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DOI: 10.1111/j.1467-8721.2008.00542.x 2008 17: 26Current Directions in Psychological Science

Malcolm D. MacLeod and Jo SaundersRetrieval Inhibition and Memory Distortion : Negative Consequences of an Adaptive Process

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Retrieval Inhibition and MemoryDistortionNegative Consequences of an Adaptive ProcessMalcolm D. MacLeod1 and Jo Saunders2

1University of St. Andrews, Fife, Scotland, and 2University of Wales Swansea, Swansea, Wales

ABSTRACTDespite the fact that misinformation effects

have long been studied by both applied researchers and

modelers of human memory, there is little consensus as to

the value of such endeavors. We argue that this may be due

to a failure to identify the underlying mechanism respon-

sible for such memory distortions. We consider novel

evidence for a relationship between retrieval-induced for-

getting and the reporting of misinformation. We also ex-

plore the extent towhich retrieval inhibition underpins this

relationship and the implications this has for the modeling

of memory and nding potential solutions to real-world

problems.

KEYWORDSmisinformation effect; eyewitnessing; retriev-

al-induced forgetting; retrieval inhibition

Little in our social world remains constant for long; relationships

change, interests develop, and people move on. In order to meet

the myriad challenges that accompany such changes, we rely

heavily upon our portable store of socially relevant informa-

tionour memory. For the most part, we encounter few dif-

culties in our ability to retrieve target information, but the

apparent ease with which this is accomplished belies its com-

plexity. Our social world is in a constant state of ux and, as

a result, memory needs to be continually updated; otherwise its

functional utility would soon be drawn into question. But there

lies the problem: How is it possible to access the information we

need when memory is being continually updated with related

material?

On rst inspection, the most obvious solution to this updating

problem might be to replace or substitute redundant material

with information that is current. Indeed, such a mechanism

would probably prove more than adequate if memory consisted

solely of telephone numbers or addresses. However, the material

stored in memory is innitely more rich and varied. In addition,

we have no way of labeling particular memories as being

potentially useful or obsolete; the material deemed redundant

for todays task may ultimately prove critical for the completion

of some future task. Thus, there is a need to retain older, po-

tentially useful material at the same time as storing current or

new information.

RETRIEVAL COMPETITION

Continually adding material to other related memories, however,

creates a tricky problem for effective information retrieval. Our

memories are highly interrelated in terms of their semantic

and/or episodic content. Yet, the cues used to access particular

memories tend not to be well specied. Thus, we access not only

the information we wish to retrieve but also other related ma-

terial that, in turn, represents an unwanted source of competi-

tion. If this went unchecked, it could mean the difference

between retrieval success and failure. So how do we deal with

unwanted retrieval competition?

A possible solution has emerged from studies concerned with

retrieval-induced forgettingthe forgetting of material as a re-

sult of remembering other related material (Anderson, Bjork, &

Bjork, 1994; Anderson & Spellman, 1995). The basic argument

is that, by forgetting related material, it becomes possible to

promote the retrieval of target information via a reduction in

retrieval competition. On rst inspection, the notion that

forgetting promotes remembering seems counterintuitive, but it

makes perfect sense if forgetting is targeted at specic related

memories that might otherwise compete for retrieval.

The mechanism responsible for this kind of forgetting is

considered to be retrieval inhibition (see Anderson & Spellman,

1995; Saunders & MacLeod, 2006), and it has been established

in a variety of contexts including memory for facts (Anderson &

Bell, 2001), exam performance (Macrae & MacLeod, 1999),

person perception (MacLeod & Macrae, 2001), and eyewitness

Address correspondence to Malcolm D. MacLeod, Social and AppliedCognition Lab, School of Psychology, University of St. Andrews, Fife,Scotland KY16 9JP, United Kingdom; e-mail: [email protected].

CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE

26 Volume 17Number 1Copyrightr 2008 Association for Psychological Science at Ruppin Institute on January 26, 2013cdp.sagepub.comDownloaded from

memory (MacLeod, 2002; Shaw, Bjork, & Handal, 1995).

Retrieval inhibition is a form of forgetting whereby unwanted

memories are actively inhibited or suppressed to the extent that

they are no longer available to conscious inspection (Anderson

et al., 1994; Anderson & Spellman, 1995)which is quite

different from traditional conceptions of forgetting such as trace

decay (i.e., the gradual deterioration of the memory trace over

time) or associative interference (i.e., the weakening of asso-

ciative links between particular retrieval cues and memories via

the strengthening of other related associative links).

