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Does contextual information bias bitemark comparisons?
SCIJUS-00420; No of Pages 7 Science and Justice xxx (2013) xxx–xxx
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Does contextual information bias bitemark comparisons? Nikola K.P. Osborne a,⁎, Sally Woods b, Jules Kieser b, Rachel Zajac a a b
Psychology Department, University of Otago, Dunedin, New Zealand Sir John Walsh Research Institute, Dunedin, New Zealand
a r t i c l e
i n f o
Article history: Received 18 October 2013 Received in revised form 13 December 2013 Accepted 18 December 2013 Available online xxxx Keywords: Bitemarks Contextual bias Emotional context Expertise Forensic science Forensic odontology
a b s t r a c t A growing body of research suggests that the interpretation of fingerprint evidence is open to contextual bias. While there has been suggestion in the literature that the same might apply to bitemarks – a form of identification evidence in which a degree of contextual information during the comparison phase is generally unavoidable – there have so far been no empirical studies to test this assertion. We explored dental and non-dental students' ability to state whether two bitemarks matched, while manipulating task ambiguity and the presence and emotional intensity of additional contextual information. Provision of the contextual information influenced participants' decisions on the ambiguous bitemarks. Interestingly, when participants were presented with highly emotional images and subliminally primed with the words ‘same’ and ‘guilty’, they made fewer matches relative to our control condition. Dental experience also played a role in decision-making, with dental students making more matches as the experiment progressed, regardless of context or task ambiguity. We discuss ways that this exploratory research can be extended in future studies. © 2013 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction Over the past decade, a number of researchers have examined the decision-making processes of those forensic experts who are responsible for determining whether two forensic patterns match. One of the most significant conclusions to emerge from this body of work has been that context can affect the judgement of those who use forensic techniques previously thought to be based on objective analysis (for review see; [1]). The key assumption in forensic pattern analysis is that a target pattern, be it a shoe-print, bitemark or fingerprint found at a crime scene, is compared objectively with the pattern (i.e., shoe, teeth, fingerprint) from a suspect. Based on this comparison, the examiner determines whether the suspect is the source of the target pattern. Objective analysis, however, is often only possible for high clarity patterns. Ambiguous comparisons, such as distorted or partial patterns, can be highly complex, with judgement decisions being increasingly susceptible to additional influences (e.g., [2–15]). Several empirical studies have shown that contextual information does indeed influence decisions about forensic evidence; most of these studies have explored this phenomenon in the interpretation of fingerprint evidence [8,9,12,16,17]. Dror and colleagues [8], for example, presented five fingerprint experts with fingerprint pairs that they had – unknowingly – concluded to be a match five years earlier. The experts, however, were told that the prints were the erroneously matched
⁎ Corresponding author at: Department of Psychology, University of Otago, PO Box 56, Dunedin 9054, New Zealand. Tel.: +64 3 479 3989. E-mail address: [email protected] (N.K.P. Osborne).
prints in a high-profile misidentification case, thus creating the expectation that the prints did not match. Four of the five experts changed their original match decision to a non-match or inconclusive decision. Similarly, Dror and Charlton [9] found that experts could be biased towards making both match and non-match decisions when contextual information created these directional expectancies. The studies described above provided participants with particular directional cues. That is, the contextual information attempted to sway them towards a match or non-match decision. Dror and his colleagues [12], however, carried out a more subtle contextual manipulation, by using emotional images as the contextual information. Fingerprint pairs were presented alongside increasing levels of emotional context, beginning with a control condition which had no associated emotional context, and ending with a condition in which print pairs were presented with captioned photographs of high emotion crimes (e.g., rape and murder) and a subliminal prime of the words “same” and “guilty.” The authors found that match decisions increased with increasing levels of emotional context, but only when the print pairs did not contain sufficient information for a clear decision to be made. The authors suggest that emotion only influences decision-making when there is insufficient information for a clear decision to be made [12]. This research has recently been successfully replicated with a larger sample size and a modified method to show that the increase in match decisions is due to the emotional influence per se, rather than merely an increase in match decisions over time [18]. Because fingerprint evidence itself does not contain any inherent contextual information, biased interpretation has the potential to be avoided through the elimination, or at least minimisation, of additional context. For other types of evidence, however, contextual information is more difficult to avoid. One of these is bitemark evidence.
