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A further experimental investigation of thought suppression
Behav. Res. Ther. Vol. 31, No. 2, pp. 207-210, 1993 Printedin Great Britain.All rightsreserwd
Copyright0
0005-7967/93 $6.00 + 0.00 1993 PergamonPressLtd
A further experimental investigation of thought suppression DAVID M. CLARK,‘*
EMMA WINTON~
and
LENKA THYNN~
‘Department of Psychiatry, University of Oxford, Wurneford Hospital, Oxford OX3 TJX, and ~Dep~tment of Experimental Psychology, ~n~versiiy of Oxford, South Parks Road, Oxford, OXI 3UD, U.K. (Received
14 May 1992)
Sununary-4 has often been suggested that attempts to suppress a thought will lead to an immediate and/or delayed increase in its occurrence. In a recent experiment (Clark, Ball & Pape, Behaviour Research and Therapy, 29, 253-257, 1991) we obtained a delayed (rebound) effect but failed to demonstrate an immediate enhancement effect. Lavey and van den Hout (Behuvioural Psychotherapy, IS, 251-258, 1991) have suggested immediate enhancement might be observed only if subjects are instructed not to use distraction while suppressing. The present experiment tested this hypothesis. An immediate enhancement effect was not obtained but the delayed (rebound) effect was twice replicated and an artifactual explanation of this effect was discounted.
In
a recent experiment Clark, Ball and Pape (1991) investigated the hypothesis that trying to suppress a thought will lead to an immediate and/or delayed increase in its occurrence. Normal Ss listened to a taped story and then verbalized their stream of consciousness during two consecutive time periods. During the first period, one group (suppression) were asked not to think about the tape while two other groups (controls) were asked to think about anything or think about anything including the tape. During the second period, all three groups were instructed to think about anything. Results from the first period failed to support the immediate enhancement hypothesis as the suppression group reported less thoughts about the tape than the controls. However, results from the second period supported the delayed (rebound) hypothesis as Ss who had previously suppressed reported more thoughts about the tape than Ss who had not. Although Clark er al. (1991) failed to find evidence for an immediate enhancement effect, it is possible that such an effect may be observed under special circumstances. In particular, Lavey and van den Hout (1990) have suggested that immediate enhancement only occurs if Ss are instructed not to use distraction while trying to suppress a thought. Consistent with this suggestion, Lavey and van den Hout (1990) found that subjects who were asked to suppress thoughts about vehicles without using distraction reported more vehicle-related thoughts than a control group to whom vehicles were simply mentioned. However, a direct test of Lavey and van den Hout’s h~othesis would require compa~ng subjects who were instructed to suppress without using distraction with Ss who were simply asked to suppress. The present experiment provides such a test. METHOD
Experimental
design
Table 1 summarizes the experimental design. As in Clark et al. (1991) Ss listened to a tape recorded story about a green rabbit and were then allocated to one of three groups. Each group verbalized their stream of consciousness during two consecutive 2 mm periods. During the first period, Group A (suppression alone condition) were instructed not to think about the story on the tape, Group B (control) were instructed to think about anything including the tape, and Group C (suppression without distraction) were instructed not to think about the story on the tape and not to do this by specifically thinking of something else. Tire exact instructions for Group C were based on a direct translation of the instructions given by Lavey and van den Hout (1990). During the second period, all three groups were instructed that they could think about anything including the tape. Frequency of thoughts about the tape was assessed in each time period. During the first time period, Lavey and van den Hout’s (1990) hypothesis would predict that Group C {supp~on without distraction) would report more thoughts about the tape than Group B (control), who in turn might be expected to report more thoughts about the tape than Group A (suppression alone), given Clark et al.‘s (1991) findings. The rebound hypothesis relates to the second time period. Clark et ui.‘s (1991) design included Groups A and B and found that during the second period Group A reported more thoughts about the tape than Group B. It was expected that this effect would replicate. As there are no theoretical reasons for predicting that the rebound effect would be influenced by whether or not Ss are asked to refrain from distracting during suppression, it was also predicted that Group C (suppression without distraction) should report more thoughts about the tape during the second period than Group B (control). Subjects 60 undergraduates were allocated to Groups A, B and C until there were 20 (10 male, 10 female) Ss per group. No S was paid and none were known to be receiving psychiatric treatment.
