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Forgetting as a consequence of remembering: Retrieval-induced forgetting and
the malleability of memory
Malcolm D. MacLeod
University of St. Andrews
&
Jo Saunders
University of Swansea
Total word count (including refs) = 10,065
Address for correspondence:
Malcolm D. MacLeod
Social & Applied Cognition Lab
School of Psychology
University of St. Andrews
St. Andrews
Fife, Scotland
KY16 9JU
Tel:
+44 (0)1334-462064
Fax: +44 (0)1334-463042
Email: [email protected]
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Abstract
This article considers some of the most recent advances in experimental psychology
regarding the role of inhibitory control in the production of misinformation effects in memory;
that is, the tendency to report misleading post-event information in preference to original
material. Specifically, we report our findings from a series of studies using the retrieval practice
paradigm (cf. Anderson, Bjork & Bjork, 1994) that explore how retrieval-induced forgetting (i.e.,
the tendency to forget material as a consequence of retrieving other related material) may be
involved in promoting misinformation effects. We then consider how to establish experimentally
that the mechanism underlying this relationship is inhibitory. In doing so, we outline the
independent probe technique (Anderson & Spellman, 1995); that is, the use of novel retrieval
cues at test rather than those cues used during the initial stages of the retrieval practice paradigm.
Our findings clearly indicate that misinformation effects emerged only under conditions where
retrieval-induced forgetting remained active and that the retrieval-induced forgetting observed in
our studies was due to inhibitory control. Implications for our understanding of how memory is
updated; the design of experimental paradigms that consider the role of active forgetting in
memory; and the development of police investigative techniques are also explored.
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Forgetting as a consequence of remembering: Retrieval-induced forgetting and the
malleability of memory
In our quest to understand how memory works, researchers often face the problem of
how to make everyday phenomena the focus of empirical study. Foremost amongst these myriad
challenges is the design of appropriate experimental paradigms that permit memory to be
explored in a controlled manner yet retain sufficient realism to allow extrapolation to the real
world. For those who are primarily interested in solutions to applied problems, a high level of
control can appear artificial and remote from the contexts and factors associated with memory in
the real world. For other researchers, however, experimental control is paramount (e.g., Banaji &
Crowder, 1989).
While arguments can be made regarding the utility of studies that have high
ecological validity, we are of the opinion that this should never be at the expense of experimental
control. We wish to be clear that this preference is not due to our lack of interest in applied
problems - in fact, the converse is true. Rather, we believe that good applied research is, without
exception, based on well-designed and tightly controlled experimentation. Thus, in our view,
highly controlled studies offer the greatest potential - both in terms of how to advance theory and
the development of solutions to real world problems. Indeed, there is much to commend the
sentiment expressed by Banaji and Crowder (1989) that ‘…the complexity of a phenomenon is a
compelling reason to seek, not abandon the laboratory’ (p.1112). Unfortunately, all too often the
complexity of the phenomena of interest is given as the raison d’etre for poorly controlled
research.
In order to illustrate our point of view, the present article details some of the most
recent experimental studies concerning the mechanisms underlying one of the most researched of
all memory phenomena - the misinformation effect. This effect refers to the bias towards
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unwittingly recalling misinformation (i.e., information inconsistent with that originally
presented) in preference to original material (Loftus, 1979a; Loftus, Miller & Burns, 1978). In
considering this body of research, we hope to demonstrate how, through controlled
experimentation, it is possible to enhance our understanding not only of those conditions most
likely to give rise to misinformation effects in the real world but also the complex relationship
that exists between forgetting and remembering.
The Misinformation Effect Despite the fact that much of the pioneering work on the
misinformation effect was carried out almost thirty years ago (Loftus, 1979a; Loftus, et al., 1978;
Loftus & Palmer, 1974), its influence in cognitive psychology remains strong. The standard
misinformation paradigm (cf. Loftus, et al., 1978) involves the presentation of a target incident
such as an accident or crime scene in the form of a slide sequence, video, or piece of prose. On
encoding this material, participants are presented with a series of questions about the target
event. Unknown to the participants, however, a piece of misinformation (i.e., information
inconsistent with that originally presented) is embedded within one of the questions. For
example, participants could be asked about a car that passed by a ‘Yield’ sign whereas, in fact,
the car in question had actually passed by a ‘Stop’ sign or vice versa (cf. Loftus, et al., 1978).
Following a short delay, participants are asked to choose which slides they had seen previously
in a two-alternative forced-choice recognition task. For non-critical items, participants are
required to choose between a novel slide and the original whereas, for the critical item (i.e.,
where misinformation had been introduced), participants have to choose between the original
and the misinformation item (e.g., ‘Yield’ versus a ‘Stop’ sign). In their seminal study, Loftus
and colleagues found that 59% of participants who were misled in this way chose the
misinformation item in contrast to only 25% of non-misled participants. Although the magnitude
of this particular effect can be modified by a range of cognitive and social factors (see Wright &
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Davies, 1999 for a review), its robustness has never been in question (see e.g., Belli, 1989;
Bekerian & Bowers, 1983; Chandler, Gargano & Holt, 2001; Christiaansen & Ochalek, 1983;
Dodson & Reisberg, 1991; Lindsay & Johnson, 1989a, 1989b; Pirolli & Mitterer, 1984).
There are several reasons why this work has proved to be important. Although it had
been appreciated for some time that eyewitness reports could be modified by the form of
questions employed to elicit information from witnesses (Binet, 1900, 1905; Harris, 1973; Loftus
& Zanni, 1975; Stern, 1939), Loftus’ work has proved instrumental in prompting us to think
about whether the unwitting introduction of misinformation may actually result in the distortion
of memory rather than simply influencing its report. Loftus et al (1978) identified that one of the
problems with the leading question paradigm was a mismatch between the stimuli used during
the study phase and final test. Specifically, while the study phase generally consisted of a visual
presentation, verbal tests were usually employed at final test that may have biased retrieval
towards the verbal misinformation. From a theoretical viewpoint, therefore, the extent to which
the leading question paradigm could reveal anything about the nature of memory was somewhat
limited. By introducing a recognition test as the final phase of the misinformation paradigm,
however, it became possible to determine the availability of the memorial representation for the
original item following the introduction of misinformation. Arguably, it is the attention to
experimental details such as this that has kept the study of the misinformation effect at the
forefront of applied cognitive research for almost thirty years.