1355-0306/$ – see front matter © 2013 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scijus.2013.12.005
Please cite this article as: N.K.P. Osborne, et al., Does contextual information bias bitemark comparisons?, Sci. Justice (2013), http://dx.doi.org/ 10.1016/j.scijus.2013.12.005
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With a lack of established scientific principles for its use, the interpretation of bitemark evidence continues to be one of the most controversial of the forensic sciences [19–26]. Furthermore, a recent National Research Council (NRC) report on the state of many commonly used forensic science techniques recognises that bitemark analysis can be tainted by examiner bias [27]. Despite this, there is currently no empirical research investigating how context might influence the interpretation of bitemark evidence. There are two main reasons to suspect that bitemark analysis might be vulnerable to contextual bias. The first is that, in the forensic analysis of a bitemark made in skin, a high degree of emotional context is inherent within the evidence. That is, because bitemark impressions are often associated with sexual assault, child abuse, and homicide [28–30], it is unlikely that all emotive context – such as the injuries that the offender has inflicted – can be removed. In cases where there is significant trauma or injury, a forensic odontologist's emotional reaction to the evidence – whether conscious or subconscious – could play a significant role in his or her subsequent forensic decision-making [31]. Second, bitemarks are rarely clear impressions that allow for straightforward analysis. Instead, the appearance of the bite might change over time, or contain bruising, swelling, and broken skin that will make for ambiguous patterns [27,32]. As we know from previous research, contextual influences have the greatest effect when the information to be interpreted is ambiguous (e.g., [2–15]). Despite the potential for contextual bias in bitemark analysis, most recently noted by Page and colleagues [31], researchers have yet to empirically evaluate this possibility. In the present study, we provide an exploratory investigation of this issue. We had two primary objectives. First, we set out to examine whether there would be an influence of an additional emotional context when the evidence itself presupposes an emotional context. Second, we wanted to know whether we would observe similar results for forensically-trained dental students, compared with non-dental students (undergraduate psychology students). In fingerprint analysis, we know that both experts and non-experts can be influenced by contextual information [8,9,12,16]. However, research suggests that people with at least some training and experience will process information differently to those with no training and experience; the latter tend to make more intuitive-like judgements, especially in cases of uncertainty [33–38]. Therefore, familiarity with dental features and a forensic knowledge base could lead our forensically trained dental sample to be more influenced by context than the nondental sample. The undergraduate dental curriculum at the University of Otago is unique in that it provides an in depth grounding in forensic odontology. Hence it was felt that, although they were not experts in bitemark interpretation, these students have considerable knowledge and experience with the analysis of teeth and dental structures for forensic purposes. In addition, a dentist with little to no forensic training is able to present expert testimony in actual bitemark casework in New Zealand. By comparing forensically-trained dental students to non-dental students, we aim to explore whether the level of expertise will lead to differences in vulnerability to contextual bias. To address these two objectives, we employed a modified version of Dror and colleagues' [12] paradigm. 2. Method 2.1. Participants Participants were recruited from one of two populations; dental and non-dental students. Our dental sample (n = 178; M age = 22.4 years, SD = 3.07; range = 19.1 to 46.4 years; 68 males) comprised students enrolled in the Bachelor of Dental Surgery, Bachelor of Dental Technology, and Bachelor of Oral Health programmes at the School of Dentistry, University of Otago, New Zealand. While Dental Surgery students receive a full forensic odontology course interwoven throughout their four clinical years of study, Dental Technology and Oral Health students