MEMORY DISTORTION

Most recently, our studies have sought to explore the possibility

that retrieval inhibition may also underlie misinformation ef-

fects. Misinformation effects have traditionally been explored

using the misinformation paradigm (cf. Loftus, Miller, & Burns,

1978). After viewing a target event (e.g., a carpedestrian

accident), subjects are typically presented with a narrative or

questionnaire that includes some misleading information about

the event (e.g., suggesting that the car in question was green

when in fact it had been blue). In this setup, misled subjects

reliably choose the misinformation item in preference to the

original in a subsequent test of memory, whereas nonmisled

subjects tend to choose the original information.

Explanations for this kind of memory distortion range from

the operation of a destructive updating process whereby older

memories are erased by newer material (e.g., Loftus et al., 1978)

to the inadequate monitoring of memory sources at encoding

(e.g., Lindsay & Johnson, 1989). Others have argued that the

absence of effective retrieval cues during the nal test reduces

retrieval accessibility for the original material relative to more

recent misinformation (e.g., Bekerian & Bowers, 1983). Some

have even argued that such studies reveal less about how human

memory works than about biases inherent within the paradigm

(e.g., McCloskey & Zaragoza, 1985).

We considered an alternative: namely, the possibility that the

apparent lack of clarity on this issue may be symptomatic of a

failure to identify the basic cognitive mechanism underlying

the misinformation effect. In doing so, we explored the possi-

bility that retrieval-induced forgetting may be such a mecha-

nism. We also explored the possibility that an increased

susceptibility to suggestion may represent the cost of an other-

wise adaptive inhibitory mechanism that serves to facilitate the

retrieval of target memories.

RETRIEVAL-INDUCED FORGETTING

Retrieval-induced forgetting has been explored via the retrieval-

practice paradigm (cf. Anderson et al., 1994), in which subjects

are typically shown sets of categoryexemplar pairs (e.g., tree

sycamore, treespruce . . . shsalmon, shhaddock . . . sport

tennis, sportfootball . . . fruitbanana, fruitpear . . .). After

initial presentation, subjects are given selective retrieval

practice (usually three repetitions) on a subset of items from a

subset of categories using cued stem tests (e.g., treesy____;

shsa____). Following a distracter task that lasts up to 20

minutes, subjects are asked to recall all the exemplars originally

presented.

Consistent with expectations, a facilitation effect is evident for

practiced items frompracticed sets (designatedRp1 items); that

is, Rp1 items (e.g., sycamore, salmon) are better remembered

than either unpracticed items from the practiced sets (Rpitems; e.g., spruce, haddock) or unpracticed items from the

unpracticed sets (Nrp items; e.g., sport and fruit exemplars).

What is much less intuitive, however, is that Rp items tendto be more poorly recalled than Nrp items (see Fig. 1). Thus,

despite the fact that neither Rp items nor Nrp items receiveany retrieval practice, Rp items are more poorly recalled thanNrp items are.

This pattern of recall performance is consistent with an in-

hibitory account whereby Rp items are actively suppressed orinhibited in order to minimise unwanted competition during the

repeated retrieval of Rp1 items. Nrp items, in contrast, tend to

be better remembered than are Rp items because they do notrepresent a source of retrieval competition and therefore there is

no need for inhibitory control (see Anderson et al., 1994).

RETRIEVAL-INDUCED FORGETTING AND THE

MISINFORMATION EFFECT

The selective-retrieval procedure is critical to the emergence of

retrieval-induced forgetting. Where subsets of exemplars are

0

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0.2

0.3

0.4

0.5

0.6

0.7

0.8

Rp+ Rp NrpItem type

*

* = retrieval-induced forgetting

Prop

ortio

n re

call

Fig. 1. Retrieval-induced forgetting of nonpracticed items from prac-ticed categories. Rp15 practiced items from practiced category; Rp5unpracticed items from practiced category; Nrp5 unpracticed items fromunpracticed category. Despite Rp and Nrp items being treated in thesame manner (i.e., neither item type receives retrieval practice), Rpitems tend to be more poorly recalled than Nrp items. FromMacLeod andMacrae (2001, Study 1).