Having identified the possibility that the study of misinformation effects could
provide important insights into how memories are subject to destructive updating by newer,
inconsistent material (Loftus, 1979ab; Loftus et al., 1978), Loftus also sought to eliminate
alternative accounts for her findings. In doing so, her research indicated that motivational factors
and demand characteristics appeared to have little impact on the production of such effects.
Monetary incentives failed to improve recall accuracy (Loftus, 1979a) as did the provision of
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opportunities to change initial responses (Loftus, 1979) and warnings to participants that false
information may have been introduced (Greene, Flynn & Loftus, 1982).
In addition to eliminating competing explanations for her findings, Loftus also sought
to gather additional supportive evidence that was consistent with a destructive updating account
of the misinformation effect. In particular, Loftus’ use of reaction time methodology allowed her
to infer whether integration between memory for the original item and memory for the
misinformation is likely to have taken place. The rationale behind this approach was that, if
misled participants could be shown to be slower in choosing an item at test, this would be
consistent with the extra time needed to resolve two conflicting representations. If, on the other
hand, response times could be shown to be fast, it would suggest that any conflict between the
original and the misinformation had already been resolved and that the misinformation is likely
to have been integrated into memory for the target event shortly after the introduction of the
misinformation. The fact that Loftus and colleagues found misled participants to react quickly to
critical items provided support for the thesis that the misinformation effect is a product of
memory change (see Cole & Loftus, 1979; Loftus, Donders, Hoffman & Schooler, 1989).
Loftus also considered the possibility that the introduction of misinformation could
disrupt the original memory trace in such a way that a blended representation of the original item
and misinformation may be produced (Loftus, Schooler & Wagenaar, 1985; Metcalfe, 1990).
This idea derives from earlier work by Loftus (1975; 1977) in which participants who had been
misled about the colour of an object, tended towards a compromise option (i.e., some
combination of the original and misinformation) rather than choosing the original or
misinformation item. Thus, if the original target item had been ‘green’ and the misinformation
suggested the target item had been ‘blue’, participants tended towards a ‘bluish-green’
compromise at test. On further detailed experimentation, however, it became apparent that the
empirical evidence for such a blending mechanism was not particularly strong. Memory blending
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is not an easy phenomenon to study given that there are relatively few instances of items that can
be blended to form a single object. For example, there is no obvious blend for ‘Yield’ and ‘Stop’
signs, or for Coke and 7-Up cans. Even when exploring memory for materials that had the
potential to be blended (e.g., colour), Belli (1988) found that misled participants tended to favour
either the original or the misinformation item at test rather than any compromise option.
As with many lines of experimental enquiry, the paradigms employed by researchers
tend to be influenced by what has gone before; that is, we tend to build upon existing knowledge
and techniques. This approach has the advantage that we don’t have to keep ‘re-inventing the
wheel’ but it can also have the disadvantage that some of the problems associated with earlier
paradigms can be unwittingly transferred to current ones. Chandler (1989) has made the point
that the misinformation paradigm is arguably no different in this regard given that parallels can
be drawn between it and the A-B, A-D paired associate paradigm which was extensively
employed during the classic interference era. According to Chandler’s rationale, the target
incident in the misinformation paradigm can be considered the equivalent of the A stimulus, the
original target item the equivalent of the B response, and the misinformation item the equivalent
of the D response. Chandler and colleagues have argued (e.g., Chandler, 1989, 1993; Chandler &
Gargano, 1995) that misinformation effects can best be viewed as a form of retroactive
interference; that is, the learning of new information during an interpolated task (e.g.,
questionnaire or post-event narrative) can interfere with our ability to recall original target items.
In other words, the standard misinformation paradigm fosters retroactive interference and that
this interference is the cause of the misinformation effect.
In a series of carefully constructed studies, Chandler demonstrated that the retrieval
availability of the original item increases as retroactive interference dissipates, thereby
suggesting that memory for the original item has neither been erased nor altered. Consistent with
this interference interpretation, Chandler and Gargano (1995, 1998) have also demonstrated that
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misinformation dissimilar to the original target item creates less interference and a consequent
reduction in the level of misinformation reported at test. As a way of minimising such
interference effects, Chandler et al (2001) emphasised the importance of reinstating retrieval
cues that specifically access information contained in memory for the original event. By
encouraging participants to reinstate contextual cues that are unique to the original event,
memory for the original item can be accessed in preference to memory for the misinformation.
Consistent with this view, Bekerian and Bowers (1983) have argued that the
misinformation effect may be a function of a mismatch between encoding and retrieval cues.
Specifically, thematic cues associated with the temporal order of the original presentation may
provide vital cues for the subsequent retrieval of information about the target incident. They
pointed out that, in the standard misinformation paradigm, the slides in the final test phase are
presented in random order thereby eliminating thematic cues. Bekerian and Bowers
demonstrated that when retrieval conditions at test are sufficiently strong to reinstate the retrieval
conditions present at encoding (i.e., when the original temporal order is reinstated), the original
item tends to be chosen at test. Conversely, when retrieval conditions at test fail to match
conditions at encoding (i.e., random order of presentation at final test), participants are more
likely to rely upon their most recent memory for the target event and consequently choose the
misinformation item. Here then is empirical evidence to suggest that, given the provision of
appropriate retrieval cues, it is possible to access memory for the original item; that is, the
memorial representation for the original item still exists and appears not to have been subject to
destructive updating (see also Eakins, Schreiber & Marshall, 2003; Lindsay & Johnson, 1989b)
Amongst other significant challenges to Loftus’ destructive updating account,
Lindsay and Johnson (1989a) demonstrated that, where participants were encouraged to attend to
information concerning memory source, it was possible to eliminate misinformation effects.