*Author for correspondence. BRT 31/Z--F
207
208
CASE HlSTORlES
AND SHORTER
COMMUNiCATlONS
Table I. Experimental design Gr0up A B c
First period
Second period
Think anything except tape Think anything including tape Think anything except tape but do not distract
Think anything including tape Think anything including tape Think anything including tape
Thoughts about the tape were measured in two ways. First, by dividing Ss’ verbalizations during the 2-min periods into ideation units and classifying them as tape-related or not tape-related. Second, by Ss’ own ratings of the amount of time they spent thinking about the tape during each 2 min period. Details of these measures can be found in Clark et ni. (1991). Two further 5 point scales assessed the extent to which Ss made an effort to suppress thoughts about the tape during the first time period and the extent to which Ss attempted to distract themselves from thoughts about the tape during the first time period. Procedure
This was the same as in Clark et al. (1991) with the addition that at the end of the second period Ss completed the extra scales described above. In the first period subjects in Group A (suppression alone) were told “. . to keep from your mind thoughts of anything which you heard on the tape. You can think about what you like except for material from the tape. It is absolutely essential that you try to suppress this material from your mind . .“. Group B (control) were told “. . . to think about absolutely anything, with no restrictions, including material you heard on the tage . .” and Group C (suppression without distraction) were told “. . to keep from your mind thoughts of anything which you heard on the tape. It is absolutely essential that you try to suppress this material from your mind. Now, of course, we known it can be rather easy to ‘not think about something’ if you deliberately concentrate your thoughts on something else. It is very important for this experiment that you do not use this trick. Doing so would make the data worthless. It is therefore of the utmost importance that you don’t deliberately focus your thoughts on something else .“. During the second period all groups were told “. it doesn’t matter what you think about, you can think about the tape or anything else you like .“. All Ss were told that it was essential to say out loud whatever came into their minds in each period. Statistical analysis
Several measures failed Levene’s test for equality of variance. For these measures, comparisons between groups within each time period were based on Brown-Forsythe’s one-way analysis of variance (Dixon, Brown, Engelman, Hill & Jennrich, 1988), which does not assume equal variance. When the overall F-test was significant, separate variance t-tests, with Bonferroni correction for multiple comparisons, were used to isolate the source of any significant differences between means. Other measures were analyzed using traditional analysis of variance followed by Tukey tests. RESULTS Thinking during the f;rst period
Table 2 shows the total number of thoughts (irrespective of content), percentage of thoughts about the tape, and ratings of time spent thinking about the tape during the first period. There were no differences between groups in the total number of thoughts during the first period. In order to investigate whether either suppression condition produced an immediate enhancement effect, percentages of thoughts about the taped story were compared. Contrary to Lavey and van den Hout’s (1990) hypothesis, the suppression without distraction group had a significantly (P < 0.001) smaller percentage of thoughts about the tape than the control group and did not differ from the suppression alone group. In a replication of Clark et al.% (1991) finding, the suppression alone group (Group A) also had a significantly (P < 0.001) smaller percentage of thoughts about the tape than the control group. It therefore appears that in the short-term Ss were able to suppress thoughts about the tape, even when asked not to use distraction in order to achieve this aim.
fable 2. Thouaht tneasures durina first and second neriods Grouv
Measure
suppress alone (A)
Total number of thoughts Number of thoughts abut tape Percentage thoughts about tape Time spent thinking about taupe
Pii 16.1 2.8’ 18.68 1.3”
Total number of thoughts Number of thoughts about tape Percentage thoughts about tape Time soent thinkinn about tane
17.1 4.5” 28.7’ 1.8’
Suppress without Control (B)
distraction (C)
F
13.8 6.1b 45.7b 2.lb
14.8
II.OP 0.9”
1.0 17.98 25.2’ 8.9’
13.3 l.lb 8.3+J 0.Q
13.7 4.8” 34.1s I .4=
3.0 14.58 12.7’ 8.0’
period
I.@
Second period
More: *P < 0.001. Within each tneasure and time period means with different superscripts are significantly (P < 0.05 or less) different from each other.