receive specific instruction in bitemark analysis at the start of their programme. These participants were reimbursed with the choice of petrol voucher, supermarket voucher or a movie ticket. Our non-dental sample (n = 182; M age = 20.3 years, SD = 2.82, range = 17.8 to 49.9 years; 60 males) comprised undergraduate Psychology students, also from the University of Otago. These participants were given the opportunity to earn a small amount of course credit following the completion of an online worksheet based on the experiment. All potential participants were warned that they might be presented with graphic crime scene photographs as part of the experiment; this warning was reiterated immediately prior to informed consent procedures. 2.2. Materials 2.2.1. Bitemark impressions Bitemark impressions were collected from 15 volunteers. To maximise the visibility of the bite impression in the skin, all volunteers were Caucasoid. They were instructed to press a maxillary orthodontic dental cast firmly on their forearm for approximately 60 s, producing a clear impression of the dental cast on the skin. The impression was then photographed with a Cannon Powershot G11 camera, which was positioned on a stationary Cullmann 40180 Induro Alloyflex 6 m AX114 tripod, 32 cm above the table on which the volunteer's arm was placed. The bitemark impression was photographed at 90° with an ABFO No. 2 scale positioned through the first premolars of each bitemark. This process was repeated ten times on each volunteer to build a database of 150 photographed bitemark impressions. 2.2.2. Dental overlays We utilised Adobe Illustrator to create digital dental overlays of each dental cast used to form the impressions. Each dental cast was photographed in colour with the ABFO No. 2 scale. The photograph was inverted along its vertical axis. Using the pen tool, a Bezier curve was drawn around the cutting surface of the teeth on the photograph of the dental cast. The Bezier curves were placed over the bitemark photographs on skin and repositioned to match the positioning of the teeth on the bitemark; this was done to account for the fact that the skin is a relatively poor impression material [39–41]. Both the dental cast and bitemark photographs were removed to reveal a transparent trace outline of the cutting edge of the teeth. This process was repeated for all 150 dental casts. 2.2.3. Bitemark pairs From the database of 150 bitemark impressions and dental overlays, 96 bitemark pairs were created. Each pair comprised one bitemark impression and one dental overlay. The bitemark impression was labelled “crime scene” and the digital overlay was labelled “suspect dental overlay”. Forty-eight of the pairs contained clear bottom-up information. Twenty-four of these were unambiguous (i.e., clear) matches, in which the bitemark impression and the dental overlay had been created using the same dental cast (see Fig. 1A). Twenty-four of the pairs were unambiguous mismatches, in which the bitemark impression was paired with a dental overlay from a set of teeth that were clearly different from those that created the impression (see Fig. 1B). The remaining 48 pairs were ambiguous pairs; these did not contain enough information to make a clear decision as to whether or not the bitemark impression and suspect dental overlay were from the same source. The ambiguous bitemark pairs were designed to be representative of a real world situation, where bitemark impressions might be photographed in poor light, the ABFO No. 2 scale may be missing, or some of the teeth impressions may be missing or distorted due to movement on skin during the biting process ([39–41]; see Fig. 1C). The distortions in the ambiguous pairs were made or enhanced using tools in Adobe Photoshop.
Please cite this article as: N.K.P. Osborne, et al., Does contextual information bias bitemark comparisons?, Sci. Justice (2013), http://dx.doi.org/ 10.1016/j.scijus.2013.12.005
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(A)
(B)
(C) Fig. 1. Example of an unambiguous match (A), an unambiguous mismatch (B) and an ambiguous (C) bitemark stimulus pair.
2.2.4. Contextual information Captioned crime-related colour photographs were used to induce high and low emotional states. The photographs depicted victims of – or crime scenes involved in – violent crimes (e.g., rape or murder; see Fig. 2). All victims in the photographs were Caucasoid to match the
colour of the skin on which bitemark impressions were produced. The photographs used in this experiment were obtained from the International Affective Picture System (IAPS) – a database of emotion inducing photographs used in emotion research [42] – and by conducting internet image searches using words such as “crime scene” and “murder.”