Although this can be interpreted as another co-existence account of the misinformation effect, it
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differs in that it does not propose any difficulty in retrieving the original item. Rather, the source
monitoring explanation suggests that misled participants believe that the misinformation had
actually occurred in the original target event. Such misattributions are likely to occur because the
original and misinformation items both concern the same event. As the two sources for these
items are highly similar, the standard misinformation paradigm arguably fosters ideal conditions
for source misattribution errors to occur. Where similarity of source is reduced (by increasing the
temporal distinctiveness of the material), a concomitant reduction in misinformation effects is
observed (Lindsay, 1990).
Lindsay and Johnson (1989b) also showed that it was possible to produce
misinformation effects under conditions in which the misinformation was presented before the
target event. They argued that the demonstration of this ‘reversed suggestibility effect’ is
difficult to reconcile with a destructive updating account in which original material is updated by
later (i.e., newer) material. Their work also suggested that the two-alternative forced-choice
recognition test employed in the standard misinformation paradigm may actually encourage
participants to base their judgements on feelings of familiarity. For critical slides, misled
participants may choose the misinformation slide because it seems more familiar to them and
mistakenly believe that the item had occurred in the original presentation. Related work by
Zaragoza and colleagues (Zaragoza & Lane, 1994; Zaragoza & Koshmider, 1989), however, has
indicated that misled participants can choose the misinformation item at test despite being aware
that they could not remember seeing it in the original event. Thus, it would seem that participants
can be aware of the source of memories but still favour the misinformation item, thereby
indicating that source confusion cannot always provide an adequate account of the
misinformation effect.
Some researchers have taken the view that the standard misinformation paradigm (cf.
Loftus, et al., 1978) tells us little about memory updating because of the problems of demand
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characteristics (i.e., implicit biases within the paradigm itself). McCloskey and Zaragoza (1985)
demonstrated that where the two-alternative forced choice recognition test comprised a choice
between an original item (e.g., a spanner) and a completely novel item (e.g., a screwdriver)
rather than the standard procedure where the choice would be between an original item (e.g., a
spanner) and a misinformation item (e.g., a hammer), memory performance for misled
participants was little different from that of non-misled controls. The rationale behind
McCloskey and Zaragoza’s modified test is that, if the introduction of misinformation results in
the destructive updating of memory for original material (cf. Loftus, et al., 1978), misled
participants could be expected to choose the original item less often than would participants who
had not been misled. The fact that there was no difference between misled participants and
controls led McCloskey and Zaragoza to surmise that Loftus’ misinformation paradigm provides
no conclusive evidence that the introduction of misinformation modifies memory for original
material. See also Zaragoza, McCloskey & Jamis (1987) and Weinberg, Wadsworth & Baron
(1983) for further discussion of demand characteristics and how they affect the production of
misinformation effects.
There can be little doubt that social factors have the potential to influence report. The
effect of others on social judgements is well-documented in the social literature (e.g., Asch,
1951). Great care, therefore, needs to be taken to minimise the effects of demand characteristics
in misinformation studies. In our view, however, it is unlikely that misinformation effects can be
attributed entirely to demand characteristics. On the one hand, we have a number of compelling
studies that indicate the co-existence of memories for both the original and the misinformation
item. In other words, one’s ability to access original material from memory is largely seen as a
function of employing the most appropriate retrieval cues. On the other hand, we have equally
compelling reaction time data which indicate that some form of integration is likely to have
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taken place shortly after the introduction of the misinformation item (Loftus, Donders, et al.,
1989).
Of course, this body of research may simply suggest that the misinformation effect
can be multiply determined - a possibility that has already been acknowledged by Loftus (e.g.,
Loftus & Hoffman, 1989; Loftus, Korf & Schooler, 1989). In our view, however, it is also likely
that we have revealed only part of the picture. Traditionally, memory has been characterised by
passive processes such as interference and decay – the same processes that have been implicated
in the production of the misinformation effect. We are currently engaged in a programme of
research, however, which indicates that more active forgetting mechanisms may provide a
credible alternative explanation as to why memorial representations for original material appear
lost under certain conditions but available to conscious inspection under others. In the remainder
of this article, therefore, we detail an active forgetting mechanism that we believe may contribute
to the production of misinformation effects and help to explain this complex pattern of findings.
Retrieval-Induced Forgetting
Our current programme of research has led us to consider the
possible role of retrieval-induced forgetting in the production of misinformation effects.
Retrieval-induced forgetting (Anderson, Bjork & Bjork, 1994; Anderson & Spellman, 1995)
refers to a form of forgetting that occurs as a consequence of remembering other related material.
The basic rationale for this phenomenon stems from the observation that the cues we typically
employ during memory retrieval are insufficiently specified. This means that there is a tendency
to access not only the material we wish to remember but also unwanted related material that, in
turn, provides unwelcome competition for recall. The problem for memory, therefore, is how to
deal with unwanted competition from related memories during retrieval. Anderson and
colleagues have suggested that memory accomplishes this feat via a form of suppression or
active inhibition that renders unwanted competitors unavailable to conscious inspection. In much
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the same way that inhibition is thought to be responsible for resolving competition at other levels
of information processing, inhibition may be responsible for resolving competition during
memory retrieval (see Anderson & Bjork, 1994; Anderson & Neely, 1996).