Volume 17Number 1 27

Malcolm D. MacLeod and Jo Saunders


simply re-presented, forgetting effects for Rp items fail toemerge (Anderson, Bjork, & Bjork, 2000). Given this pattern

of forgetting, our basic premise is that the postevent question-

naire typically used to introduce misleading information in the

misinformation paradigm is also likely to give rise to the same

kinds of retrieval conditions known to elicit retrieval inhibition.

By giving subjects a set of questions about a target event as a

means of introducing misleading material, conditions are ripe

for the selective retrieval of information (Saunders & MacLeod,

2002). The misinformation paradigm therefore may unwittingly

incorporate a selective-retrieval-practice procedure not unlike

that found in the retrieval-practice paradigm. It is important to

note, however, that we are not arguing that the misinformation

effect is an artifact of the paradigmfar from it. Rather, we

are saying that the retrieval conditions created by the postevent

questionnaire are very similar to those that exist in the real

world (i.e., the selective retrieval of information) and that such

conditions, in turn, are likely to elicit retrieval inhibition. Given

this, are misinformation effects more likely to emerge where the

original target items have been subject to retrieval-induced

forgetting?

To explore this possibility, we adapted the standard retrieval-

practice paradigm in order to incorporate key phases from the

misinformation paradigm. Specically, after reading two narra-

tives containing information about a series of stolen household

items, subjects engaged in selected retrieval practice for items

about one of the burglaries. This took the form of questions

concerning half of the stolen items from one of the houses.

Subjects were prompted to retrieve each item three times. A

manipulation check to measure the presence of retrieval-

induced forgetting followed this retrieval-practice procedure:

Subjects were rst asked to recall all the stolen items from both

houses and then were asked additional questions regarding

scene-setting information contained within the original narra-

tives. Included in one of these questions was a piece of misin-

formation about either an Rp1 item (i.e., a practiced item from

the practiced burglary), an Rp item (i.e., a nonpracticed itemfrom the practiced burglary), or an Nrp item (i.e., a nonpracticed

item from the nonpracticed burglary). Finally, memory for both

events, including themisleading item, was tested using a forced-

choice recognition task that required subjects to choose between

the original item and two novel items for noncritical items, and

between the original, a misleading item, or a novel item for the

misinformation item.

Using this paradigm, we found that misinformation effects

occurred only where misleading information had been intro-

duced about items that were subject to retrieval-induced for-

getting; that is, memory distortions occurred only for non-

practiced items that were in the same set as practiced items (i.e.,

Rp items). When misinformation had been introduced abouteither Rp1 or Nrp items, the level of misinformation reported at

the nal test was no different from that for controls who had

received no relevant retrieval practice (see Fig. 2).

If retrieval-induced forgetting plays such a pivotal role in the

production of misinformation effects, these memorial errors

should emerge only where retrieval-induced forgetting remains

demonstrably active. In a test of the boundary conditions asso-

ciated with retrieval-induced forgetting, MacLeod and Macrae

(2001) had established that the timing of the retrieval-practice

procedure is critical. Specically, they found that a delay of

24 hours between retrieval practice and nal test resulted in the

dissipation of retrieval-induced forgetting (i.e., recall perfor-

mance for Rp items had recovered), whereas Rp perfor-mance remained impaired following a 24-hour delay between

original presentation and retrieval practice (i.e., retrieval-

induced forgetting was still evident). Our studies not only con-

rmed MacLeod and Macraes earlier ndings but also showed

that the interval between the selective-retrieval-practice pro-

cedure and the introduction of misleading material was critical

in producing misinformation effects (Saunders & MacLeod,

2002).

IS IT INHIBITION?

Having established a link between retrieval-induced forgetting

and the production of misinformation effects, there remains

the thorny issue of whether the mechanism is truly inhibitory or

not. In order to explore this, we devised a set of studies that

incorporated the independent-cue technique rst devised by

Anderson and Spellman (1995). This technique involves

prompting subjects with novel cues at test, rather than using the

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Prop

ortio

n of

mis

info

rmat

ion

effe

cts

MisRp+ MisNrp MisControl MisRpCondition

Fig. 2. Effects of introducing postevent misleading information followingselective-retrieval practice. When misleading postevent information isintroduced concerning an item subject to retrieval-induced forgetting(MisRpcondition), the majority of subjects (M 5 .60) choose the mis-leading item on a forced-choice recognition test. When misinformation isintroduced about items not subjected to retrieval-induced forgetting (i.e.,practiced items from practiced categories, MisRp1 condition, and un-practiced items from unpracticed categories, MisNrp condition), the levelof misinformation reported is no different from that of controls where norelevant retrieval practice has taken place (i.e., MisControl condition).From Saunders and MacLeod (2002, Study 1).

