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Social cognition research has indicated that attempts to suppress thoughts can lead to a paradoxical increase in the frequency of that thought. This phenomenon has been a central component of cognitive-behavioural models of obsessive-compulsive disorder (OCD); however, research has yet to demonstrate deficient thought suppression ability in OCD patients. We examined whether individuals with OCD (OCs) exhibit a deficit in the ability to suppress thoughts. In Experiment 1, attempted thought suppression led to a paradoxical increase in self-reported thoughts for OCs, but not for nonanxious controls (NACs) or anxious controls (ACs). In order to rule out self-report biases, in Experiment 2 we utilized a lexical decision paradigm that measured priming strength of a target word under thought suppression conditions. Results paralleled those of Experiment 1: OCs showed decreased lexical decision latency of the ‘suppressed’ thought (thought to reflect either increased priming strength or disrupted processing of nonsuppressed thoughts), thus exhibiting a paradoxical effect of thought suppression. This effect was not seen in NACs or ACs. These findings suggest that deficits in cognitive inhibitory processes may underlie the intrusive, repetitive nature of clinical obsessions.

A critical feature of obsessive-compulsive disorder (OCD) is that sufferers attempt to ignore, neutralize, or suppress their intrusive thoughts (American Psychiatric Association, 1994). However, the use of such strategies is rarely successful and may even lead to increases in the frequency of intrusive thoughts. Indeed, the cognitive-behavioral model of OCD (e.g. Rachman, 1998 and Salkovskis, 1996) posits that failures to control or suppress intrusive thoughts underlie the frequent and intense nature of obsessions. Specifically, this model suggests that whereas unwanted, upsetting thoughts are a universal experience, individuals with OCD (OCs) attribute irrational, negative meaning to these kinds of cognitions, and are thus motivated to suppress them. These suppression attempts lead to the paradoxical effect of increasing, rather than decreasing, the frequency of the unwanted thought. In an early investigation of the thought suppression paradox, Wegner, Schneider, Carter, and White (1987) observed that participants who were instructed to suppress thoughts of a white bear subsequently reported more white bear thoughts than did participants who had not been instructed to suppress. Increased thought frequency during attempts to suppress was termed the ‘immediate enhancement effect’, and increased thought frequency after suppression attempts had been relaxed was termed the ‘rebound effect’. Wegner's ( Wegner, 1994a; Wegner, Erber, & Zanakos, 1993) ‘ironic process’ theory suggests that attempts to suppress thoughts involve two cognitive processes: an intentional search for a distracter thought, and an automatic search for the target (suppressed) thought. Because the automatic monitoring process searches for occurrences of the target thought, it paradoxically increases the accessibility of that thought to consciousness. Thus, in consciously trying to suppress thoughts of a white bear, the monitoring process searches for white bear thoughts, thus undermining suppression attempts. Subsequent studies of the thought suppression paradox have yielded mixed results. A comprehensive review of the thought suppression literature is beyond the scope of this paper; more information can be found in published reviews (e.g. Abramowitz, Tolin, & Street, 2000; Purdon & Clark, 2000; Wegner, 1994a and Wegner, 1994b). In a meta-analytic review, we (Abramowitz et al., 2000) found little evidence of an immediate enhancement effect across studies (mean weighted effect size =−0.35). Contrary to Wegner's model, across studies, participants were generally successful in suppressing thoughts. Despite the absence of an initial enhancement effect, a significant rebound effect was found (mean weighted effect size =0.30). That is, across studies, participants who had previously suppressed a thought subsequently reported more occurrences of the thought after suppression instructions were withdrawn, compared to participants who had not suppressed. Despite its mixed results, the thought suppression paradox remains a mainstay of cognitive-behavioral theories of OCD (Rachman, 1998 and Salkovskis, 1996). However, thought suppression among OCs has received little empirical attention to date. Typically, experimental investigations of thought suppression have utilized samples of healthy (or unselected) undergraduate volunteers. Less frequently, clinical analogue participants selected from larger nonclinical samples have been studied (e.g. Salkovskis & Campbell, 1994). These analogue participants may be selected for high scores on measures of psychopathology, but differ from clinical samples in that they do not necessarily meet diagnostic criteria for psychiatric disorders and are not seeking treatment. Questionnaire-based studies have shown that OCD patients and student volunteers with sub-clinical OCD symptoms report attempting to suppress their thoughts (Freeston & Ladouceur, 1997; Wegner & Zanakos, 1994), and report more suppression than do nonanxious control participants (Amir, Cashman, & Foa, 1997). It is important to note, however, that the causal role of thought suppression can only be determined using laboratory-based studies in which crucial variables are manipulated experimentally. It may be hypothesized that the increased thought suppression attempts of OCs may paradoxically intensify their obsessions; alternatively, the negative emotions associated with obsessions may be so distressing as to cause suppression attempts, or both phenomena may be caused by an unknown third variable. In the only published experimental investigation of thought suppression among OCD patients, Janeck & Calamari (1999) asked OCs and nonanxious control participants (NACs) to suppress personally relevant, negative thoughts over a three-stage sequence. In stage 1, participants were asked merely to monitor occurrences of the target thought by ringing a bell each time it occurred (a procedure termed ‘event marking’). During stage 2, participants were randomly instructed either to suppress the target thought or to simply monitor target thought occurrences (event marking continued in both groups). In stage 3, all participants were again instructed only to monitor the thought's occurrences. OCs reported a higher frequency of target thoughts, regardless of condition or period. However, no significant differences between suppression and monitor-only groups were detected. Thirty-one percent of OCs vs. 19% of NACs showed an initial enhancement effect, and 25% of OCs vs. 0% of NACs showed rebound effects. Thus, Janeck and Calamari's study provides some limited evidence for enhanced paradoxical effects of thought suppression among OCs. However, several methodological problems limit the conclusions that can be drawn from Janeck and Calamari's study. First, their main finding (increased rebound effect among OCs) was based on only four OC participants. Second, because an anxious control (AC) group was not used, their study lacked the data to conclude that group differences indicated specific OCD deficits, as opposed to characteristics of people with anxiety disorders in general. Third, the use of only personally relevant, negative thoughts could be tantamount to asking OCs to suppress their obsessions. As cognitive-behavioral theory posits that they are already doing so on an ongoing basis, the instruction might have little impact on OCs. Fourth, although using personally relevant, negative thoughts may increase ecological validity, it decreases internal validity by introducing the potential confound of systematic between-group differences in the nature of the target thoughts. Indeed, recent studies suggest substantial differences between obsessions and non-OC worries (Langlois, Freeston, & Ladouceur, 2000). Fifth, the study relied exclusively on self-reports, which are largely susceptible to experimental demand characteristics (Abramowitz et al., 2000; Purdon & Clark, 2000). The present experiments were designed to investigate whether OCs are characterized by an inability to suppress thoughts. In Experiment 1, we used event marking (asking participants to press a key indicating the presence of the target thought) as the primary dependent variable. Participants were asked to indicate occurrences of the target thought during a 5-min monitoring period, a 5-min suppression period, and another 5-min monitoring period. In Experiment 2, in order to minimize the influence of self-report bias, our next step was to implement a new paradigm that would be sensitive to the effects of thought suppression without relying on self-report. We hypothesized that thought suppression would enhance priming of the suppressed word, thus rendering it more accessible to cognitive operations. To this end, we designed a lexical decision task, in which participants were asked to indicate as quickly as possible whether they were being shown a word or a nonword. Participants were asked to suppress a target word, which was then embedded in the lexical decision task (immediate enhancement effect). Next, participants were instructed to think about anything they wanted, and completed the same lexical decision task again (rebound effect). 2. Experiment 1 Experiment 1 was designed to investigate whether OCs show diminished ability to suppress thoughts, compared to those without OCD. The study design was similar to that of Janeck & Calamari (1999), with two main differences. First, whereas Janeck and Calamari's participants were asked to suppress personally relevant, negative thoughts, we instructed all of our participants to suppress the same neutral thought (a white bear). This was because prior research (e.g. Amir et al., 1997) suggests that OCs are already suppressing their obsessions; hence, instructions to do so would not be expected to have any effect. If OCs have a global deficit (as opposed to a bias) in their ability to suppress thoughts, then this deficit should be evident even for neutral thoughts. Second, whereas Janeck and Calamari used only OCs and NACs, we also included an AC group consisting of patients with generalized social phobia. This allows us to determine whether any obtained effects were specific to OCD, or were characteristic of anxious patients in general. We predicted that OCs would show a greater immediate enhancement and rebound effect than would NACs and ACs.