusions), it would produce a later resurgence of suppressed thoughts in the form of a “rebound effect”. This pattern of results was hypothesized owing to the similarity between the methods and design of this study and that of studies by Clark et al., 1991 and Clark et al., 1993who found a similar pattern. To provide a comparison, the experiment also investigated the effects of suppressing a stimulus that has been employed in earlier studies, that of a polar bear. It was expected that the rebound effect would be greater for analogue post-traumatic intrusions than for polar bear thoughts.