Please cite this article as: N.K.P. Osborne, et al., Does contextual information bias bitemark comparisons?, Sci. Justice (2013), http://dx.doi.org/ 10.1016/j.scijus.2013.12.005
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Fig. 2. Example of a photograph intended to induce a high level of emotion. The caption presented alongside this photograph was “A woman is found dead at the [hotel name], Lubbock. Police believe the attack may have been random.”.
2.3. Experimental procedure
3. Results
Participants were given brief background information about the ‘physical comparison method’ of bitemark analysis; this background information comprised seven Powerpoint slides describing the process of photographing, inverting, and tracing the dental cast to create the bitemark overlays. Participants then completed six practice trials. On each trial, participants saw one bitemark impression on skin and one suspect dental overlay (from this point, these will be referred to as stimulus pairs) and were asked to indicate, using the computer keyboard, whether the bitemark impression and suspect dental overlay were from the same source (match; by pressing the S key) or a different source (mismatch; by pressing the D key). After each practice trial, participants received feedback as to whether their decision was correct or incorrect before the next trial was presented. Stimulus pairs presented during the practice trials were always clear matches or clear mismatches (i.e., unambiguous stimuli). Participants were then presented with the 96 test trials. One stimulus pair was presented at a time. Once the participant made a decision on a trial, the next stimulus pair was presented; no feedback was given on response accuracy. The 96 stimulus pairs were presented in three blocks of 32 pairs. Within each expertise group, participants were randomly assigned to one of two conditions. In the no-context condition (dental n = 90; non-dental n = 91), the 96 stimulus pairs were presented with no accompanying information on all three blocks. In the context condition (dental n = 88; non-dental n = 91), emotional contextual information was introduced on two of the trial blocks. The first block comprised stimulus pairs with no accompanying information. In the second block, the pairs were preceded with a high emotion captioned photograph. In the final block, high emotion captioned photographs were accompanied by a subliminal prime, in which the words “same” and “guilty” were presented for 88 ms immediately before the presentation of each stimulus pair. The three blocks were always presented in the same order, so that there were no carryover effects of emotional context across the blocks. In both conditions, the order of presentation of the stimulus pairs was counterbalanced both within and across the blocks, such that each participant saw all 96 pairs and no pair was presented twice. Presentation of the 16 ambiguous and 16 unambiguous pairs (eight clear matches and eight clear mismatches) was also randomised within each block.
All analyses described are based on the number of match (i.e., “same”) decisions that participants made. Initial analyses were planned comparisons based on our hypothesis that context would only affect decisionmaking when the stimuli were ambiguous. We submitted the data to a 2 (expert level; dental, non-dental) × 2 (context condition; nocontext, context) × 3 (block; 1, 2, 3) ANOVA with repeated measures across block, separately for the ambiguous and unambiguous stimulus types. For the unambiguous stimulus pairs, the results were in line with our expectation that there would be no interactions between expert level and context condition. The only significant result in this analysis was a main effect of Block, F(2) = 6.85, p b .05, with a small increase in match decisions over progressive blocks (Block 1: M = 8.01, SD = 0.71; Block 2: M = 8.05, SD = 0.91; Block 3: M = 8.21, SD = 0.95). For the ambiguous stimulus pairs, however, the ANOVA resulted in a significant Block × Expert Level interaction, F(2) = 3.81, p b .05, and a Block × Context condition interaction, F(2) = 4.09, p b .05. The remaining analyses investigate the interaction effects observed when decisions were made about ambiguous stimulus pairs.
3.1. Block × Context condition interaction In the absence of context, match decisions increased as the experiment progressed, while the rate of making match decisions remained consistent throughout all blocks of trials for those participants who received contextual information. Independent-samples t-tests for each block, with context as the between-subjects factor, revealed that participants in the no context condition made significantly more match decisions than those in the context condition on Block 3, t(358) = 2.13, p b .05, but not Blocks 1 and 2 (ps N .05; see Fig. 3).
