Vulnerability to False Memory: The Effects of Stress, Imagery, Trait Anxiety, and Depression PAUL ROBERTS Murdoch U n i v e r s i t y There is now abundant evidence that false memories can be created inside and outside the laboratory, and much is known about the kinds of techniques and manipulations that encourage the creation of false memories. However, rather less is known about the influence of naturally occurring variables over which the experimenter or clinician has little or no control. The current study investigated the influence of some of these variables on false memory creation. The influence of individual differences in depression, trait anxiety, imagery vividness and the interaction of these factors with stress was studied. Subjects studied series of pictures and words and then attempted to recall all the pictures presented, at a low-stress time and then later at a high-stress time. Intrusions from words at recall were the false memories in this study. Subjects who experience more vivid imagery were more vulnerable to false memories when they were stressed. Low trait anxious subjects were more likely to assign remember judgement to a false memory when they were stressed. These findings are discussed in terms of the source monitoring framework and implications for clinical practice.
here is now a considerable literature documenting the creation of false memories in the laboratory (see Lindsay and Read, 1994 and Roediger, 1996 for reviews). But do these laboratory findings tell us anything about the creation of false memories outside the laboratory? A number of authors (e.g., Freyd and Gleaves, 1996; Pezdek, 1994) have argued that these laboratory studies are so lacking in external validity that the answer to this question must be no. For example, Freyd and Gleaves point out that the experimental paradigm employed by Roediger and McDermott (1995) is not a good analogue for memory recovery in therapy (note that in their reply to Freyd and Gleaves, Roediger and McDermott, 1996 point out that they did not claim otherwise). Indeed, it is unlikely that this paradigm is a good analogue for any common everyday m e m o r y situation. But this does not mean that the results from such laboratory studies are irrelevant to the understanding of everyday memory. The fact that false memories can be created in the laboratory tells us that there are cognitive processes that can give rise to false memories. It would be very suprising if these processes never resulted in the creation of a false memory without the intervention of a memory researcher (cf. Roediger and McDermott, 1996). The real question is not whether false memories occur outside the laboratory, but which naturally occurring variables act to increase (or decrease) the probability of false memory creation.
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In a recent survey, Poole, Lindsay, Memon and Bull (1995) documented the fact that many therapists employ techniques that memory researchers have suggested are likely to increase the risk of creating a false memory (see e.g., Lindsay & Read, 1994). One such technique sometimes employed by therapists is guided imagery. In this procedure, clients who are having difficulty recalling information are encouraged to form a visual image of the relevant situation. This image often cues other images and recollections and is supposed to have a general enhancing effect on memory recovery. In order to investigate the effect of imagery on the probability of false memory creation, Hyman and Pentland (1996) asked people to attempt to recall actual childhood events that had been described by their parents and false events that were constructed by the experimenters. If they were unable to recall an event, subjects in one group were asked to imagine the event and to describe their image to the experimenter. Subjects in a control group were required to sit and think about the event for one minute. The results showed that the imagery manipulation increased the probability of recovering true events that were previously unable to be recalled. However, this beneficial effect of imagery came at the cost of an increased probability of creating a false memory of the event constructed by the experimenters. This is a very important result and confirms that therapists should be wary of employing guided imagery when the veracity of their client's memory is important. However, in a sense, the question of what techniques therapists should and should not use is not the main problem. We already have a pretty good idea what kinds of therapeutic practices are likely to create memory distortions (e.g., Hyman and Pentland, 1996; Lindsay and Read, 1994) and many people have advised therapists what not to do on this basis. A less tractable problem concerns the impact of variables on false memory creation over which the therapist has little or no control, but which are likely to be ubiquitous. There seem to be (at least) two classes of such variables. Firstly, there are individual differences that may increase (or decrease) vulnerability to false memory and secondly, there are mental state variables. In even the simplest false memory experiment some people produce false memories and some people do not. One reason for this may be that there are individual differences that predispose some people to create false memories more often. Since instructions to engage in imagery increases the probability of creating a false memory (Hyman & Pentland, 1996), and since there are individual differences in the use of imagery (Richardson, 1994), it may be that people who are more likely to engage in imagery spontaneously are also more susceptible to false memory creation. In fact, there is some research on the relationship between this individual difference and false memory occurrence.