CASE HISTORIES
AND
SHORTER
Yes (Group
No A)
Prior
209
COMhWNlCATIONS
(Group
I31
Yes (Group C)
Suppression
Fig. 1. Percentage of thoughts about tape during the second period as a function of prior suppression with distraction (Group A) and without distraction (Group C). Thinking during the second period
Table 2 also shows the total number of thoughts (irrespective of content), percentage of thoughts about the tape, and ratings of time spent thinking about the tape duting the second period. Once again there were no differences between groups in the total number of thoughts. To determine whether either type of suppression instruction was associated with a subsequent rebound effect, percentages of thoughts about the tape were compared. Figure 1 illustrates these data. Consistent with Clark et ai.% (1991) results, Ss who had been instructed to suppress thoughts about the tape during the first period (Group A) had a significantly (P < 0.01) greater percentage of tape related thoughts during the second period than Ss who had not been instructed to suppress during the first period (Group B). In addition, Ss who were instructed to suppress thoughts about the tape but not to do so by intentionally thinking of other things (Group C) also had a significantly (P <: 0.01) greater percentage of tape related thoughts during the second period than the control group. Mean ratings of time spent thinking about the tape followed a similar pattern. Ratings
of suppression and distraction
Table 3 shows Ss’ ratings of the extent to which they attempted to suppress thoughts about the tape during the first phase and the extent to which they attempted to distract themselves from such thoughts by ~n~ntionally thinking about something else during the first phase. In order to determine whether Ss who were instructed to suppress thoughts about the tape actually did so, the suppression ratings of each group were compared. Both suppression groups reported making significantly (P < 0.01) greater attempts to suppress thoughts about the tape than the control group and did not differ from each other. In order to determine whether Ss complied with the experimental instructions concerning the use of distraction, distraction ratings for each group were compared. The suppression alone group reported making significantly (P -=z0.01) greater attempts to distract themselves from thoughts about the tape than the control group (Group B). However, the suppression without distraction group failed to differ from either the suppression alone group or the control group. Further analyses
As the present experiment used Lavey and van den Hout’s (1990) instructions in the suppression without distraction condition, the failure of this condition to show an immediate enhancement effect contradicts Lavey and van den Hout’s hypothesis that instructing Ss not to use distraction is sufficient to produce an enhancement effect. However, it is possible that although Ss were instructed not to use distraction, some did so and this may have undermined the chances of observing an enhancement effect. The fact that the suppression without distraction group’s distraction rating was not significantly lower than the suppression alone group’s distraction rating is consistent with this view. To further explore this ~ssibility, two additional analyses were performed. First, the 25% of S’s with the lowest distraction ratings in the suppression alone condition and the 25% of Ss with the highest distraction ratings in the suppression without distraction condition were excluded and the data re-analysed. By excluding these Ss, the suppression without distraction group scored significantly Table 3. Ratings of suppression and distraction during first period Group Measure Attempts to suppress thoughts about tape Attempts to distract self from thoughts about tape
Suppress alone (A)
Control (B)
Suppress without distraction (C)
F
2.1s
0.7b
1.8’
14.3’1
29
1.2b
2.0”.b
i5.2+
Note: *P -z 0.01, l*P t0.001. Within each measure and time period means with different superscripts are significantly different from each other.