Retrieval-induced forgetting is typically demonstrated using the retrieval practice
paradigm (Anderson et al., 1994) which comprises four phases: study, retrieval practice,
distractor, and final test. In the study phase, participants are presented with a series of categoryexemplar pairs and instructed to memorize the items (e.g., fish-bream, fish-trout, fish-flounder,
fish-salmon, flower-tulip, flower-anemone, flower-rose, flower-daffodil). Participants are then
instructed to perform retrieval practice on half the studied exemplars from half the categories
using cued stem tests (e.g., fish-br____, fish-tr____). Each of these cued exemplars is typically
presented three times. Following a distractor task, participants are then cued with each category
name (e.g., fish, flower) and asked to recall all the exemplars originally presented. In these kinds
of retrieval practice studies, recall performance for three types of exemplars is assessed: Rp+
items (i.e., practiced exemplars from practiced categories, bream); Rp- items (i.e., unpracticed
items from practiced categories, salmon); and Nrp items (i.e., unpracticed items from unpracticed
categories, tulip). The basic idea here is that if remembering some items from a category inhibits
memory for other related material, then we should see poorer recall performance for unpracticed
items from the same categories as practiced items than for unpracticed items from previously
unpracticed categories. In other words, despite Rp- items and Nrp items being treated in the same
way (i.e., both are unpracticed), Rp- items should be recalled more poorly than Nrp items by
virtue of sharing the same retrieval cue as practiced items. See Figure 1 for an example of
retrieval-induced forgetting (Macrae & MacLeod, 1999, Study 1). Importantly, the mere
presentation of items without retrieval practice is insufficient for retrieval-induced forgetting to
emerge (Anderson, Bjork & Bjork, 2000). Rather, the act of retrieval is a necessary prerequisite
to set up the inhibition of related unpracticed items.
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Figure 1 about here
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Applicability to the real world On reviewing the retrieval-induced forgetting literature,
however, it would be easy to gain the impression that these effects might be limited to the rather
esoteric world of learning category-exemplar pairs. With this in mind, Macrae and MacLeod
(1999) set out to explore whether retrieval-induced forgetting also applied to the social world.
They argued that it is one thing to forget previously encountered items of fruit or drink but that it
may be an entirely different matter to inhibit the retrieval of information encountered in
meaningful social contexts. Also, given that participants in retrieval-induced forgetting studies
are typically instructed to remember target material prior to retrieval practice, the encoding
conditions could be critical for inhibition to occur; that is, the initial instruction to remember the
presented material could prove central for inhibition to take place. This point is of particular
relevance to our understanding of eyewitnessing given that the inherent ambiguity of many
criminal episodes means that we are seldom aware that what has been witnessed is of any
importance until after the event.
Using a set of traits describing two hypothetical individuals (Bill and John), Macrae
and MacLeod showed that selective retrieval practice on traits about one of the individuals (i.e,
the target) resulted in poorer recall performance for unpracticed traits about that target person
relative to unpracticed traits about the non-target person, despite the fact that no explicit
instructions had been given to remember the presented material. They also showed that retrievalinduced forgetting can be elicited even when participants are motivated to remember all the
presented material (e.g., under mock examination conditions). Macrae and MacLeod presented
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participants with facts about two fictitious islands, Tok and Bilu (e.g., the main cash crop on Bilu
is maize). Despite the fact that the participants were all university students and well-accustomed
to studying and sitting examinations, participants still showed the typical retrieval-induced
forgetting effect. Thus, it would appear that even when people are well-motivated – as honest
eyewitnesses are likely to be – retrieval-induced forgetting still emerges. Finally, Macrae and
MacLeod demonstrated that the amount of retrieval practice had little effect on the magnitude of
the retrieval-induced forgetting effect. From an applied point of view, this is a particularly
important point given that its relevance to understanding forgetting in the real world would have
been severely limited if large amounts of retrieval practice had been deemed necessary for the
effect to emerge.
Given the apparent robustness and ease with which retrieval-induced forgetting can
be produced, it is easy to appreciate its possible impact on everyday life. Arguably, nowhere is
this more apparent than in the field of eyewitnessing where naturally occurring conditions (e.g.,
repeated questioning by police, lawyers, family members and colleagues about a witnessed
event) may give rise to exactly those conditions that promote the inhibition of related memories.
In order to explore this possibility, Shaw, Bjork and Handal (1995) examined memory for
visually presented stimuli under conditions analogous to those experienced by eyewitnesses.
Specifically, participants were told to imagine that they had attended a party and that, on leaving
the party, they noticed that their wallet was missing. Participants were instructed to watch a
series of slides of a student’s flat and to pay close attention to the details contained therein in
order to assist police with their enquiries. The slides contained a number of household items plus
two categories of target items (i.e., college sweatshirts and college schoolbooks). The retrieval
practice phase of the study comprised questions about the target items which, in turn, produced
evidence of retrieval-induced forgetting. In other words, guided retrieval practice on one of the
classes of items (e.g., college sweatshirts) resulted in a significant decrease in recall performance
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for unpracticed items from the same category relative to recall performance for unpracticed items
from the unpracticed category (e.g., college schoolbooks).
The relevance of this form of active forgetting to the real world was further explored
in a recent set of studies by MacLeod (2002). In the first study, participants were asked to
imagine that they were police officers attending the scene of a crime and to pay close attention to
a series of slides depicting a number of items ostensibly stolen from two houses (House A and
House B). Guided retrieval practice resulted in retrieval-induced forgetting. Thus, it would
appear that for retrieval-induced forgetting to occur, well-established semantic links are not
required as in the case of exemplars of fruit, plants, or animals. Rather, it would appear that
retrieval-induced forgetting can occur for recently learned semantic information. This study also
provided clear evidence that the observed effects could not be accounted for by output
interference; that is, the tendency for items recalled first to interfere with the recall of subsequent
items (see also MacLeod & Macrae, 2001; Macrae & MacLeod, 1999).
The second study by MacLeod (2002) explored the relevance of retrieval-induced
forgetting to eyewitnessing by considering the effects of guided retrieval practice on memory for
details about a fraudulent event (i.e., a bogus charity collection). In this study, however,
participants were neither provided with information that drew attention to particular items for
encoding nor any explicit category information with which to organise the witnessed material. In
doing so, this study set out to mimic the kind of visual information typically available to real-life
witnesses. If inhibitory mechanisms in memory have importance beyond the confines of the
laboratory, then it should be possible to demonstrate retrieval-induced forgetting for information
that can be mapped on to either pre-existing schemata or where the method of organising the
material is implicit within the witnessed episode.