3.2. Block × Expert Level interaction Relative to non-dental students, dental students made an increasing number of match decisions as the experiment progressed. Independentsamples t-tests for each block, with expert level as the between-subjects factor, revealed that dental students made significantly more match decisions than non-dental students on Block 2, t(358) = 3.13, p b .05 and Block 3, t(358) = 4.13, p b .05, but not Block 1 (p N .05; see Fig. 4).
Please cite this article as: N.K.P. Osborne, et al., Does contextual information bias bitemark comparisons?, Sci. Justice (2013), http://dx.doi.org/ 10.1016/j.scijus.2013.12.005
Mean number of match decisions
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No Context Context
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Block Fig. 3. Mean number of match decisions (±1SE) made on ambiguous stimulus pairs, shown as a function of context condition (no context, context) and block (1, 2, 3).
4. Discussion Our findings fall into two main categories: the effect of context and the effect of dental experience.
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is that our participants were suspicious about whether the bitemarks were recovered from the crimes depicted in the photographs. After all, the bitemark impressions displayed to participants were photographed under ideal conditions, inflicted on similarly coloured skin, and never broke the skin's surface. In support of this hypothesis, similar results to ours have been obtained in at least one other study of contextual bias [16]. In that study, experts and non-experts were required to judge fingerprint pairs, with the knowledge of a previous examiner's decision. The supposed previous examiner was well-known and prominent (high bias condition), or unknown (low bias condition). Experts in the high bias condition were actually less biased and more accurate than their control counterparts — a finding that the authors attributed to experts becoming aware of the experimental manipulation and the reasons behind it. It is possible that similar factors were at play in our study. We must emphasise here that significant effects of context only occurred when the bitemark pairs were ambiguous. When the bitemark pairs contained clear bottom-up information, context did not exert any effect on matching decisions. This finding is highly consistent with previous literature (e.g., [2–15]). Ambiguity, however, is a hallmark of forensic evidence. 4.2. Effect of dental experience
4.1. Effect of context
Mean number of match decisions
Providing contextual information clearly influenced participants' decisions. As a result, this is the first empirical study to show that bitemark decisions are susceptible to factors outside of the bitemarks themselves. The direction in which this effect occurred, however, was somewhat unexpected. In the context condition, the only difference between Block 2 (where there was no group effect) and Block 3 (where there was a significant group effect) was the addition of the subliminal presentation of the words ‘same’ and ‘guilty’ — words that we might have expected to bias participants towards making a match. Why, then, did our contextual information decrease matching decisions relative to the control group? One explanation for this finding could be that participants who were exposed to contextual information might have felt more accountable for the decisions that they made. Accountability refers to implicit or explicit expectations that our decisions or beliefs will need to be justified to others [43–45]. When people feel that they need to justify their decisions, they become more cautious [46,47], less complacent [48], use more information to inform their decisions [49,50], and use less top-down information [49]. Accountability research therefore provides a highly plausible explanation for our findings. Why might participants in this experiment have felt more accountable when exposed to context, when it appears that those in previous fingerprint research did not (e.g., [12,18])? The most likely explanation
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Dental
The dental students increased their rate of match decisions as they progressed through the trials. There is more than one way to explain this finding. For example, we know from previous literature that experts are inclined to apply intuitive-like judgements that can rely on topdown processing [51–53]. Although our dental sample did not have extensive experience in analysing bitemarks, their knowledge and experience with the structure of teeth could have led them to apply judgements of this kind. These types of judgments are more likely to occur as a person's experience with a given task increases [54], as would have occurred after multiple exposures to the bitemark stimuli. Alternatively, multiple exposures combined with previous dental experience could have led dental students to gain confidence in their ability over time. Empirical data suggest that experts are inclined to be overconfident in their decisions, more so than non-experts [55]. We also know that, as people become more confident, they tend to use less of the available information to inform their decision-making [56–58], thereby discounting potentially relevant information or its importance [56,59]. In the current study, if a dental overlay had six teeth and the bitemark impression had seven, but both patterns contained highly similar information, a dental student might explain this discrepancy by inferring that the tooth of the suspect had been removed after the bite took place. In contrast, non-dental students might be less likely to make this type of inference. Although we did not directly examine participants' confidence or the inferences that they made, this would be a relatively simple task for future research. Another potential explanation for dental students' increase in match decisions over time is that dental students were becoming complacent in their decision-making. Complacency can occur when a task is repetitive (as our procedure undoubtedly was) and becomes automated [60]. Although both experts and non-experts are vulnerable to complacency [61], experts appear to be especially vulnerable [62,63], possibly because they are more automatic in their decision-making [63,64].
