Eyewitness testimony and memory distortion

Transcription

1 Japanese Psychological Research 1996, Volume 38, No Special Issue: Eyewirness Tesrimony Eyewitness testimony and memory distortion CHARLES G. MANNING and ELIZABETH F. LOFTUS Department of Psychology, University of Washington, Seattle, Washington , USA Abstract: Do memories change as we acquire new information? Recent research on memory distortion using implicit tests along with research using confidence is reviewed and new studies are presented. Two new studies asked misinformed subjects to provide reasons for their answers. In each study 15% to 27% of subjects said they remembered seeing items they had only read about. In another study subjects were asked to identify the source of misleading items they had seen in slides or read in misleading questions. Subjects were more likely to say they had seen in slides something they read about in the questions than they were to confuse information from two nearly identical sets of slides. Recent work shows that, not only is it possible to distort memory for events, it is possible to implant an entire memory for something that never happened. The evidence is now clear that we can become mentally tricked into making large as well as small changes in the way we recall the past. Key words: memory, misinformation, memory distortion, cues, attribution. -- The modem work on memory distortion comes from a distinguished heritage in psychology, which can be found under the rubric of interference theory. The basic idea is that memories do not exist in isolation but rather in a world of other memories that can interfere with one another (Greene, 1992). In particular, a phenomenon known as retroactive interference has shown that memory can be especially fragile in the face of subsequent events. Over the last two decades the fragility of memory has been repeatedly demonstrated in a large number of interference studies. In these studies, subjects first witness a complex event such as a simulated violent crime or automobile accident. Some time later, half of the subjects receive misleading information about the event, while the other half do not. During the third phase of the study, all subjects try to recall the original event, and the extent to which the misinformation leads to changes in recollection is assessed. Taken as a whole, these studies leave little doubt that misleading information can produce errors in what subjects report having seen. Some studies have produced dramatic performance deficits, with difference exceeding 30% after exposure to misinformation. With a little help from misinformation, subjects have recalled seeing stop signs when they had actually seen yield signs, hammers when they had seen screwdrivers, and bearded culprits when they actually were clean shaven. Subjects have also recalled nonexistent items such as broken glass, tape recorders, and even something as large and conspicuous as a barn in a scene that contained no buildings at all. Why do people make these memory mistakes? Do errors in memory arise because the misinformation causes representations (or traces) to be partially or completely lost from the memory system, or is it because the misinformation causes those representations to be less accessible at the time the memory test is given? Japanese Psychological Association. Published by Blackwell Publishers Ltd, 108 Cowley Road, Oxford OX4 1JF. UK and 238 Main Street, Cambridge, MA 02142, USA.

2 6 C. G. Manning and E. F. Loftus Wording the issue slightly differently, do the errors arise because misinformation causes a loss of information from the person s prior memory, or is it because misinformation causes a loss of access to that prior memory? An excellent review by Brainerd, Reyna, Howe, and Kingma (1990) begins with Hoffding in 1891, and then Freud, Kohler, Wulf, and others in the early part of the 20th century and provides the earliest theoretical positions on this issue. As Brainerd et al. note, Hoffding, Freud and others believed that once representations are formed they do not get changed. According to this view, forgetting is a matter of retrieval failure. Conversely, other prominent early theorists assumed that representations do not remain crystallized, but rather degenerate through decay, reorganization, substitution, or some other mechanism. As noted by Brainerd and Ornstein (1991), this fundamental question about memory has captivated scholars throughout the 20th century. Some associationists claimed that memory traces remain intact once they get into longterm memory. This idea constituted an important feature of models from a quarter-century ago that conceived of long-term memory as a permanent system and attributed forgetting to failure of retrieval. Gestalt psychologists, on the other hand, thought the traces were altered with time. Throughout this century, similar ideas can be seen in the writings of memory theorists such as Bartlett (1932), Alba and Hasher (1983). and Brewer and Nakamura (1984). These theorists held that forgetting occurs, in part, because people are continually processing new information through mental structures that are built up from previous knowledge and beliefs about the world. A number of recent formal models seem to favor storage changes. Consider recent theories that are heavily based on associationist ideas. For example, CHARM (composite holographic associative recall model; MetcalE, 1990) demonstrates a formal mechanism by which new inputs can impair ability to remember earlier details. Although the mathematics underlying this model are unique to holographic memory models (Schooler & Tanaka, 1991), the representations that result from the model are similar to those of other distributed memory models such as those developed by McClelland and Rumelhart (1986) (see also, McClelland, 1988). In the modern memory distortion literature, investigators