Dobson and Markham (1993) presented high- and low-imagery subjects with a short film depicting a petrol station holdup. Subjects were then given a series of statements to read that described the film, some of which were accurate and some of which were inaccurate. At a later date they were given a second series of statements, some of which referred to information presented in the film only, some of which referred to information presented in the first statements only, some of which referred to information presented in both the film and the first statements, and some containing
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information not presented in either the film or the earlier statements. The subjects' task was to indicate whether the information in these new statements had been presented previously, and if so in what form. While high- and low-imagery subjects were able to discriminate previously presented information from new information to an equivalent degree, high-imagery subjects were significantly poorer at discriminating whether that information had been presented in the film or in a subsequent statement. Using the ubiquitous Deese (1959) paradigm, Wilkinson and Hyman (1998) found that erroneous recognition of the critical lures was correlated with imagery measures. Subjects who reported using more imagery were more likely to falsely recognize a lure and subjects with more vivid imagery were more likely to make a remember judgement to a lure. Winograd, Peluso and Glover (1998) found a trend towards a similar relationship between individual differences in imagery vividness and remember judgements to critical lures. While there have been a number of studies that have investigated the relationship between various personality factors and false memory occurrence (e.g., Hyman and Billings, 1998; Ward and Loftus, 1985), there do not appear to be any studies that focus on the two personality dimensions that are likely to be most important in a clinical setting: trait anxiety and depression. For this reason the current study focused on these two individual differences, in addition to individual differences in imagery. There are undoubtedly many mental state variables that are related to false memory occurrence. The most obvious of these is hypnosis and we now know that recollections under hypnosis should be treated with caution (e.g., Putnam, 1979). In terms of variables that are likely to be ubiquitous in a clinical setting and over which therapists have little control, perhaps the most important is likely to be stress. This is because, almost by definition, people entering therapy are likely to be experiencing stress. However, while there is an enormous literature on memory accuracy for stressful events (see e.g., Christianson, 1992), there is comparatively little written on the effect of stress on memory accuracy. This latter category of phenomena is probably more relevant for the current concerns as we are interested in how being in a stressed state influences susceptibility to false memory creation, rather than how (actual) stressful memories are remembered. One study which did investigate this relationship was that of Siegal and Loftus (1978). Using an eyewitness testimony paradigm, they found a significant correlation between eyewitness accuracy and state anxiety. The more anxious a subject, the less accurate they were. Gudjonsson (1988) also found a similar relationship between suggestibility and state anxiety. To summarize, the current study was designed to investigate the influence of a number of naturally occurring variables, outside therapist control, on susceptibility to false memory creation. Two kinds of variables were identified; individual differences and mental state variables. Three individual difference variables were identified as important within a clinical setting: imagery, trait anxiety and depression. State anxiety was identified as an important mental state variable within a clinical setting. While the influence on false memory creation of some of these variables has been previously studied, many of the previous studies have been conducted within an eyewitness testimony/suggestibility framework. While suggestibility is an important issue in the
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clinical setting, nevertheless it is something over which therapists have some control. In the current study interest centred on the influence of factors over which therapists have no control. Thus the element of suggestibility was removed entirely. The current study utilized an experimental paradigm designed to tap a particular kind of source monitoring error thought likely to be important in a clinical setting. That is, having read about or heard tell of an event, a person comes to believe that they have experienced that event themselves (Lindsay and Read, 1994). Subjects studied pictures and words and then later attempted to recall all the pictures they had seen. Presented words recalled as pictures were the false memories in this study. The relationship between these word intrusions and the variables discussed above was examined. METHOD
Design Subjects all filled out the same questionnaires and participated in the same memory task. Stress was manipulated in a quasi experimental manner, within subjects, by testing subjects early in the first semester and then late in the second semester, just prior to the end of year examinations.