Non-Dental
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4.3. General discussion
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Block Fig. 4. Mean number of match decisions (±1SE) made on ambiguous stimulus pairs, shown as a function of expert level (dental, non-dental) and block (1, 2, 3).
This study is the first to investigate whether contextual information can influence the interpretation of bitemark evidence, and represents an exploratory investigation of this issue using a paradigm previously tested with fingerprint evidence [12,18]. We observed a significant effect of context, but one that differed in direction to that observed in fingerprint research using the same methodology. In light of this
Please cite this article as: N.K.P. Osborne, et al., Does contextual information bias bitemark comparisons?, Sci. Justice (2013), http://dx.doi.org/ 10.1016/j.scijus.2013.12.005
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difference, it is important to consider two key differences between fingerprint and bitemark evidence. First, fingerprint evidence is obtained by comparing whether two patterns made in the same way share a common source. That is, a pattern is compared directly with another pattern. In contrast, bitemark evidence is obtained by comparing an impression made in skin or other soft substance (e.g., food) with a set of teeth. Regardless of how a suspect's pattern of dentition is extracted, it will not be a direct comparison to the skin or food impression [65]. For this reason, bitemark evidence is considered impression evidence, rather than pattern evidence [27]. Research into other forms of impression evidence – such as shoeprints [2] and bullet comparisons [4] – have failed to find evidence for contextual bias. Future research should aim to uncover why some types of evidence might be more – or differentially – vulnerable to contextual bias than others. Perhaps, for example, the process of directly comparing patterns is cognitively different to that used when two patterns represent an inverse of each other or, as in the current study, an indirect comparison. The other key difference between bitemark evidence and fingerprint evidence is that bitemark evidence on skin presupposes an emotional context. That is, because bitemarks on skin are generally the result of significant force, it is impossible to view them without some contextual knowledge that pain has been inflicted. Consequently, we might expect that additional emotional information would make little difference to decisions about the stimuli. Our findings provide some support for this proposition. In contrast to findings from the fingerprint literature [12,18], we only observed an effect of context when we added the subliminal prime to the emotional stimuli; on their own, the emotional stimuli did not exert a significant effect on participants' decisions. Researchers could further investigate this issue by having participants compare a suspect's dentition to a bitemark recovered from a nonskin surface (i.e., a food item). As well as differences between fingerprint and bitemark evidence per se, it is also possible that we failed to make our ambiguous bitemark stimuli ambiguous enough to see the effect we expected. It is well documented that contextual bias increases as task difficulty increases (e.g., [2–15]). Although we manipulated various factors to increase ambiguity (e.g., poor lighting, the absence of a scale bar, and low quality images), it was impossible to make the actual bite impressions appear as distorted as those that would be encountered in field work [39–41]. That is, the factors that make bitemark stimuli ambiguous – such as wounding or severe bruising – could not be replicated in our stimuli. Furthermore, the impressions were always made on the flat hard surface of the arm, and therefore are not likely to reflect how a bite would look on the buttocks or breast – where bites are most likely to occur [39,41,66]. It is important to acknowledge that participants in our study did not have the option of making an ‘inconclusive’ decision. Although this was done to amplify any effects of context, it is possible that it might have influenced participants' decision-making. We know from the fingerprint literature, however, that analysts rarely utilise this response option [8,9]. Furthermore, it was suggested in the NRC Report [27] that the pressure to solve the types of crimes in which a bitemark is found – usually child abuse, sexual assault and homicide – could render experts even less likely to display uncertainty. 