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Attempted suppression of traumatic memories was investigated in survivors of motor vehicle accidents with acute stress disorder (ASD; n=24) and without ASD (n=24). Participants monitored their trauma-related thoughts for three 5-minute periods. In Period 1, participants were instructed to think about anything. For Period 2, participants were administered suppression or non-suppression instructions relating to thoughts of the trauma. In Period 3, participants were again instructed to think about anything. ASD participants reported higher ratings of anxiety, frequency of trauma-related thoughts, and attempted suppression of trauma-related thoughts than non-ASD participants. Participants who were given suppression instructions demonstrated a delayed increase in trauma-related thoughts in the period subsequent to suppression.

Researchers have recently employed personally relevant thoughts and clinical populations in order to ascertain the applicability of thought suppression paradigms to clinical phenomena. The attempted suppression of naturally occurring negative intrusions has been shown to result in an increase in the occurrence of the suppressed thoughts (Salkovskis and Campbell, 1994; Trinder and Salkovskis, 1994; McNally and Ricciardi, 1996). Further, a recent study comparing spider phobics and controls found that while attempted suppression was associated with an increase in spider-related thoughts, the contribution of attempted suppression to the number of spider-related thoughts was relatively minimal (Muris et al., 1997). Muris et al. (1997)concluded that attempts to suppress are not central to the maintenance of intrusions in spider phobia, but might play a role in intrusive thoughts that are characterised by more ``ruminative preoccupation'' such as those that may occur posttrauma (p. 773). This reasoning is consistent with several other commentaries that have argued that the thought suppression paradigm may have utility for understanding the maintenance of posttraumatic intrusions and avoidance (Trinder and Salkovskis, 1994; Ehlers and Steil, 1995; Gold and Wegner, 1995). To date, there has been no investigation of attempted suppression in traumatised individuals. Accordingly, the present study represents the first investigation of the relationship between attempted suppression and intrusions in individuals diagnosed with acute stress disorder (ASD). Intrusive thoughts and cognitive avoidance are hallmark symptoms of ASD (American Psychiatric Association, 1994). The study was designed to investigate the two types of ironic effects that have been previously reported in the literature, namely, the immediate increase in the target thoughts when suppression is attempted (e.g., Lavy and van den Hout, 1990) and the delayed increase in the number of intrusive thoughts after attempted suppression has ceased (e.g., Clark et al., 1991 and Clark et al., 1993). Considering the salience of intrusions (Rachman, 1982) and the potentially greater cognitive load associated with ASD (Wegner, 1994), it was expected that ironic effects of thought suppression would be stronger for ASD than non-ASD participants.