Pilot work demonstrated that participants who viewed the bogus charity incident
organised their recollection via the two women portrayed in the sequence of slides (i.e., a blonde
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and a brunette). This pilot work also offered the opportunity to identify the most common
descriptive items recalled about each woman. These, in turn, formed the subject of the guided
retrieval practice procedure. Despite the absence of any explicit organising principal around
which the material could be organised, participants clearly organised material around the most
salient perceptual dimensions (i.e., the two women). Guided retrieval practice again resulted in
retrieval-induced forgetting effects. Thus, this study indicates that retrieval-induced forgetting is
possible for recently learned semantic information even in the absence of explicit organising or
grouping information.
It would appear from these studies that, at the very least, there is a need to be as
comprehensive as possible in the questioning of witnesses because of the risk that selective
retrieval can result in the forgetting of other items that may ultimately prove important to the
successful investigation or prosecution of a case (see Shaw et al., 1995). MacLeod has also
raised the possibility that the kind of forgetting produced by guided retrieval practice may also
be implicated in the production of misinformation effects.
Retrieval-induced forgetting and the misinformation effect
In the standard misinformation
paradigm (cf. Loftus et al., 1978), participants are presented with a post-event questionnaire that
probes memory for the previously witnessed target event. Inevitably, however, such questions do
not constitute an exhaustive retrieval of all information known about that target event but rather
focuses on a subset of items. The selective retrieval of information in the standard
misinformation paradigm, therefore, may constitute exactly those conditions likely to promote
retrieval-induced forgetting for items that were not the subject of the initial enquiry.
If we consider this possibility further it is possible that, should misinformation be
inadvertently introduced about an inhibited item, subsequent tests of memory for that item might
result in a preference for the post-event misinformation given that memory for the original
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material may no longer be available for retrieval. Thus, if we return to our earlier example from
Loftus et al (1978): ‘Did another car pass the red Datsun while it was stopped at the stop sign?’,
the request for information concerns a second possible car. In addressing this question, there is
no requirement to retrieve information about the type of sign at which the Datsun was stopped.
Assuming that no other question focuses on memory for the type of sign, memory for a ‘Yield’
sign may subsequently become inhibited to the extent that only post-event information (i.e., that
it was a ‘Stop’ sign) remains available for retrieval.
In the real world, insufficient time or resources may mean that extensive police
interviews may not be carried out. Incomplete retrieval of the material known about a witnessed
event may, in turn, produce retrieval-induced forgetting for those items of information that had
not been the subject of retrieval. Our thinking goes one stage further; that is, the incomplete
retrieval of information about an event may also produce exactly those conditions that promote
the incorporation of misinformation into memory by honest eyewitnesses. In other words, where
misinformation has inadvertently been introduced about an item that has been the subject of
retrieval-induced forgetting, only the misinformation may remain available to conscious
inspection.
In order to explore this possibility, we embarked on a set of studies that examined the
relationship between retrieval-induced forgetting and the production of misinformation effects
(see Saunders & MacLeod, 2002). Using a variant of the basic retrieval practice paradigm, we set
out to investigate whether misinformation effects would be apparent where inconsistent postevent information had been introduced about Rp- items (i.e., items that had been inhibited as a
result of selected retrieval practice of other related items). If our reasoning is correct, we could
also expect that where misinformation is introduced about non-inhibited items (i.e., Rp+ or Nrp
items) significant misinformation effects would fail to emerge.
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Study 1
Study Phase: Participants were presented with items ostensibly stolen from two houses (i.e.,
Jones’ and the Smith’s). Information was presented in the form of two narratives about two
separate burglaries. Order of presentation of the two narratives was counterbalanced throughout.
Information about the burglaries was contained within a booklet that also contained a number of
distractor tasks in addition to the retrieval practice questions. Participants were prompted through
each stage of the booklet by the experimenter. The first part of each narrative contained scenesetting information about when and where the burglary had occurred. Ten items were described
as having been stolen from the Smith’s house and ten items from the Jones’ house. Each item
was embedded within a set of sentences describing where the stolen item had originally been in
located within the house. (e.g., ‘The television had been in the sitting room, which is at the front
of the house. It was sitting in the corner of the room. The remote control for it hadn’t been
taken.’) Earlier pilot work had established that the items chosen for each household were
considered believable as potential stolen items in a burglary.
On completing the first narrative, participants were instructed to read the second
narrative. The stolen items were presented in blocked format (i.e., all items about the Jones’
house followed by the Smith’s house or vice versa) and their presentation fully randomized
within each block. The information sets for each house were divided into two subgroups (each
containing five items) for the purpose of creating a practiced (i.e., Rp+) and an unpracticed (i.e.,
Rp-) set of items for each theft.
Retrieval Practice: Immediately after the study phase, participants were presented with a series
of questions about one of the subsets of stolen items from one of the houses, thereby creating
Rp+, Rp-, and Nrp items sets. This series of questions comprised three sets of questions about
18
five of the stolen items (i.e., 15 questions in total). In order to maximise the effects of retrieval
practice, we adopted the procedure suggested by Landauer and Bjork (1978) whereby, on each
successive set of retrieval practice questions, the inherent difficulty of the question sets
increased. Question difficulty had been determined in earlier pilot work. Counterbalancing
ensured that each item appeared equally often as Rp+, Rp- and Nrp items. Participants were
required to write down all the stolen items they could remember from those presented originally.
This served as a manipulation check that retrieval-induced forgetting had occurred in all
experimental conditions.
We also included a control condition in which participants received retrieval practice
for the names of capital cities (e.g., ‘The capital city of Cuba is Ha____’ ). In doing so, we
ensured that participants in the control were engaged in the same type of task (i.e., retrieval) but
that it was ‘non-relevant’ in the sense that the questions concerned material unrelated to the
items presented in the study phase. Each of the three retrieval practice phases was followed by a
distractor task. On completion of the retrieval practice phase, participants were presented with a
piece of misinformation about an Rp+, Rp-, or Nrp item. Participants in the control condition
also received a piece of misinformation about one of the thefts, thereby providing a baseline
level of misinformation reported in the absence of relevant retrieval practice. Only one
misinformation was incorporated into each set of 12 questions so as not to arouse suspicions
about the true nature of the study. Critical items that were the subject of misinformation were
counterbalanced throughout for each condition.