5. Conclusions Bitemark analysis has been criticised for not having its basis firmly within the scientific community [22,24,27,67–70]. However, its continued use in a forensic capacity means that it is imperative to conduct scientific research to understand the cognitive processes involved in bitemark decisions. Our study is the first to show that bitemark comparisons – whether they are made by people with or without dental experience – are susceptible to contextual influences, although the mechanisms by which this occurred are by no means clear. While
it looks likely that we increased people's threshold for making a match decision, it is unclear whether this was a direct result of the contextual information per se, an indirect effect mediated by accountability, or merely a result of suspicion about our experimental manipulation. We encourage researchers to address the questions raised by these findings to gain further insight into the mechanisms that underlie the interpretation of bitemark evidence. Funding This study was funded by grants awarded to Rachel Zajac by the University of Otago and by the Marsden Fund Council (from the Government funding administered by the Royal Society of New Zealand). Support for the preparation of this manuscript was granted to Nikola Osborne by the Graduate Research Committee, by means of the University of Otago Postgraduate Publishing Bursary (Doctoral). Acknowledgements The authors would like to acknowledge Matt Blair for his contribution to stimulus construction, Hadyn Youens for his assistance in computer programming, and Dr. Brian Niven for the statistical advice. References [1] I.E. Dror, S.A. Cole, The vision in blind justice: expert perception, judgement, and visual cognition in forensic pattern recognition, Psychon. Bull. Rev. 17 (2010) 161–167. [2] J.H. Kerstholt, R. Paashuis, M. Sjerps, Shoe print examinations: effects of expectation, complexity and experience, Forensic Sci. Int. 165 (2007) 30–34. [3] J.H. Kerstholt, A.R. Eikelboom, Effects of prior interpretation on situation assessment in crime analysis, J. Behav. Decis. Mak. 20 (2007) 455–465. [4] J.H. Kerstholt, A. Eikelboom, T. Dijkman, R. Stoel, R. Hermsen, B. van Leuven, Does suggestive information cause a confirmation bias in bullet comparisons? Forensic Sci. Int. 198 (2010) 138–142. [5] W.C. Thompson, Interpretation: Observer Effects, Wiley Encyclopedia of Forensic Science, John Wiley & Sons, Ltd., 2009 [6] W.C. Thompson, S. Ford, T. Doom, M. Raymer, D. Krane, Evaluating forensic DNA evidence: essential elements of a competent defense review, The Champion 27 (2003) 16–25. [7] W.C. Thompson, S.A. Cole, Psychological aspects of forensic identification evidence, in: M. Costanzo, D. Krauss, K. Pezdek (Eds.), Expert Psychological Testimony for the Courts, Lawrence Erlbaum & Associates, New York, 2006, pp. 31–68. [8] I.E. Dror, D. Charlton, A.E. Péron, Contextual information renders experts vulnerable to making erroneous identifications, Forensic Sci. Int. 156 (2006) 74–78. [9] I.E. Dror, D. Charlton, Why experts make errors, J. Forensic Identif. 56 (2006) 600–616. [10] I.E. Dror, G. Hampikian, Subjectivity and bias in forensic DNA mixture interpretation, Sci. Justice 51 (2011) 204–208. [11] T.A. Busey, I.E. Dror, Special abilities and vulnerabilities in forensic expertise, in: A. McRoberts (Ed.), The Fingerprint Sourcebook, National Institute of Justice, Washington, DC, 2011, pp. 1–23. [12] I.E. Dror, A.E. Péron, S.-L. Hind, D. Charlton, When emotions get the better of us: the effect of contextual top-down processing on matching fingerprints, Appl. Cogn. Psychol. 