28 Volume 17Number 1

Retrieval Inhibition and Memory Distortion


cues originally linked to exemplars during the retrieval-practice

phase of the retrieval-practice paradigm.

The logic behind this modication is this: If retrieval-induced

forgetting is due to noninhibitory means (such as associative

interference or blocking, which affect the utility of the retrieval

route and cue), the presentation of novel cues at test should

prove sufcient to circumvent any such interference. Thus, as

Anderson and colleagues have argued (Anderson et al., 1994;

Anderson & Spellman, 1995), if Rp items are simply blockedrather than inhibited, the use of novel cues at test should ensure

that Rp items are as well remembered as are Nrp items. But ifRp items are inhibited, they should remain unavailable toconscious inspection and should be more poorly recalled, de-

spite the use of novel cues at test (see Fig. 3). Recently, Storm,

Bjork, Bjork, and Nestojko (2006), using an impossible re-

trieval practice condition, found that the effort to retrieve Rp1

items even when no actual Rp1 items could be recalled still

produced retrieval-induced forgetting of Rp itemsa resultthat strongly argues against a blocking explanation.

Consistent with Andersons thesis on inhibitory control, we

found that Rp items remained below Nrp baseline perfor-mance despite the use of such novel cues at test (MacLeod &

Saunders, 2005; Saunders &MacLeod, 2006). We also surmised

that, if such effects are due to retrieval inhibition, Nrp items that

are semantically related to either Rp1 or Rp items should alsobe susceptible to the introduction of misleading information. We

conrmed that misinformation effects emerged where misinfor-

mation had been introduced about items that had been subject to

rst-order inhibition (i.e., Rp items), cross-category inhibition(i.e., Nrp items related to Rp1 items), and even second-order

inhibition (i.e., Nrp items related to Rp items). In contrast, Nrpitems that were not semantically related to items from the

practiced category (i.e., unrelated to either Rp1 or Rp items)were susceptible neither to inhibition nor to misinformation.

CONCLUSIONS

The current studies suggest that some memory distortions may

be a function of a normally adaptive inhibitory mechanism that

is activated in response to unwanted competition from related

memories. This kind of retrieval-induced forgetting creates

conditions wherein misleading postevent information can un-

wittingly be reported instead of original material. By exploring

the inhibitory mechanisms underlying this relationship, it be-

comes possible to predict when such memory distortions are

likely to occur.

Our research points to two potentially important principles in

interviewing witnesses. The rst is that the initial search for

information about a witnessed event should be as exhaustive as

possibleotherwise conditions that lead to the selective re-

trieval of information may be created, which, in turn, give rise to

retrieval inhibition for items that were not probed. The second is

that sufcient time should elapse between initial information

retrieval and any nal statement so that, should retrieval-in-

duced forgetting have occurred during the selective retrieval of

information, its effects will have had time to dissipate. In this

way, it may be possible to minimize the risk of witness suscep-

tibility to postevent suggestion.

Finally, although our research is consistent with an inhibitory

account of retrieval-induced forgetting, we are aware that there

is still much to be done to establish how such inhibitory mech-

anisms actually work. In particular, we need to understand the

levels at which inhibitory control operates (see Saunders &

MacLeod, 2006). For example, does inhibition operate at the

level of the retrieval cue, the retrieval route (i.e., the link be-

tween the retrieval cue and memory), or the memorial repre-

sentation itself? We also need to explore more thoroughly the

effects of repeated inhibition over longer periods of time and the

extent to which resultant memory distortions become permanent

features of memory. Through an improved understanding of such

mechanisms, wemay nd ourselves better placed not only to nd

effective solutions to real-life problems but also to gain impor-

tant insights into fundamental issues concerning memory

structure and updating.

Recommended ReadingAnderson, M.C. (2003). Rethinking interference theory: Executive

control and the mechanisms of forgetting. Journal of Memory andLanguage, 49, 415445. A clearly written, user-friendly, andcomprehensive review for readers who wish to expand their

knowledge on inhibitory processes in memory.