Final Test The final task comprised a series of multiple-choice questions that probed memory
for the stolen items. For non-critical items (i.e., where no misinformation had been presented),
participants had to choose between the originally presented items and two new erroneous items.
For the critical item, however, participants were required to choose between the original, the
19
misinformation and a new item. On completion, participants were debriefed, thanked for their
participation, and dismissed. (See Saunders & MacLeod, 2002 for further details of the materials
and procedures employed).
Results Retrieval practice rates were 90.6%, 88.8% and 87.7% for the MisRp+ (i.e., where
misinformation had been introduced about a practiced item), MisRp- (i.e., where misinformation
had been introduced about a non-practiced item from the practiced set), and MisNrp conditions
(i.e., where misinformation had been introduced about a non-practiced item from the unpracticed
set), respectively. In order to establish an adequate test of our hypothesis that retrieval-induced
forgetting facilitates the production of misinformation effects, we first confirmed that retrievalinduced forgetting had occurred in all the experimental conditions (see Figure 2). Indeed, across
all treatment conditions, mean recall performance for unpracticed items from the unpracticed set
was .43 whereas for unpracticed items from the practiced set mean recall performance was only
.26. The magnitude of the difference between Rp- and Nrp items (i.e., the retrieval-induced
forgetting effect) was also found to be highly comparable to other studies of retrieval-induced
forgetting (Anderson et al., 1994; Anderson & Spellman, 1995; MacLeod, 2002; MacLeod &
Macrae, 2001; Macrae & MacLeod, 1999).
__________________________________________
Figure 2 about here
__________________________________________
Additionally, we confirmed that Nrp recall performance in each of the treatment
conditions (overall M = 0.43) was significantly lower than that achieved in the control condition
20
(M = 0.56). Thus, we can be confident that the observed differences between Rp- and Nrp items
in the treatment conditions was not due to an inflation of the Nrp baseline performance but rather
to a real drop in Rp- recall performance.
Having demonstrated the presence of retrieval-induced forgetting in all three
treatment conditions, we then turned to consider the critical issue of whether the misinformation
effect is dependent upon retrieval-induced forgetting. The principal comparison of interest here
is between the level of misinformation reported in the MisRp- condition where misinformation
was introduced about an item that was subject to retrieval-induced forgetting versus the levels of
misinformation reported in the MisRp+ and MisNrp conditions where misinformation was
introduced about items that had not been subject to retrieval-induced forgetting. We found that in
the MisRp- condition, 60% of participants chose the misinformation in comparison to 16% in the
MisRp+ and 20% in the MisNrp conditions (see Figure 3). Chi-square analysis confirmed that
there was no significant difference in the proportion of participants reporting misinformation in
either the MisRp+ or MisNrp conditions compared with controls where no relevant retrieval
practice had taken place.
_________________________________________
Figure 3 about here
_________________________________________
We also noted that there was no significant difference in the final test performance
across conditions for non-critical items (85%, 89%, 86% and 91% correct for the MisRp+,
MisRp-, MisNrp, and control conditions, respectively). Thus, it would seem unlikely that the
misinformation effect produced in the MisRp- conditions was due to poorer overall recall
performance at time of test.
21
In summary, this study provides us with a clear illustration that the retrieval practice
of a subset of items can create conditions ideal for the promotion of misinformation effects. It
would appear that when misinformation is introduced about items that have been subject to
retrieval-induced forgetting, participants are significantly more likely to choose the misleading
information during final test. In contrast, where misinformation is introduced about an item that
has not been subject to retrieval-induced forgetting, the level of misinformation reported is no
different from controls where no relevant retrieval practice had occurred.
Study 2
In order to test our hypothesis further, we posed an even more stringent test. As we
and others have argued that retrieval-induced forgetting is the result of an active inhibitory
mechanism (Anderson & Spellman 1995; Anderson & Green, 2001; Saunders & MacLeod, in
press), it could be expected that misinformation effects would emerge only where that inhibition
remains active. MacLeod and Macrae (2001) had previously demonstrated that, at least for
certain kinds of information, the effects of retrieval-induced forgetting are transient. Specifically,
they showed that where a 24-hour delay had been inserted between retrieval practice and final
recall, the typical pattern of retrieval-induced forgetting disappeared. In contrast, the insertion of
a 24-hour delay between original presentation and retrieval practice produced the typical
retrieval-induced forgetting effect. MacLeod and Macrae argued that the transient nature of
retrieval-induced forgetting reflects the need for flexibility in the selection of the material
retrieved from memory in order to meet current or anticipated information processing goals.
From one day to the next, we have little way of knowing what tasks we will be required to
perform and what information from memory will ultimately prove critical for the completion of
these tasks. Thus, MacLeod and Macrae have argued that it would not be particularly adaptive to
22
render some forms of information permanently inaccessible given that this information may be
just what is required in order to complete a future task.
We used the transient nature of retrieval-induced forgetting to test our hypothesis
further that misinformation effects are dependent upon retrieval-induced forgetting. Specifically,
we predicted that where misinformation was introduced about a critical item that was still subject
to retrieval-induced forgetting, the typical misinformation effect would emerge. In contrast, the
introduction of misinformation about an item that had been subject to retrieval-induced
forgetting but which was no longer inhibited, should fail to produce a misinformation effect.
In order to test our hypothesis, we used the same materials as those employed in our
first study. In this particular study, however, misinformation was introduced about Rp- items
only (i.e., those items that were subject to retrieval-induced forgetting). Misinformation was
introduced either following a free recall procedure in which participants experienced a 24-hour
delay between study and retrieval practice or following a 24-hour delay between retrieval
practice and free recall. Manipulation checks were consistent with MacLeod and Macrae’s
original findings. Importantly, we found that misinformation effects emerged only where
retrieval-induced forgetting remained active; that is, where no delay had been inserted between
any stages of the retrieval practice paradigm, or where a 24-hr delay was inserted between study
and retrieval practice. No significant misinformation effects emerged where a 24-hour delay had
been inserted between retrieval practice and recall (see Figure 4).