are now asking questions analogous to those posed 50 years ago. When people are exposed to misinformation about an event, is there any impairment of original memory? After all, people can give an erroneous misinformation response for reasons, such as demand characteristics, that have nothing to do with impairment of original memories. If impairment of event memories can be shown to occur, what kind of impairment is it? Two classes of memory impairment hypotheses have been addressed in the literature (Belli, Windschitl, McCarthy, & Winfrey, 1992; Brainerd et al., 1990). The first is retrieval-based memory impairment, and it holds that the stored representation of an event remains intact, but misinformation renders it more difficult to access. The other class of hypotheses are storagebased memory impairment hypotheses. These hypotheses hold that misinformation changes the stored event information in some way. If evidence is found for storage-based memory impairment, the next question to be answered is how this process should be characterized (Brainerd & Ornstein, 1991). Does it principally involve a mechanism of trace destruction, trace fading, reorganization, or what? The preceding discussion raised two of the most pressing questions about the impact of misinformation on a person s ability to recall the past accurately. Does misinformation impair a person s ability to remember details? And, once misinformation is embraced and reported, do people genuinely believe in their misinformation memories? The fate of the original memory Does misinformation alter pre-existing memory traces? Some investigators have suggested that even abundant errors following receipt Psychological Association 1996

3 Eyewitness testimony and memory distortion 7 misinformation do not provide evidence for impairment of prior traces. Demand characteristics and response biases could readily lead subjects to perform more poorly in the face of misinformation. For example, subjects who do not remember some event detail (say, a hammer) could report a suggested detail (say, a screwdriver) in order to go along with the perceived desires of the experimenter (McCloskey & Zaragoza, 1985), or they could report a suggested detail because they remember reading something about it, not because they think they saw it. While it is clear that these mechanisms are partially responsible for erroneous reporting, the question remains: Does misinformation ever lead to memory impairment? Several procedural innovations have been developed to explore this issue. One of these involves an adaptation of Jacoby s logic of opposition (Jacoby & Kelley, 1992; Lindsay, 1990, 1993). Subjects saw an event that contained critical details such as a hammer, then they read an erroneous narrative that mentioned, say, a screwdriver. Finally, they were tested on what they saw. The experimental innovation involved informing subjects before they were tested that the postevent information that they read did not include any correct answers to the test questions (if you read it, it is not accurate). Half the subjects were exposed to conditions that made it easy to remember the postevent suggestions and the other half were exposed to conditions that made it difficult. Before taking their test, all subjects were warned to report what they saw, and that anything they remembered reading was not accurate. The results showed that subjects in the easy condition were quite able to identify the source of their memories of suggested details and they refrained from reporting seeing these in the event. Despite this ability, the misleading suggestions still interfered with their ability to report the event details correctly. In other words, they knew it was not a screwdriver, but they were not so sure it was a hammer. The argument that subjects are trying to respond to experimenters desires cannot account for these findings, and Lindsay (1993) has argued that this finding constitutes powerful evidence that misleading suggestions can impair the ability to remember event details. Implicit testing Most tests to assess memory distortion are explicit tests of memory, in which subjects are instructed to remember recent events and try to do so. Implicit measures, on the other hand, are those in which subjects are not told to remember particular events, but rather are asked to perform some other task, such as completing a word fragment. A memory exists to the extent that it influences later performance on the implicit task relative to some baseline. Implicit measures have been known to reveal evidence of memory where explicit measures fail to do so (Graf & Schacter, 1987; Roediger, 1990; Schacter, 1987). Suppose a hypothetical subject, Mary, has seen a hammer and been misled about a screwdriver. On an explicit test, she reports the screwdriver, with high confidence, and insists that she did not see a hammer. In other words, the explicit tests produce no evidence of memory for a hammer. What would happen if she were given an implicit test instead? For example, assuming that prior exposure to the hammer in the absence of misinformation primes her performance on the implicit test. Would performance still be primed in the face of misinformation? A number of attempts to address this issue have appeared in the recent literature - with mixed results. Dodson and Reisberg (1991) found no evidence for impairment of memories with an implicit memory test, but interpretation of their results is compromised because their subjects were first given an explicit test. On the other hand, Birch and Brewer (1990) found evidence for impairment of memories with an implicit test, but their materials were somewhat unusual. Kilmer and Loftus (cited in Loftus, 1991) also found evidence for impairment of memories using an implicit test, but the particular test that they used makes the results open to many interpretations.