210
CASE HISTORIES
AND
SHORTER
COMMUNlCATlONS
(P < 0.001) lower on the distraction scale than the suppression alone group and did not differ from the control group (P > 0.3). However, both suppression groups continued to report a significantly smaller percentage of thoughts about the tape during the first period than the control group. Second, new suppression alone and suppression without distraction groups were created by dividing all the Ss who were asked to suppress (Groups A and C) above and below the median rating on the distraction scale. Once again, this guaranteed that the suppression without distraction group gave significantly (P < 0.001) lower distraction ratings than the suppression alone group, and did not differ from the control group (P > 0.8). However, as in the first analysis, both suppression groups continued to report significantly (Ps < 0.05) less thoughts about the tape during the first period than the control group. In discussing the rebound effect, Clark er al. (1991) suggested that the effect they observed in their experiment may not be a function of prior suppression, but instead could simply be a consequence of the control group having experienced more thoughts about the tape during the first period. To evaluate this suggestion, the thought measures during the second period were re-analysed using the first period scores as covariates. For both the percentage of thoughts about the tape, and ratings of time spent thinking about the tape, both suppression groups (Groups A and C) still had significantly (Ps < 0.001) more thoughts about the tape during the second period than the control group. It therefore appears that the rebound effect is not simply an artefactual consequence of the number of thoughts each group experiences during the first phase. Instead, it appears to be a direct consequence of attempting to suppress thoughts during the first period. DISCUSSION
Lavey and van den Hout (1990) have suggested that an immediate enhancement effect occurs if subjects are instructed not to use distraction while trying to suppress a thought. In order to test this hypothesis, we compared suppression without distraction instructions with suppression alone instructions and instructions which controlled for simply mentioning the tape. Contrary to Lavey and van den Hout’s (1990) hypothesis, suppression without distraction instructions did not lead to an immediate enhancement effect. In order to investigate whether this was simply a result of Ss not complying with the instructions, further analyses were conducted using empirically defined groups. These additional analyses also failed to support the Lavey and van den Hout (1990) hypothesis. Lavey and van den Hout (1990) did not include a suppression alone group in their experiment. However, they did include a control for simply mentioning the target material and found that this group reported less thoughts about the target (vehicles) than the suppression without distraction group. By contrast, in the present experiment the control group reported more thoughts about the target during the first phase than the suppression without distraction group. The explanation for this discrepancy is unclear. One possibility is a slight difference in instructions between the two studies. In particular Lavey and van den Hout’s (1990) instructions may have made the target thought (vehicles) more salient to the suppression without distraction group than to the control group and this increased salience, rather than attempted suppression, may have lead to the former group reporting more vehicle-related thoughts. There are two ways in which vehicles may have been made more salient for the suppression without distraction group. First, for both groups a specific alternative to vehicles was mentioned-animals in a zoo. However, in the suppression without distraction instructions vehicles were mentioned first, whereas in the control group instructions animals in the zoo were mentioned first. This may have given Ss the implicit message that in the suppression without distraction instructions the experimenter was particularly concerned with them thinking about vehicles, whereas in the control group instructions the experimenter was particularly interested in them thinking about animals in a zoo. Second, one could argue that the category of vehicles was mentioned more frequently in the suppression without distraction group than in the control group. This is because in the instructions for the former group the category ‘vehicles’ was elaborated by giving subjects four instances of this category (“cars, trolleys, bikes and related articles”). By contrast, the control group instructions mentioned vehicles but do not give any examples of the category. More research is required to resolve this issue. In contrast to our negative findings with respect to the immediate enhancement effect, the present experiment provided a double replication of the delayed (rebound) hypothesis. The suppression alone group in the present experiment was identical to the suppression condition used by Clark et al. (1991). As in the previous study, this condition produced a rebound in thoughts about the tape. In addition, a new suppression condition (suppression without distraction) was investigated and this also produced a rebound in thoughts about the tape. Further analyses indicated that these rebound effects were not simply an artefactual consequence of dillerences in the number of thoughts about the tape during the first period. It therefore appears that a subsequent increase in thought frequency is a reliable consequence of attempts to suppress a thought. The clinical implications of this effect are discussed elsewhere (Clark et al., 1991; Salkovskis, 1989; Wegner, 1989). REFERENCES
Clark, D. M., Ball, S. & Pape, D. (1991). An experimental investigation of thought suppression. Behauiour Research and Therapy, 29, 253-257.