FRUIT

baseball orange

ROUND

Retrieval practice cue Novel test cue

Rp+ Rp

Fig. 3. Inhibitory account of retrieval-induced forgetting. Noninhibitoryaccounts of forgetting assume that the association between the retrievalpractice cue (ROUND) and the practiced (Rp1) item (baseball) isstrengthened (bold arrow) through retrieval practice while the associationbetween the retrieval practice cue and the non-practiced (Rp) item(orange) is weakened (dashed arrow). If retrieval-induced forgetting isdue tononinhibitory factors, the presentation of novel cues at test (FRUIT)should circumvent any interference between the retrieval practice cue andthe Rp item. If, however, retrieval-induced forgetting is due to inhibi-tion, then Rp items should still remain unavailable to conscious inspec-tion despite the use of novel cues at test (i.e., designated by dashed lines).

Volume 17Number 1 29

Malcolm D. MacLeod and Jo Saunders


Bjork, E.L., Bjork, R.A., & MacLeod, M.D. (2006). Types and conse-

quences of forgetting: Intended and unintended. In L-G. Nilsson&

N. Ohta (Eds.), Memory and society: Psychological perspectives(pp.134158). New York: Psychology Press. Discusses the con-

sequences of forgetting in more detail than the current article.

MacLeod, M.D., & Saunders, J. (2005). (See References). A represen-

tative study that illustrates original research about the role of in-

hibition in misinformation effects.

REFERENCES

Anderson, M.C., & Bell, T. (2001). Forgetting our facts: The role of

inhibitory processes in the loss of propositional knowledge.

Journal of Experimental Psychology: General, 130, 544570.

Anderson, M.C., Bjork, E.L., & Bjork, R.A. (2000). Retrieval-induced

forgetting: Evidence for a recall-specicmechanism.PsychonomicBulletin & Review, 7, 522530.

Anderson, M.C., Bjork, R.A., & Bjork, E.L. (1994). Remembering can

cause forgetting: Retrieval dynamics in long-term memory. Jour-nal of Experimental Psychology: Learning, Memory & Cognition,20, 10631087.

Anderson, M.C., & Spellman, B.A. (1995). On the status of inhibitory

mechanisms in cognition: Memory retrieval as a model case.

Psychological Review, 102, 68100.

Bekerian, D.A., & Bowers, J.M. (1983). Eyewitness testimony: Were we

misled? Journal of Experimental Psychology: Learning, Memory &Cognition, 9, 139145.

Lindsay, D.S., & Johnson, M.K. (1989). The eyewitness suggestibility

effect and memory source. Memory and Cognition, 17, 349358.

Loftus, E.F., Miller, D., & Burns, H. (1978). Semantic integration of

verbal information into a visual memory. Journal of ExperimentalPsychology: Learning, Memory & Cognition, 4, 1931.

MacLeod, M.D. (2002). Retrieval-induced forgetting in eyewitness

memory: Forgetting as a consequence of remembering. AppliedCognitive Psychology, 16, 135149.

MacLeod, M.D., & Macrae, C.N. (2001). Gone but not forgotten: The

transient nature of retrieval-induced forgetting. PsychologicalScience, 12, 148152.

MacLeod, M.D., & Saunders, J. (2005). The role of inhibitory control in

the production of misinformation effects. Journal of ExperimentalPsychology: Learning, Memory & Cognition, 31, 964979.

Macrae, C.N., & MacLeod, M.D. (1999). On recollections lost: When

practice makes imperfect. Journal of Personality and Social Psy-chology, 77, 163473.

McCloskey, M., & Zaragoza, M. (1985). Misleading postevent infor-

mation and memory for events: Arguments and evidence against

memory impairment hypotheses. Journal of Experimental Psy-chology: General, 114, 116.

Saunders, J., & MacLeod, M.D. (2002). New evidence on the suggest-

ibility of memory: The role of retrieval-induced forgetting in

misinformation effects. Journal of Experimental Psychology:Applied, 8, 127142.

Saunders, J., & MacLeod, M.D. (2006). Can inhibition resolve retrieval

competition through the control of spreading activation? Memory& Cognition, 34, 307322.

Shaw, J.S., Bjork, R.A., & Handal, A. (1995). Retrieval-induced for-

getting in an eyewitness paradigm. Psychonomic Bulletin andReview, 2, 249253.

Storm, B.C., Bjork, E.L., Bjork, R.A., & Nestojko, J. (2006). Is retrieval

success necessary for retrieval-induced forgetting? PsychonomicBulletin & Review, 13, 10231027.

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