4 8 C. G. Manning and E. F. Loftus Do people really believe in their misinformation memories? The fate of original memories in the face of misinformation remains an issue of continuing interest to scholars of the misinformation effect. Whether the underlying traces remain intact or are modified, subjects often choose the misinformation option. So, a separate issue to explore is the extent to which overt reports reflect the subject s genuine belief that the misinformation was actually part of the original experience. It is tempting to claim that people genuinely believe in their misinformation memories because they often report those memories with a great deal of confidence. However, some subjects might use high confidence ratings simply because they believe the misinformation is right and assume that they must have seen it (Zaragoza & Lane, 1994). Fortunately, there are other techniques for showing that subjects really believe in their misinformation memories. One finding that is consistent with this idea is that subjects will bet money on those memories (Weingardt, Toland, & Loftus, 1994). On the other hand, it could be argued that subjects might be willing to bet money on a particular item that they do not remember seeing but, for other reasons, conclude is the right answer. These reasons are developed below. Clearly, there are many reasons why a subject who sees a hammer and receives misinformation about a screwdriver will subsequently come to report seeing a screwdriver. Some subjects could have failed to encode the hammer in the first place, and could choose the screwdriver on the test because they remember reading about it ( misinformation acceptance ). Other subjects might remember both hammer and screwdriver, and choose the screwdriver on the test because they trust their memory for the misinformation more than their memory for the original hammer ( deliberation ). Other subjects might simply be guessing. In other words, multiple process histories could be responsible for reports of different subjects to the same item, screwdriver. Most of the previous research on the misinformation effect simply examines the proportion of subjects who report screwdriver, or the average confidence or speed with which they do so. While such data are often useful, they sometimes mask important results because they are averages from subjects who have given the same answer, but who have come to that answer through different processes. So, the procedure of averaging performance across subjects and trials can be especially misleading if subjects vary in the strategies they use to perform a given task (Newell, 1973). If one wants to know something about how often the various strategies are used in a typical misinformation study, another method is needed. One new approach to gaining information about individual strategies relies on a simple but clever method used by Siegler (1987, 1989) in a completely different cognitive domain - namely the study of children engaged in addition and subtraction. In the subtraction study, for example, when the data were averaged from all trials, and over all strategies, the conclusion reached was that children solved subtraction problems mostly by counting down from the larger number or counting up from the smaller one. However, when the data were analyzed separately according to the particular strategy that a child claimed to use, a different picture was suggested. It is no wonder Siegler (1987) included the phrase the perils of averaging data in the title of his paper, and in a later (1989) paper he referred to the hazards of the practice of averaging. A similar research strategy would yield useful data in the misinformation domain. If data were analyzed separately, according to some retrospective indication of the specific strategy subjects said they used on the trial, more informed conclusions about the impact of misinformation might be reached. Subjects who arrive at a misinformation response via one strategy (e.g.. deliberation) versus another (e.g., misinformation acceptance) might express different levels of confidence in their memories, might describe the memories in different ways, and might be differentially resistant to being convinced that their memory is wrong.