Dixon, W. J., Brown, M. B., Engelman, L., Hill, M. A. & Jenmich, R. I. (1988). BMDPsfatisticalsoftware manual. Berkeley: University of California Press. Lavey, E. H. & van den Hout, M. A. (1990). Thought suppression induces intrusions. Behauioural Psychotherapy, 18, 251-258.
Salkovskis, P. M. (1989). Cognitive-behavioural
factors and the persistence of intrusive thoughts in obsessional problems.
Behaviour Research and Therapy, 27, 677-687. Wegner, D. M. (1989). White bears and other unwanted thoughts: suppression, obsession, and
Viking Press.
thepsychology of mental control.
Copyright0
0005-7967/93 $6.00 + 0.00 1993 PergamonPressLtd
A further experimental investigation of thought suppression DAVID M. CLARK,‘*
EMMA WINTON~
and
LENKA THYNN~
‘Department of Psychiatry, University of Oxford, Wurneford Hospital, Oxford OX3 TJX, and ~Dep~tment of Experimental Psychology, ~n~versiiy of Oxford, South Parks Road, Oxford, OXI 3UD, U.K. (Received
14 May 1992)
Sununary-4 has often been suggested that attempts to suppress a thought will lead to an immediate and/or delayed increase in its occurrence. In a recent experiment (Clark, Ball & Pape, Behaviour Research and Therapy, 29, 253-257, 1991) we obtained a delayed (rebound) effect but failed to demonstrate an immediate enhancement effect. Lavey and van den Hout (Behuvioural Psychotherapy, IS, 251-258, 1991) have suggested immediate enhancement might be observed only if subjects are instructed not to use distraction while suppressing. The present experiment tested this hypothesis. An immediate enhancement effect was not obtained but the delayed (rebound) effect was twice replicated and an artifactual explanation of this effect was discounted.
In
a recent experiment Clark, Ball and Pape (1991) investigated the hypothesis that trying to suppress a thought will lead to an immediate and/or delayed increase in its occurrence. Normal Ss listened to a taped story and then verbalized their stream of consciousness during two consecutive time periods. During the first period, one group (suppression) were asked not to think about the tape while two other groups (controls) were asked to think about anything or think about anything including the tape. During the second period, all three groups were instructed to think about anything. Results from the first period failed to support the immediate enhancement hypothesis as the suppression group reported less thoughts about the tape than the controls. However, results from the second period supported the delayed (rebound) hypothesis as Ss who had previously suppressed reported more thoughts about the tape than Ss who had not. Although Clark er al. (1991) failed to find evidence for an immediate enhancement effect, it is possible that such an effect may be observed under special circumstances. In particular, Lavey and van den Hout (1990) have suggested that immediate enhancement only occurs if Ss are instructed not to use distraction while trying to suppress a thought. Consistent with this suggestion, Lavey and van den Hout (1990) found that subjects who were asked to suppress thoughts about vehicles without using distraction reported more vehicle-related thoughts than a control group to whom vehicles were simply mentioned. However, a direct test of Lavey and van den Hout’s h~othesis would require compa~ng subjects who were instructed to suppress without using distraction with Ss who were simply asked to suppress. The present experiment provides such a test. METHOD
Experimental
design
Table 1 summarizes the experimental design. As in Clark et al. (1991) Ss listened to a tape recorded story about a green rabbit and were then allocated to one of three groups. Each group verbalized their stream of consciousness during two consecutive 2 mm periods. During the first period, Group A (suppression alone condition) were instructed not to think about the story on the tape, Group B (control) were instructed to think about anything including the tape, and Group C (suppression without distraction) were instructed not to think about the story on the tape and not to do this by specifically thinking of something else. Tire exact instructions for Group C were based on a direct translation of the instructions given by Lavey and van den Hout (1990). During the second period, all three groups were instructed that they could think about anything including the tape. Frequency of thoughts about the tape was assessed in each time period. During the first time period, Lavey and van den Hout’s (1990) hypothesis would predict that Group C {supp~on without distraction) would report more thoughts about the tape than Group B (control), who in turn might be expected to report more thoughts about the tape than Group A (suppression alone), given Clark et al.‘s (1991) findings. The rebound hypothesis relates to the second time period. Clark et ui.‘s (1991) design included Groups A and B and found that during the second period Group A reported more thoughts about the tape than Group B. It was expected that this effect would replicate. As there are no theoretical reasons for predicting that the rebound effect would be influenced by whether or not Ss are asked to refrain from distracting during suppression, it was also predicted that Group C (suppression without distraction) should report more thoughts about the tape during the second period than Group B (control). Subjects 60 undergraduates were allocated to Groups A, B and C until there were 20 (10 male, 10 female) Ss per group. No S was paid and none were known to be receiving psychiatric treatment.