______________________________________________
Figure 4 about here
______________________________________________
23
Discussion Our programme of research was designed to explore the extent to which retrievalinduced forgetting could promote the production of misinformation effects. In the present article
we have detailed two of our initial studies in this field (Saunders & MacLeod, 2002). Both of
these studies provide strong inference that retrieval-induced forgetting is a potent mechanism in
facilitating misinformation effects.
It is unlikely that the misinformation effects observed in our study can be attributed to
demand characteristics (cf. McCloskey & Zaragoza, 1985) as it could have been expected that
misinformation effects would have emerged across all conditions given that any demand
characteristics would have been equivalent. Instead, we found in Study 1 that misinformation
effects emerged only where misinformation had been introduced about items that had been
subject to retrieval-induced forgetting. Similarly, it would be difficult to argue that the
misinformation effect observed in the MisRp- condition (Study 1) was due to a failure to encode
the critical item given the high level of recall performance for non-critical items across all
conditions, or that some form of spontaneous forgetting might have occurred given that new
erroneous information tended not to be chosen at test.
Our interpretation that misinformation effects are facilitated by retrieval-induced
forgetting is further supported by our second study in which we demonstrated that
misinformation effects emerged only where retrieval-induced forgetting remained active. Also,
the observed pattern of retrieval-induced forgetting observed in this study supports MacLeod and
Macrae’s (2001) earlier findings that the effects of retrieval-induced forgetting are, at least under
certain conditions, transient. One interpretation for this transient aspect of inhibitory processing
is that memory needs to be flexible in order to meet the demands of a complex and constantly
changing social world. For many kinds of information, it would make little sense to inhibit
information on a more permanent basis as unknown future processing goals may require the
processing of such information. This feature of inhibitory processing, however, also permitted us
24
to explore further the role of retrieval-induced forgetting in the production of misinformation
effects. From our studies it is clear that misinformation effects were produced only where there
was evidence that retrieval-induced forgetting remained active. Where it had dissipated, in
contrast, misinformation effects failed to emerge.
Inevitably, however, our findings raise further questions and issues of theoretical and
applied interest. The most obvious of these concerns the nature of the mechanism underlying
retrieval-induced forgetting. As we intimated earlier, retrieval-induced forgetting can be caused
by both non-inhibitory and inhibitory means. The present studies illustrate a strong relationship
between retrieval-induced forgetting and the production of misinformation effects but do not
provide conclusive evidence that the underlying mechanism is inhibitory. While there is good
reason to think that the kind of retrieval-induced forgetting found using the retrieval practice
paradigm is inhibitory and that output interference and source confusion were not significant
contributors to the observed effects (see Saunders & MacLeod, 2002), there remains a need to
provide more compelling data that the misinformation effects produced using this modified
misinformation paradigm are a function of inhibitory control.
In order to establish if this is the case, we must first determine the nature of the
processes underlying the retrieval-induced forgetting effect found with our materials. To date,
the best means of establishing whether retrieval-induced forgetting is caused by inhibition or not
is via the independent cue technique (cf. Anderson and Spellman, 1995). This procedure involves
the use of novel retrieval cues at final test rather than employing those retrieval cues used during
initial study and retrieval practice. Anderson and Spellman’s rationale is that if the drop in recall
performance for Rp- items is a consequence of non-inhibitory mechanisms (e.g., blocking) then
it could be expected that the use of novel cues at final test would permit the Rp- items to be
accessed (i.e., they should become retrievable). In other words, the typical retrieval-induced
forgetting should fail to emerge if non-inhibitory mechanisms are responsible for its production.
25
Anderson and Spellman and others (Anderson & Green, 2001; Saunders & MacLeod, in press),
have demonstrated that Rp- items fail to emerge at final test despite the use of such novel
retrieval cues, thereby providing strong inference that the Rp- items themselves had been
inhibited. See Figure 5 for an example of independent cueing.
_______________________________________________
Figure 5 about here
_______________________________________________
Of course, if the active inhibition of the memorial trace is a necessary prerequisite for
the production of misinformation effects, then the Rp- item is not the only item that may
potentially be vulnerable to misleading post-event information. Anderson and Spellman (1995)
previously demonstrated that items from an unrelated and unpracticed category can be inhibited
due to their relationship with items from the practiced category. Specifically, items from the
unpracticed category that share a retrieval cue with Rp- items (i.e., Nrp-similar items) are
themselves vulnerable to inhibition. While these Nrp-similar items may not directly compete
with Rp+ items for retrieval they are, nevertheless, related to items that do compete (i.e., Rpitems) and, thus, Nrp-similar items may suffer the same fate as that of Rp- items. Conversely,
items from the unpracticed category that do not share a retrieval cue with any of the exemplars
from the practiced category (i.e., Nrp-dissimilar items) are not subject to inhibitory control,
presumably because such items neither compete for retrieval with Rp+ items, nor are they related
to items inhibited due to retrieval competition (i.e., Rp- items).
Extending this line of research, Saunders and MacLeod (in press) have recently
determined (using the independent cue technique, cf. Anderson & Spellman, 1995) that items
26
from the unpracticed category that share retrieval cues with items from the practiced category are
susceptible to inhibitory effects. Consistent with Anderson and Spellman, we found that Nrp
items that share retrieval cues with Rp- items were vulnerable to inhibitory effects but,
additionally, we found that Nrp items that share retrieval cues with Rp+ items were also
inhibited. Recall performance for items from the unpracticed category that were semantically
dissimilar to items from the practiced category were not subject to inhibitory control and were
recalled at a level similar to that observed for the between-subjects baseline control.
Having determined which items are most likely to be inhibited, our next step is to
evaluate the inhibitory account of misinformation effects by adapting our misinformation
paradigm for use with the independent cue method. To address this goal, we need to add the
necessary misinformation and recognition phases to the retrieval practice paradigm that we
employed in Saunders and MacLeod (in press). If we are correct in our assertion that the
inhibition of the original item facilitates the introduction of post-event misleading information,
then all the items we determine to be inhibited should also be susceptible to misleading postevent information. Specifically, misinformation effects should occur in those conditions where
misleading information is introduced about Rp- items, and also Nrp items that are semantically
related to either Rp+ or Rp- items. Conversely, misinformation effects should not arise in
conditions where misleading information is introduced about non-inhibited items (i.e., Nrpdissimilar items).