5 Eyewitness testimony and memory distortion 9 Like Siegler, we asked subjects to describe the strategy they used to arrive at their response for each trial. The technique of asking people immediately after each response how they generated their answers has been successful in a variety of domains (Ericsson & Simon, 1984). It is particularly useful when the processing episode being asked about is not extremely brief. Although protocol analysts are well aware of the argument that verbal reports can give a misleading picture of what people are doing (Nisbett & Wilson, 1977), Siegler has argued convincingly that indirect methods of cognitive assessment (such as chronometric analyses) can give an equally misleading picture. Moreover, some related work in memory (Gardiner & Java, 1990) points to the potential benefits of gathering postresponse verbal reports. In this related work, subjects recognized previously presented words, and then indicated whether they actually remembered the previous Occurrence or whether they simply knew that it had occurred before but were unable to consciously recollect anything about its occurrence. These remember versus know judgments tapped qualitatively different components of memory. Analogously, we hoped that, by simply asking subjects the reason why they reported a particular item, we would discover whether subjects did so because they genuinely believed they had seen those items. A dilemma arises as to whether we should leave the subjects free to describe the reason they reported an item using their own words, or whether we should provide subjects with a list of options to choose from. Obviously, providing options would facilitate our analysis; however, we might be forcing subjects to select a reason that did not quite match their real reason. Thus, a student working in my laboratory, Richard Dashiell (Loftus, Feldman, & Dashiell, in press), did the experiment both ways. A new reasons study The subjects, 301 students, watched a series of 67 slides depicting a man visiting a local bookstore. The man interacted with a number of critical items (e.g., a screwdriver or a wrench, a can of Pepsi or Seven-Up). After the event, the subjects read a postevent narrative that contained some misinformation. Finally, subjects answered questions about what they had seen during the event. The questions took the form: Did you see a screwdriver or a wrench? One member of the pair was in the slides and the other had been given a misinformation to half the subjects. The innovation in this study is that after subjects indicated what they had seen, they indicated the reason for their choice. The results showed a strong misinformation effect. Subjects were correct less often when they had been given misinformation than when they had not (51% versus 78%). Next, we examined the reasons that subjects gave for choosing the misinformation option. The openended responses of subjects were classified into the eight following categories. 1. Read (the subject remembered reading about the item, e.g., read it in the narrative ). 2. Saw (the subject remembered seeing the item in the slides, e.g., I saw that the can was red ). 3. Deliberated (the subject remembered one item in the slides and another in the narrative, and finally opted for the narrative). 4. Guessed (the subject indicated a pure guess). 5. Educated guess (the subject made a guess based on some piece of apparently relevant information other than the item itself, e.g., Since I think he was screwing something, it must have been a screwdriver ). 6. Remember (the subject indicated remembering without stating a source for the memory, e.g., I remember it, or I memorized it ). 7. Familiar (the subject indicated a sense of familiarity about the item, e.g., I hear it in my mind ). 8. Reject one (the subject rejected one option and chose the other by default, e.g., I would have remembered a Mickey Mouse shirt ). Six percent of the time, no reason was given or else the response given was uncodable. The remaining 94% of responses were categorized 0 Japanese Psychologtcal Assoclatlon 1996

6 10 C. G. Manning and E. F. Loftus into one of the eight categories. The major reason subjects gave for choosing the misinformation option was because they remembered having read it (43%). A smaller, but sizable, minority (190/) chose the misinformation item because they simply remembered it from somewhere (17%) or because it just seemed familiar (2%). But, it is also important that 15% of the time subjects explicitly said they chose the misleading item because they remembered seeing it. One potential concern about the study is that subjects were asked for their strategies about one item (e.g., hammer or screwdriver) immediately after revealing which item they thought they had seen but before they were asked about the next set of items (e.g., Seven-Up or Pepsi). Thus, they knew at the time they made their choices that they would be pressed for information about their strategy. Perhaps this requirement affected the choices they made or the strategy they used to choose. For this reason, in the next study, subjects first indicated for all items which ones they thought they had seen. Later they went back and revealed for each item the reason they had selected it. The first strategy gave us some indication of the types of reasons subjects gave when permitted to express those reasons in their own words In the second strategy study, we gave subjects a list of six possible reasons for selecting the misinformation option, and urged them to select one of the six. The second reasons study used 282 students as subjects. They viewed the same event, and read a postevent narrative that contained some misinformation. Finally they were tested. They responded by using a black pen. When they were finished, the black pens were collected and blue pens were distributed. With the blue pens the subjects indicated the reason for their earlier choices. The change in pens prevented subjects from changing their earlier answers. The strategy instructions urged the subjects to choose one of the following categories: 1. Saw (e.g., I remember seeing it in the slides ) Saw and read (e.g., I remember it was in both