Subjects The subjects were 60 Murdoch University undergraduates who were paid AS10 to participate in the two testing sessions. There were 46 females and 14 males, with a mean age of 29.7 (SD = 8.77) years. Forty one subjects returned for the second testing session; 30 females and 11 males with a mean age of 27.24 (SD = 8.96). Materials and Apparatus. Initially, a set of 60 concrete nouns and 60 line drawings depicting these words were collected. In order to ensure that each drawing was unambiguously identified with its associated verbal label 5 judges were asked to name each picture. Pictures that were incorrectly named by any of the judges were discarded from the stimulus set, along with the associated word. The final stimulus set consisted of 50 words and pictures. Half of the words were randomly assigned to Set A and half to Set B. The pictures not represented by a Set A word became members of Set A and similarly for Set B. Thus, there were two stimulus sets, each containing 25 words and 25 pictures representing 50 objects. Stimuli were transferred to slides and projected via a Kodak Carousel S-AV 2010 slide projector so that the resulting image just fit within a 300mm square area on the screen. Four separate questionnaires were administered. The Speilberger (1969) State-Trait Anxiety Inventory (STAI), the Beck (1987) Depression Inventory (BDI), the Questionnaire on Mental Imagery (QMI, Sheehan, 1967) and the Verbalizer-Visualizer Questionnaire (VVQ, Richardson, 1994). The STAI measures both trait and state anxiety. The measure of state anxiety it yields was the index of stress used in this study. The BDI measures depression. The version used here probably measures both state and trait aspects of depression. The QMI is a measure of the vividness with
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which people experience imagery. The VVQ is a measure of the tendency to think habitually in a verbal versus a visual way.
Procedure The experiment consisted of two testing sessions, one early in the first semester and one late in the second semester, just prior to the subjects' first examination in the end of year examination period (between seven and eight months later). Subjects were tested in groups of two to 15 in a quiet room. They were told that they were participating in an experiment on how people classify objects. The procedure was identical for both testing sessions, except that if subjects were exposed to Set A in the first testing session they were exposed to Set B in the second testing session and vice versa. In the first testing session, half the subjects were randomly assigned Set A stimuli while the other half were assigned Set B stimuli. This counterbalancing was approximately maintained in the second testing session, with subject attrition producing 22 subjects who had previously been exposed to Set A in the first testing session (who now received Set B) and 19 who had previously been exposed to Set B (and who now received Set A). Each testing session consisted of three phases. In the first phase the slides of pictures and words were presented for five seconds each, with a blank screen present for 1 second between each slide. Subjects were instructed to keep a mental tally of the number of animate versus inanimate objects presented. They were told that their accuracy on this task would be scored. Following presentation of all 50 slides subjects were instructed to record their tally of animate and inanimate objects in the space provided on the response sheet. In the second phase of the experiment subjects filled out the QMI, VVQ, STAI and BDI, in that order. Subjects proceeded at their own pace but always took less than 30 minutes to complete the questionnaires. In the third phase of the study subjects were asked to recall all the pictures that had previously been presented and to write the names of those pictures in the space provided on the response sheet. They were cautioned not to write the names of words previously presented. Two minutes was allowed for this task. Subjects were then given a short break of two minutes before again being given the instruction to recall previously presented pictures. These names were recorded on a separate response sheet that obscured the first set of responses. Finally, subjects were asked to make a "remember" or "know" judgment to each of the items they had recalled at the second attempt. The instructions for the remember&now judgements were adapted from Rajaram (1993) and Dewhurst and Conway (1994). The duration of the entire experiment was approximately 45 minutes. RESULTS
Subjects Attrition The first concern in analyzing these data was to investigate whether there were any differences between the 41 subjects who returned for the second testing session, seven
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to eight months after attending the first testing session, and the 19 subjects who did not return for the second testing session. Table 1 shows the scores obtained on the various questionnaire measures for these two groups of subjects. T-tests comparing the two groups of subjects on each of the questionnaire measures did not approach significance (ps > .5). The subsequent analyses are conducted on the 41 subjects who did return for the second testing session.
Reliability and Validity of the Predictors Pearson correlations between the four questionnaire measures and between the first and second testing session for each questionnaire measure were calculated. Reliability coefficients are shown in Table 2. Subjects' scores on the BDI and the STAI (trait) were quite highly correlated in both the first (r=.54, p < .0001) and the second (r=-.55, p < .0001) testing session. Scores on the QMI and the VVQ were not significantly correlated in either the first (r=--.24, p >.05) or the second (r=-.29, p > .05) testing session. Neither QMI or VVQ scores were significantly correlated with BDI or STAI (trait) scores at the first or the second testing session (r < .3 in every case, p > .05 in every case).
