In 1987 Wegner and colleagues published a study describing the effects of intended thought suppression (Wegner, Schneider, Carter, & White, 1987). The group found that intended thought control produces paradoxical effects by enhancing the frequency of the thought to be suppressed after the suppression instruction is withdrawn. This unexpected result was named rebound effect.
Since then, many studies have investigated thought suppression, with variations in instructions for the expression period, in the method of thought sampling, and with different stimuli to suppress (ranging from personally irrelevant material to recollections of personally relevant events in the past). With regard to the rebound effect, these studies produced mixed results: some authors found the rebound effect for personally non-relevant material (Clark, Ball, & Pape, 1991, Rutledge, Hollenberg, & Hancock, 1993 and Kelly & Kahn, 1994, Study 2; Rutledge, Hancock, & Rutledge, 1996 and Wegner & Gold, 1995; Wegner, Schneider, Carter, & White, 1987 and Wegner, Schneider, Carter, & White, 1987; Wegner, Schneider, Knutson, & McMahon, 1991 and Wenzlaff, Wegner, & Klein, 1991), while others found an increase in thought frequency only for personally relevant material (Smári, Birgisdóttir, & Brynjólfsdóttir, 1995). Some authors found no rebound effect at all, but did demonstrate an increase in thought frequency during the suppression period: an immediate enhancement effect (Merckelbach, Muris, van den Hout, & de Jong, 1991, Muris, Merckelbach, & de Jong, 1993 and Salkovskis & Campbell, 1994). Finally, some studies found neither of the two possible paradoxical effects of thought suppression (Kelly & Kahn, 1994, Study 1; Muris, Merckelbach, van den Hout, & de Jong, 1992, Muris, Merckelbach, Horselenberg, Sijsenaar, & Leeuw, 1997, Muris, de Jongh, Merckelbach, Postema, & Vet, 1998, Salkovskis & Campbell, 1994 and Smári, Sigurjónsdóttir, & Sæmundsdóttir, 1994).
Regardless of the mixed evidence for rebound or enhancement effects following thought suppression, these effects have been used to explain the development and maintenance of psychological disorders characterized by unwanted thoughts. Most models concern obsessive–compulsive disorder and generalized anxiety. The assumed central mechanism relates to prolonged efforts to suppress unwanted thoughts—which produce paradoxical effects. As the person tries harder and harder to suppress, a circulus vitiosus is set. For simple phobia, Muris et al. (1998) reported that dental phobics showed higher levels of intrusive and negative thinking during dental treatment than non-phobics. They also demonstrated that dental phobics engaged a lot more in thought suppression during dental treatment than a control group. They conclude that their results are “. . . consistent with the notion that thought suppression plays a role in psychopathological conditions such as the anxiety disorders” (p. 285) as previously formulated by Wegner (1989). In a second study, conducted with spider phobics, the authors withdraw from this general view. Their study of thought suppression in spider phobics (Muris et al., 1997) found only minimal effects of thought suppression increasing the frequency of spider-related thoughts. They argue that this unexpected result may be because spider phobics cope with their fears predominantly in a behavioural way, so that effects in cognitive variables would be small. This is plausible, as a spider is a very concrete object of fear. But what would happen in phobic patients with more vague objects of fear or with a wide variety of stimuli which evoke fear, as is the case with social phobia and agoraphobia?
So far, no studies using these populations have been published. The present study aimed to fill the gap. Two clinical groups with social phobia and agoraphobia respectively were compared with a healthy control group. An important question was whether problems with thought suppression exist only for a disorder-related topic, or if there is a generally impaired mental control. This distinction is relevant in determining the role of the impaired mental control within the model of development and maintenance of the disorder.