the slides and the narrative ). Read (e.g., I read it in the narrative ). Conflict (e.g., I thought I saw one choice in the slides, but I thought that I read the other in the narrative ). Guess (e.g., I couldn t remember which it was so I guessed or made an educated guess based on what I did remember ). Familiar (e.g., I remembered my answer from the experiment, but I don t remember what the source was (slides or narrative)). Subjects were told that if they could not classify a strategy using this system, they should try to explain their strategy in their own words. Again, the results showed a reliable misinformation effect. Misled subjects were more likely to choose the misinformation option, and thus perform more poorly than control subjects. What reasons did subjects give for picking the misinformation? This time, the largest percentage of misinformation responses resulted from guesses (34%). Next largest was the option Read (30%). Relatively few subjects indicated that they chose the misinformation after a conflict (7%) or because it just seemed to be familiar (2%). Of greatest interest, however, was the finding that over a quarter of the subjects claimed to have seen the misinformation item, either by checking Saw (14%) or Saw and read (13%). Taken together, these studies are consistent with the idea that at least some of the subjects pick the misinformation item because they remember seeing it. Even when given the choice of saying that they remember both seeing and reading the item, one in seven subjects still claim that they only saw it. A source attribution study In another study, using the same series of slides as the previous studies, subjects were asked to identify the source of items they had seen or read about. In one condition subjects were shown the slides and then asked misleading questions about the slides and in another condition subjects saw two sets of slides. In the

7 Eyewitness testimony and memory distortion 11 later condition, the two sets of slides were identicai except for a few critical details. Some of these critical details were changed in the second set of slides and some of the critical details were left out. Similarly, in the slidesquestions condition, some of the misleading questions contained new information and some of the misleading questions contained information that conflicted with the slides. Later, subjects were given a list of items and were asked to indicate which items they had seen in slides or read about in the questions. Subjects were informed that they had seen some of the items in the first set of slides and they had read about some of the items in the misleading questions. Or, in the slides-slides condition, subjects were told some of the items had appeared in the first set of slides and some of the items had appeared in the second set of slides. In addition, subjects were told that none of the critical items had been in both the slides and the questions or both in the first and second sets of slides. They were then asked to go back to the items they recognized and to indicate where they had seen each item (in the first set of slides, in the misleading questions, or in the second set of slides). Did subjects say they had seen items they had only read about in misleading questions? When subjects said they recognized something that had only been mentioned in the questions, 56% of the time they said they had seen the items in the slides. Was this result due to subjects inability to identify the source of any items that they recognized? Or, is there something special about information from misleading questions? Subjects were more likely to say they had seen something they read about in the questions than they were to confuse information from a second, nearly identical, set of slides. Subjects who recognized items from the second set of slides incorrectly said the item came from the first set of slides only 18% of the time. And, subjects misidentified the source of information from the first set of slides only 16% of the time. What about the fate of the original information? Were subjects less likely to recognize an item they had seen in the first set of slides when conflicting information was presented in the questions or in the second set of slides? An important feature of this study was that subjects were given a chance to indicate whether or not they recognized both the item from the original set of slides and the item from the subsequent slides or questions. And subjects were informed that different items may have come from different sources. This was done so that subjects who remembered both items were not forced to choose only one. Subjects recognized items from the first set of slides 72% of the time when no additional (i.e., conflicting) information was given but only 61 YO when the item was followed by another, conflicting, item. Besides providing support for the idea that subjects confuse information from different sources, this study also supports the idea that subsequent misinformation affects the ability to remember the original information. As in the reasons study, subjects in the source attribution study remembered seeing things they only read about. In addition, subjects were less likely to recognize an item from the original event when it was followed by misinformation than when it was not. Whether or not the underlying memory trace of the original event is affected by subsequent information about that event is, at least for now, unanswerable. What we do have strong support for is that memory for past events is influenced by subsequent events. The creation of false memories It is one thing to change memory for a detail about some recently experienced event, and quite another thing to implant an entire memory for something that never happened. To determine whether it is even possible to implant entire memories that come to be believed, one project suggested to five individuals that they had been lost for an extended time when they were about the age of 5 years. With the help of some prodding from a trusted family member (a mother, an older brother, an aunt), these individuals, aged 8 to 42, were convinced they had been lost. (See Loftus & Ketcham, 1994.)