Stress Manipulation State anxiety was measured at the two testing sessions to provide a manipulation check on the assumption that stress had in fact increased from the first to the second testing session. In the first testing session the mean STAI (state) score was 36.02 (11.76). This increased to 41.37 (11.94) in the second testing session, and this difference was reliable It(40) = 2.77, p < .01]. Although there were numerical increases in BDI scores (6.17 to 6.37) and STAI (trait) scores (39.24 to 40.54) these differences were not reliable (ps> .05).
Predicting Correct Recall The median scores on the VVQ, QMI, BDI and STAI (trait) from the first testing session were computed and subjects assigned to one of two groups on the basis of their score on each of these four questionnaires. Subjects scoring above the median on a particular questionnaire were assigned to the "high" group while subjects scoring below the median were assigned to the "low" group. Subjects obtaining the median score were dropped from analyses. Table 3 gives the mean number of pictures correctly recalled as a function of group membership and testing session. It also shows the proportion of these responses that were assigned a remember judgement rather than a know judgement. (Note that since subjects were required to make either a remember or a know judgement to every response that the proportion of know judgements is equal to one minus the proportion of remember judgements). Four separate 2 X 2 ANOVAs were conducted on the number of pictures correctly
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TABLE 1 Means and Standard Deviations (in Parentheses) for Subjects Who Did, versus Did Not, Return for the Second Testing Session.
BDI STAI (Trait) STAI (State) QMI VVQ
Returned
Did not return
6.17 (5.34) 39.24 (10.61) 36.02 (11.76) 93.27 (35.53) 8.54 (2.34)
6.68 37.63 34.53 95.89 8.47
(4.94) (7.91) (8.04) (35.07) (2.61)
TABLE 2 Test-retest Reliability for the Four Questionnaire Measures (r)
Questionnaire measure BDI
STAI (trait)
QMI
VVQ
.60***
.90***
.91"**
.66***
***p< .0001
recalled, with testing session (first versus second) as one factor and VVQ, QMI, BDI or STAI (trait) group membership (low versus high) as the other factor. There was a main effect of testing session (p < .01 in every case), with subjects recalling correctly more pictures at the second testing session than at the first. The only group main effect to achieve significance was the QMI [F(1,38) = 6.57, p < .02]. Subjects who had low scores on the QMI recalled correctly, more words than those subjects who had high scores. Since a low score on the QMI indicates more vivid imagery experience this means that subjects who rated their imagery as more vivid recalled more pictures than subjects who rated their imagery as less vivid. None of the interactions achieved significance (all Fs < 1). Analyses of variance conducted on the proportion of remember judgements revealed no reliable effects (all ps > . 1).
Predicting False Memories A false memory in this task was defined as recalling as a picture, an item that had actually been presented as a word. Table 4 presents the mean number of false memories generated as a function of group membership and testing session. The analysis of false memory production mirrored that for correct recall. There was no evidence that false memory production changed across the two testing sessions (all Fs < 1). There were no effects associated with the BDI or STAI group factors (all Fs < 1). There was a trend towards a main effect for QMI group [F(1,38) = 2.87, p < .1], with
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TABLE 3 Mean Number of Correctly Recalled Pictures and Mean Proportion of Associated Remember Judgements and Standard Deviations (in Parentheses)
BDI group Low High
First testing session
Second testing session
No. Rec. 4.47 (2.25) 4.38 (3.36)
Rem. (p) .69 (.32) .77 (.33)
No. Rec. 5.84 (2.57) 5.81 (2.23)
Rem. (p) .72 (.34) .81 (.28)
4.30 (1.95) 5.17 (3.26)
.74 (.31) .76 (.33)
5.60 (2.19) 6.17 (2.62)
.72 (.34) .80 (.26)
5.50 (3.14) 3.85 (1.79)
.75 (.30) .72 (.32)
6.75 (2.61) 5.20 (1.88)
.71 (.29) .86 (.26)
4.94 (3.47) 4.38 (1.93)
.68 (.37) .79 (.25)
5.88 (2.20) 5.96 (2.53)
.77 (.31) .77 (.28)
STAI group Low
High
OMI ~rouo Low High
VVO arouo
Low High
the subjects who experience the more vivid imagery tending to produce more false memories overall than their counterparts who experience less vivid imagery. A similar trend is apparent in relation to VVQ group [F(1,39) = 2.85, p < . 1], with subjects who are stronger visualizers tending to produce more false memories than subjects who are more strongly verbalizers. There was no VVQ group by testing session interaction (F < 1). However, there was a significant QMI group by testing session interaction [F(1,38) = 4.92, p < .05). Only the difference in false memory production between high and low groups at the second testing session was reliable [t(38) = 2.77, p < .01]. Thus, in the second testing session only, when subjects were more stressed, those subjects who experience vivid imagery were more vulnerable to false memory production than were subjects who experience less vivid imagery. The proportion of false memories assigned a remember judgement in the second testing session was significantly greater than in the first (p < .01 in each case). The