*Author for correspondence. BRT 31/Z--F
207
208
CASE HlSTORlES
AND SHORTER
COMMUNiCATlONS
Table I. Experimental design Gr0up A B c
First period
Second period
Think anything except tape Think anything including tape Think anything except tape but do not distract
Think anything including tape Think anything including tape Think anything including tape
Thoughts about the tape were measured in two ways. First, by dividing Ss’ verbalizations during the 2-min periods into ideation units and classifying them as tape-related or not tape-related. Second, by Ss’ own ratings of the amount of time they spent thinking about the tape during each 2 min period. Details of these measures can be found in Clark et ni. (1991). Two further 5 point scales assessed the extent to which Ss made an effort to suppress thoughts about the tape during the first time period and the extent to which Ss attempted to distract themselves from thoughts about the tape during the first time period. Procedure
This was the same as in Clark et al. (1991) with the addition that at the end of the second period Ss completed the extra scales described above. In the first period subjects in Group A (suppression alone) were told “. . to keep from your mind thoughts of anything which you heard on the tape. You can think about what you like except for material from the tape. It is absolutely essential that you try to suppress this material from your mind . .“. Group B (control) were told “. . . to think about absolutely anything, with no restrictions, including material you heard on the tage . .” and Group C (suppression without distraction) were told “. . to keep from your mind thoughts of anything which you heard on the tape. It is absolutely essential that you try to suppress this material from your mind. Now, of course, we known it can be rather easy to ‘not think about something’ if you deliberately concentrate your thoughts on something else. It is very important for this experiment that you do not use this trick. Doing so would make the data worthless. It is therefore of the utmost importance that you don’t deliberately focus your thoughts on something else .“. During the second period all groups were told “. it doesn’t matter what you think about, you can think about the tape or anything else you like .“. All Ss were told that it was essential to say out loud whatever came into their minds in each period. Statistical analysis
Several measures failed Levene’s test for equality of variance. For these measures, comparisons between groups within each time period were based on Brown-Forsythe’s one-way analysis of variance (Dixon, Brown, Engelman, Hill & Jennrich, 1988), which does not assume equal variance. When the overall F-test was significant, separate variance t-tests, with Bonferroni correction for multiple comparisons, were used to isolate the source of any significant differences between means. Other measures were analyzed using traditional analysis of variance followed by Tukey tests. RESULTS Thinking during the f;rst period
Table 2 shows the total number of thoughts (irrespective of content), percentage of thoughts about the tape, and ratings of time spent thinking about the tape during the first period. There were no differences between groups in the total number of thoughts during the first period. In order to investigate whether either suppression condition produced an immediate enhancement effect, percentages of thoughts about the taped story were compared. Contrary to Lavey and van den Hout’s (1990) hypothesis, the suppression without distraction group had a significantly (P < 0.001) smaller percentage of thoughts about the tape than the control group and did not differ from the suppression alone group. In a replication of Clark et al.% (1991) finding, the suppression alone group (Group A) also had a significantly (P < 0.001) smaller percentage of thoughts about the tape than the control group. It therefore appears that in the short-term Ss were able to suppress thoughts about the tape, even when asked not to use distraction in order to achieve this aim.
fable 2. Thouaht tneasures durina first and second neriods Grouv
Measure
suppress alone (A)
Total number of thoughts Number of thoughts abut tape Percentage thoughts about tape Time spent thinking about taupe
Pii 16.1 2.8’ 18.68 1.3”
Total number of thoughts Number of thoughts about tape Percentage thoughts about tape Time soent thinkinn about tane
17.1 4.5” 28.7’ 1.8’
Suppress without Control (B)
distraction (C)
F
13.8 6.1b 45.7b 2.lb
14.8
II.OP 0.9”
1.0 17.98 25.2’ 8.9’
13.3 l.lb 8.3+J 0.Q
13.7 4.8” 34.1s I .4=
3.0 14.58 12.7’ 8.0’
period
I.@
Second period
More: *P < 0.001. Within each tneasure and time period means with different superscripts are significantly (P < 0.05 or less) different from each other.