8 12 C. G. Manning and E. F. Loftus A similar attempt to create entire autobiographical memory is that of Hyman, Billings, Husband, Husband, and Smith (1993). In one experiment, parents of college students supplied information about a series of personal events that occurred to their child before the age of 10 years. Subjects were asked to remember some real events, and to remember a false one. The false event was about an overnight hospitalization for a high fever with a possible ear infection. Hyman et al. (1993) found that no subjects recalled the false events during the first interview. But in the second interview, 1 to 7 days later, 20% of the subjects remembered something about the false events. Even more startling results were obtained in a study using children as subjects (Ceci, Huffman, Smith, & Loftus, 1994). These subjects were 96 children who ranged in age from 3 to 6 years who completed a minimum of seven interviews about past events in their lives. They were interviewed individually about real (parent supplied) and fictitious (experimenter contrived) events, and had to say whether each event happened to them or not. One false event concerned getting one s hand caught in a mousetrap and having to go to the hospital to get it removed; another concerned going on a hot-air balloon ride with their classmates. The children tried to recall the events on 7 to 10 separate occasions, spaced about 7 to 10 days apart. How many children agreed that the false events had happened? At the initial interview, 44% of the younger children (aged 3 to 4 years) and 25% of the older ones (aged 5 to 6 years) claimed these events had happened. By the seventh interview approximately 3 months later, about 36% of the younger children and 32% of the older ones now claimed that the events happened. Many of these children not only said that the events happened, but they greatly embellished their false memories. This study shows that it is indeed possible to suggest an entire false event to a child that can become part of the child s memory. Although repeated interviews did not significantly increase the false beliefs, in a similar study involving more interviews about different fictitious items (i.e., falling off a tricycle and getting stitches in the leg), the rate of buying the false memory was greater with more interviews (Ceci et al., 1994). So, what do we know as a result of hundreds of studies of misinformation, spanning two decades, and most of the world s continents? That misinformation can lead people to have false memories that they appear to believe in as much as some of their genuine memories, and that misinformation can lead to small changes in memory (hammers become screwdrivers) or large changes (barns that did not exist, and hospital visits that were never made). Understanding how we can become mentally tricked by suggestion may offer us a way to avoid being tricked. It also offers an important window into the malleability of the mind and helps us understand the limits and pitfalls of human memory. References Alba, J. W., & Hasher, L. (1983). Is memory schematic? Psychological Bulletin, 93, Bartlett, F. C. (1932). Remembering: A study in experimental and social psychology. London: Cambridge University Press Belli, R. F., Windxhitl, P. D., McCarthy, T. T., & Winfrey, S. E. (1992) Detecting memory impairment with a modified test procedure: Manipulating retention interval with centrally presented event items Journal of Experimental Psychology: Learning, Memory and Cognition, 18, Birch, S. L., & Brewer, W. F. (1990). Memory permanence versus memory replacement in sentence recall. Unpublished manuscript, University of Illinois at Urbana-Champaign. Brainerd, C., & Ornstein, k? A. (1991). Children s memory for witnessed events. In J. Doris (Ed.), The suggestibility of children 5 recollections (pp. 1S20). Washington, DC: American Psychological Association. Brainerd, C. J., Reyna, V. F., Howe, M. L., & Kingma, J. (1990). The development of forgetting and reminiscence. Monographs of the Society for Research in Child Development, 55 (3, Whole No. 222). Brewer, W. F., & Nakamura, G. V. (1984). The nature and functions of schemas In R. S. Wyer & T. K. Srull (Eds), Handbook of social cognition, 1 (pp ). New York: Erlbaum.

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