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TABLE 4 Mean Number of False Memories and Mean Proportion of Associated Remember Judgements and Standard Deviations (in Parentheses)
BDI group Low High
First testing session
Second testing session
No. Rec. .74 (.93) .75 (1.13)
Rem. (p) .09 (.27) .09 (.27)
No. Rec. .79 (1.23) .69 (.95)
Rem. (p) .57 (.35) .36 (.48)
.70 (.86) .67 (1.14)
.11 (.28) .11 (.32)
.80 (1.15) .83 (.98)
.72 (.11) .11 (.22)
.65 (.99) .75 (.97)
.03 (.11) .18 (.38)
1.20 (1.24) .35 (.59)
.33 (.39) .58 (.49)
.53 (.80) .79 (1.06)
.16 (.36) .06 (.22)
.47 (.62) .96 (1.23)
.50 (.50) .36 (.39)
STAI group Low
High
OMI ~rouD Low
High
VVO m'ouo Low
High
only reliable group effects involved STAI (trait). Overall, subjects low in trait anxiety assigned a greater proportion of their false memories a remember response than subjects high in trait anxiety [F(1,16) = 6.63, p < .02). However, this effect interacted with testing session [F(1,16) = 17.43, p < .001], only becoming apparent in the second testing session (p < .001). DISCUSSION Interest in this study centered on the influence of naturally occurring variables, which are outside therapist control, on false memory creation. To this end a simple task with the potential for picture/word confusions was utilized. It is thought that this task captures some elements of a process of false memory creation likely to be impor-
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tant in a clinical setting. Where people come to believe that they have experienced an event themselves, that in fact they have only read about, then they must have confused the written source of their memory with some other kind of experience. Unsurprisingly, subjects who experience more vivid imagery were able to correctly recall more of the studied pictures than subjects who experience less vivid imagery. Presumably their imagery ability makes these subjects better at encoding information in a visual form. However, this advantage for high imagery subjects became a disadvantage when vulnerability to false memory creation was considered. There was a trend for subjects who experience vivid imagery and subjects who habitually think in a visual versus verbal way to produce more false memories. This result is consistent with a number of other studies that have found a weak relationship between various kinds of false memory creation and imagery measures (Dobson and Markham, 1993; Wilkinson and Hyman, 1998; Winograd et al., 1998). Although not tested directly, it seems likely that this effect results from high imagery subjects spontaneously forming an image depicting the presented words. Since imagining something makes it more likely that subjects will believe that they have actually seen that thing (e.g., Finke, Johnson and Shyi, 1988), it is not unlikely that subjects might confuse an imagined visual representation of a word with a presented picture in this study. Interestingly, individual differences in imagery vividness had its effect largely in the second testing session when subjects demonstrated elevated state anxiety. Only in the second testing session did the difference in false memory rate between high and low imagery subjects achieve significance. Thus, being in an anxious state reduced the ability of high imagery subjects to accurately source monitor. Despite the fact that at least two other studies have found a relationship between state anxiety and eyewitness accuracy (Gudjonsson, 1988; Siegal and Loftus, 1978), in the current study, state anxiety by itself did not have any effect on false memory rate. It may be that the current study was simply not sensitive enough in some sense to detect an overall effect of state anxiety. For example, it may be that the false memory effect relied so much on subjects creating a mental image of studied words that any effect of state anxiety did not manifest unless subjects were creating a substantial number of vivid images of those words. In any case, the important point is that in vivid imagers stress had quite a powerful effect on false memory creation. The false memory rate in these subjects when they were stressed was approximately twice what it was when they were not stressed. The fact that stress increases the rate at which false memories are produced is explicable within the source monitoring framework (e.g., Johnson, Hashtroudi and Lindsay, 1993). Source monitoring accuracy depends on the quality of information encoded. Anything that disrupts that encoding, particularly in terms of the quality of perceptual properties and contextual information, is likely to interfere with the ability to accurately source monitor. Since state anxiety, via increased arousal, is likely to interfere with short-term memory effectiveness (Revelle and Loftus, 1990) then it is also likely to have some impact on the qualities of memories upon which source monitoring is based. Hence the status of a memory as a picture versus a word will become less certain. Presumably subjects who experience vivid imagery are more