CASE HISTORIES
AND
SHORTER
Yes (Group
No A)
Prior
209
COMhWNlCATIONS
(Group
I31
Yes (Group C)
Suppression
Fig. 1. Percentage of thoughts about tape during the second period as a function of prior suppression with distraction (Group A) and without distraction (Group C). Thinking during the second period
Table 2 also shows the total number of thoughts (irrespective of content), percentage of thoughts about the tape, and ratings of time spent thinking about the tape duting the second period. Once again there were no differences between groups in the total number of thoughts. To determine whether either type of suppression instruction was associated with a subsequent rebound effect, percentages of thoughts about the tape were compared. Figure 1 illustrates these data. Consistent with Clark et ai.% (1991) results, Ss who had been instructed to suppress thoughts about the tape during the first period (Group A) had a significantly (P < 0.01) greater percentage of tape related thoughts during the second period than Ss who had not been instructed to suppress during the first period (Group B). In addition, Ss who were instructed to suppress thoughts about the tape but not to do so by intentionally thinking of other things (Group C) also had a significantly (P <: 0.01) greater percentage of tape related thoughts during the second period than the control group. Mean ratings of time spent thinking about the tape followed a similar pattern. Ratings
of suppression and distraction
Table 3 shows Ss’ ratings of the extent to which they attempted to suppress thoughts about the tape during the first phase and the extent to which they attempted to distract themselves from such thoughts by ~n~ntionally thinking about something else during the first phase. In order to determine whether Ss who were instructed to suppress thoughts about the tape actually did so, the suppression ratings of each group were compared. Both suppression groups reported making significantly (P < 0.01) greater attempts to suppress thoughts about the tape than the control group and did not differ from each other. In order to determine whether Ss complied with the experimental instructions concerning the use of distraction, distraction ratings for each group were compared. The suppression alone group reported making significantly (P -=z0.01) greater attempts to distract themselves from thoughts about the tape than the control group (Group B). However, the suppression without distraction group failed to differ from either the suppression alone group or the control group. Further analyses
As the present experiment used Lavey and van den Hout’s (1990) instructions in the suppression without distraction condition, the failure of this condition to show an immediate enhancement effect contradicts Lavey and van den Hout’s hypothesis that instructing Ss not to use distraction is sufficient to produce an enhancement effect. However, it is possible that although Ss were instructed not to use distraction, some did so and this may have undermined the chances of observing an enhancement effect. The fact that the suppression without distraction group’s distraction rating was not significantly lower than the suppression alone group’s distraction rating is consistent with this view. To further explore this ~ssibility, two additional analyses were performed. First, the 25% of S’s with the lowest distraction ratings in the suppression alone condition and the 25% of Ss with the highest distraction ratings in the suppression without distraction condition were excluded and the data re-analysed. By excluding these Ss, the suppression without distraction group scored significantly Table 3. Ratings of suppression and distraction during first period Group Measure Attempts to suppress thoughts about tape Attempts to distract self from thoughts about tape
Suppress alone (A)
Control (B)
Suppress without distraction (C)
F
2.1s
0.7b
1.8’
14.3’1
29
1.2b
2.0”.b
i5.2+
Note: *P -z 0.01, l*P t0.001. Within each measure and time period means with different superscripts are significantly different from each other.