While our research provides a novel theoretical account of how misinformation
effects are produced it also has the potential to inform us about possible solutions to practical
memory problems. Specifically, experimental research that examines the influence of inhibitory
retrieval processes, and their role in misinformation effects, may help to explain some of the
memory distortions that typify honest eyewitness errors memory and even false memory
syndrome, as well as providing insight into the design of more effective interview techniques. As
27
argued elsewhere (e.g., MacLeod, 2002; Saunders & MacLeod, 2002), any interview technique
that requires the selective retrieval of information may prove vulnerable to inhibitory control,
and the production of misinformation effects. A practical concern, therefore, is that the method
employed to solicit information from witness memory may also prove to be the very procedure
that leaves witnesses vulnerable to forgetting previously unsolicited details and the subsequent
report of misleading post-event information.
Our concern with the combined effects of inhibition and misleading post-event
information seems particularly timely given the proliferation of alternative investigative
methods. Over the past few decades, interview methods such as the cognitive interview, guided
memory interview and structured interview, have been championed by basic and applied
researchers alike for their ability to enhance retrieval performance in comparison to the standard
police interview. While many studies suggest that the specific techniques employed in such
interviews can significantly increase the recall of details about a target event (e.g., Fisher,
Geiselman & Raymond, 1987; Koehnken, Thurer & Zorberbier, 1994), and the recognition
accuracy of suspects (e.g., Malpass & Devine, 1981), all these techniques tend to result in only a
sub-set of details about the witnessed event being solicited. Additionally, if questioning results in
the unintentional inhibition of memory traces that have not been subject to retrieval practice,
then such interview methods may be particularly ill-equipped for dealing with kind of problem
outlined in this article. For example, the retrieval methods employed by the cognitive interview
assume that the inability of a witness to remember certain details is due largely to cue-dependent
forgetting. Specifically, it is assumed that the failure to retrieve specific information is due to the
use of inappropriate retrieval cues or interference having occurred along the retrieval route
between the cue and the memory trace. Such cue-dependent forgetting can be overcome simply
through employing more appropriate retrieval cues that utilise retrieval routes free from such
interference. Unfortunately, if details of an event have been forgotten due to inhibition of the
28
memory trace then such retrieval methods will not prove effective. Thus, irrespective of the
quality or the quantity of the retrieval cues and retrieval routes provided, inhibited memories
may remain stubbornly unavailable to retrieval. In sum, alternative questioning methods to the
standard police interview may be just as susceptible to inhibitory control and, in turn, the
unwitting introduction of misinformation. Only through further controlled experimentation are
we likely to fully understand the complex relationship between forgetting and remembering and,
in doing so, determine the means by which misinformation effects may be minimised in the real
world.
29
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35
0.8
0.7
0.6
0.5
Proportion
0.4
recall
0.3
0.2
0.1
0
Rp+
Rp-
Nrp
Item type
Figure 1. Guided retrieval practice and retrieval-induced forgetting.
The presence of retrieval-induced forgetting is measured by comparing recall
performance for Rp- items with Nrp items (i.e., (Rp-) – Nrp). Rp+ = practiced items
from practiced category. Rp- = unpracticed items from practiced category. Nrp =
unpracticed items from unpracticed category. From Macrae and MacLeod (1999,
Study 1).
36
0.9
0.8
0.7
0.6
Proportion 0.5
recall
0.4
Rp+
0.3
Rp-
0.2
Nrp
0.1
0
MisRp+
MisRp-
MisNrp
MisControl
Condition
Figure 2. Proportion recall of each item type across misinformation conditions.
Significant retrieval-induced forgetting effects occurred in each condition where
participants engaged in relevant retrieval practice. Rp+ = practiced items from
practiced category. Rp- = unpracticed items from practiced category. Nrp =
unpracticed items from unpracticed category. From Saunders and MacLeod (2002,
Study 1).
37
0.7
0.6
0.5
Proportion of 0.4
misinformation
effects
0.3
0.2
0.1
0
MisRp+
MisRp-
MisNrp
MisControl
Condition
Figure 3. Proportion of misinformation effects reported. Significantly more
misinformation effects were found when misleading post-event information was
introduced on an item that had been subject to retrieval-induced forgetting (i.e.,
MisRp- condition) than where it was presented about items that had not been subject
to retrieval-induced forgetting (i.e., MisRp+, MisNrp, and MisControl conditions).
MisRp+ = misinformation presented about Rp+ item. MisRp- = misinformation
presented about Rp- item. MisNrp = misinformation presented about Nrp item.
MisControl = misinformation presented about Control item. From Saunders and
MacLeod (2002, Study 1).
38
0.9
0.8
0.7
0.6
Rp+
0.5
Rp-
Proportion
recall
0.4
Nrp
0.3
Misinfo
Effect
0.2
0.1
0
Delayed Test
Delayed Practice
No Delay
Condition
Figure 4. Proportion of participants reporting each item type on the free recall and
forced-choice recognition test. When free recall was delayed by 24 hours retrievalinduced forgetting and misinformation effects were absent. When retrieval practice
was delayed by 24 hours, or no delay occurred, both retrieval-induced forgetting and
misinformation effects were detected. From Saunders and MacLeod (2002, Study 2).
39
Retrieval Practice Cue
Independent Cue
FRUIT
RED
fire truck
strawberry
Rp+
Rp-
Figure 5. Cue-independent forgetting.
Inhibitory theories of retrieval-induced forgetting assume that the memory trace for
the Rp- item (e.g., RED- strawberry) is actively inhibited due to its prior competition
with the Rp+ items for retrieval. If the memory trace has been inhibited, the
employment of independent retrieval cues (e.g., FRUIT) will be unsuccessful in
overcoming inhibition.
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