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vulnerable to source monitoring errors under these conditions because they are able to more vividly imagine the object referred to by a word and therefore more likely to believe that they have actually encountered a picture of that object. It is worth pointing out that source monitoring also depends on the effectiveness of decision processes at retrieval and that stress can also interfere with the operation of those decision processes (Johnson et al., 1993). Subjects in the current study were stressed at retrieval as well as at encoding. Therefore the effect of stress could have its locus at encoding, at retrieval or at both encoding and retrieval. Investigating just where and how stress has this detrimental effect is an important subject for further research. Although there was no general effect of state anxiety on false memory creation in this study, state anxiety did have an effect on the proportion of remember judgements made to false memories. Subjects produced significantly more remember judgements to false memories in the second (high stress) testing session than in the first (low stress) testing session. However, this effect stems from the influence of low trait anxious individuals in the subject pool. High trait anxious subjects did not show an elevation in remember judgements to false memories in the second testing session. It is not easy to interpret this result. One obvious difference between high and low trait anxious individuals is that high trait anxious individuals are familiar with the experience of being state anxious, as subjects were in the second testing session. Low trait anxious individuals on the other hand have relatively less experience with being state anxious. Conceivably then, high trait anxious individuals may have developed strategies for coping with the effects of stress on memory and hence may be less likely to fall victim to memory errors when stressed. Why this should have been evident in subjects' phenomenal experience of their false memories but not in the false memory rate per se is unclear. Nevertheless, this is an intriguing result with important implications. It suggests that low trait anxious individuals are likely to be more committed to the veracity of their false memories than high trait anxious individtials, because they more often experience those false memories as having the character of a remember experience. Although this study did not attempt to provide an analogue of a clinical interaction, it did attempt to tap into some elements of the processes that might create false memories in a clinical setting. For this reason the results of the current study may have implications for clinical practice. Although, of course it would be wise to see if these findings generalise to other paradigms before firm conclusions are drawn. Contrary to what might be expected, these results suggest that it is low trait anxious individuals who may be at risk of creating compelling false memories that are difficult to distinguish from true memories. Since trait anxiety is easy to measure with a simple paper and pencil questionnaire in a clinical setting this information may help clinicians decide how much weight they should attach to a particular memory if the question of the accuracy of that memory is important. Similarly, imagery vividness and state anxiety are equally easy to measure in a clinical setting and as the current study shows, predicted elevated false memory rates in combination. Thus this knowledge might be used to allow some simple paper and pencil measures to provide a "risk factors"
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assessment in a clinical setting. Finally, both of the major effects in this study occurred when subjects were stressed. To the extent that stress really is the enemy of accurate memory (cf. Gudjonsson, 1988; Revelle and Loftus, 1990; Siegal and Loftus, 1978) it may be wise not to place too much weight on memories recalled while state anxiety is high and indeed to actively attempt to reduce state anxiety prior to eliciting recollections when the accuracy of those recollections is important. NOTES Accepted for publication: August 15, 2000. The author thanks Ainslie Lown for help in collecting and analyzing data from the first part of this study. Address correspondence to: Paul Roberts, School of Psychology, Murdoch University, South Street, Murdoch, Western Australia 6150. E-Mail:
[email protected]
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