210
CASE HISTORIES
AND
SHORTER
COMMUNlCATlONS
(P < 0.001) lower on the distraction scale than the suppression alone group and did not differ from the control group (P > 0.3). However, both suppression groups continued to report a significantly smaller percentage of thoughts about the tape during the first period than the control group. Second, new suppression alone and suppression without distraction groups were created by dividing all the Ss who were asked to suppress (Groups A and C) above and below the median rating on the distraction scale. Once again, this guaranteed that the suppression without distraction group gave significantly (P < 0.001) lower distraction ratings than the suppression alone group, and did not differ from the control group (P > 0.8). However, as in the first analysis, both suppression groups continued to report significantly (Ps < 0.05) less thoughts about the tape during the first period than the control group. In discussing the rebound effect, Clark er al. (1991) suggested that the effect they observed in their experiment may not be a function of prior suppression, but instead could simply be a consequence of the control group having experienced more thoughts about the tape during the first period. To evaluate this suggestion, the thought measures during the second period were re-analysed using the first period scores as covariates. For both the percentage of thoughts about the tape, and ratings of time spent thinking about the tape, both suppression groups (Groups A and C) still had significantly (Ps < 0.001) more thoughts about the tape during the second period than the control group. It therefore appears that the rebound effect is not simply an artefactual consequence of the number of thoughts each group experiences during the first phase. Instead, it appears to be a direct consequence of attempting to suppress thoughts during the first period. DISCUSSION
Lavey and van den Hout (1990) have suggested that an immediate enhancement effect occurs if subjects are instructed not to use distraction while trying to suppress a thought. In order to test this hypothesis, we compared suppression without distraction instructions with suppression alone instructions and instructions which controlled for simply mentioning the tape. Contrary to Lavey and van den Hout’s (1990) hypothesis, suppression without distraction instructions did not lead to an immediate enhancement effect. In order to investigate whether this was simply a result of Ss not complying with the instructions, further analyses were conducted using empirically defined groups. These additional analyses also failed to support the Lavey and van den Hout (1990) hypothesis. Lavey and van den Hout (1990) did not include a suppression alone group in their experiment. However, they did include a control for simply mentioning the target material and found that this group reported less thoughts about the target (vehicles) than the suppression without distraction group. By contrast, in the present experiment the control group reported more thoughts about the target during the first phase than the suppression without distraction group. The explanation for this discrepancy is unclear. One possibility is a slight difference in instructions between the two studies. In particular Lavey and van den Hout’s (1990) instructions may have made the target thought (vehicles) more salient to the suppression without distraction group than to the control group and this increased salience, rather than attempted suppression, may have lead to the former group reporting more vehicle-related thoughts. There are two ways in which vehicles may have been made more salient for the suppression without distraction group. First, for both groups a specific alternative to vehicles was mentioned-animals in a zoo. However, in the suppression without distraction instructions vehicles were mentioned first, whereas in the control group instructions animals in the zoo were mentioned first. This may have given Ss the implicit message that in the suppression without distraction instructions the experimenter was particularly concerned with them thinking about vehicles, whereas in the control group instructions the experimenter was particularly interested in them thinking about animals in a zoo. Second, one could argue that the category of vehicles was mentioned more frequently in the suppression without distraction group than in the control group. This is because in the instructions for the former group the category ‘vehicles’ was elaborated by giving subjects four instances of this category (“cars, trolleys, bikes and related articles”). By contrast, the control group instructions mentioned vehicles but do not give any examples of the category. More research is required to resolve this issue. In contrast to our negative findings with respect to the immediate enhancement effect, the present experiment provided a double replication of the delayed (rebound) hypothesis. The suppression alone group in the present experiment was identical to the suppression condition used by Clark et al. (1991). As in the previous study, this condition produced a rebound in thoughts about the tape. In addition, a new suppression condition (suppression without distraction) was investigated and this also produced a rebound in thoughts about the tape. Further analyses indicated that these rebound effects were not simply an artefactual consequence of dillerences in the number of thoughts about the tape during the first period. It therefore appears that a subsequent increase in thought frequency is a reliable consequence of attempts to suppress a thought. The clinical implications of this effect are discussed elsewhere (Clark et al., 1991; Salkovskis, 1989; Wegner, 1989). REFERENCES
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