19 (2005) 799–809. [13] P.C. Giannelli, Independent crime laboratories: the problem of motivational and cognitive bias, Utah L. Rev. 2 (2010) 247–266. [14] P.C. Giannelli, Cognitive bias in forensic science, Crim. Just. 25 (2010) 61–68. [15] I. Blanchette, A. Richards, The influence of affect on higher level cognition: a review of research on interpretation, judgement, decision making and reasoning, Cogn. Emot. 24 (2009) 561–595. [16] G. Langenburg, C. Champod, P. Wertheim, Testing for potential contextual bias effects during the verification stage of the ACE-V methodology when conducting fingerprint comparisons, J. Forensic Sci. 54 (2009) 571–582. [17] I.E. Dror, C. Champod, G. Langenburg, D. Charlton, H. Hunt, R. Rosenthal, Cognitive issues in fingerprint analysis: inter and intra-expert consistency and the effect of a ‘target’ comparison, Forensic Sci. Int. 208 (2011) 10–17. [18] N. Osborne, R. Zajac, An imperfect match? Emotional context influences fingerprint decisions, Appl. Cogn. Psychol. (2013) (submitted for publication). [19] J.C. Dailey, C.M. Bowers, Aging of bitemarks: a literature review, J. Forensic Sci. 42 (1997) 792. [20] M. Bowers, I.A. Pretty, Expert disagreement in bitemark casework, J. Forensic Sci. 54 (2009) 915–918. [21] A. Deitch, An inconvenient tooth: forensic odontology is an inadmissible junk science when it is used to“ match” teeth to bitemarks in skin, Wis. Libr. Rev. 1205 (2009) 1205–1236. [22] E. Beecher-Monas, Reality bites: the illusion of science in bite-mark evidence, Cardozo L. Rev. 1369 (2009) 1369–1410.
Please cite this article as: N.K.P. Osborne, et al., Does contextual information bias bitemark comparisons?, Sci. Justice (2013), http://dx.doi.org/ 10.1016/j.scijus.2013.12.005
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Please cite this article as: N.K.P. Osborne, et al., Does contextual information bias bitemark comparisons?, Sci. Justice (2013), http://dx.doi.org/ 10.1016/j.scijus.2013.12.005
Contents lists available at ScienceDirect
Science and Justice journal homepage: www.elsevier.com/locate/scijus
Does contextual information bias bitemark comparisons? Nikola K.P. Osborne a,⁎, Sally Woods b, Jules Kieser b, Rachel Zajac a a b
Psychology Department, University of Otago, Dunedin, New Zealand Sir John Walsh Research Institute, Dunedin, New Zealand
a r t i c l e
i n f o
Article history: Received 18 October 2013 Received in revised form 13 December 2013 Accepted 18 December 2013 Available online xxxx Keywords: Bitemarks Contextual bias Emotional context Expertise Forensic science Forensic odontology
a b s t r a c t A growing body of research suggests that the interpretation of fingerprint evidence is open to contextual bias. While there has been suggestion in the literature that the same might apply to bitemarks – a form of identification evidence in which a degree of contextual information during the comparison phase is generally unavoidable – there have so far been no empirical studies to test this assertion. We explored dental and non-dental students' ability to state whether two bitemarks matched, while manipulating task ambiguity and the presence and emotional intensity of additional contextual information. Provision of the contextual information influenced participants' decisions on the ambiguous bitemarks. Interestingly, when participants were presented with highly emotional images and subliminally primed with the words ‘same’ and ‘guilty’, they made fewer matches relative to our control condition. Dental experience also played a role in decision-making, with dental students making more matches as the experiment progressed, regardless of context or task ambiguity. We discuss ways that this exploratory research can be extended in future studies. © 2013 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction Over the past decade, a number of researchers have examined the decision-making processes of those forensic experts who are responsible for determining whether two forensic patterns match. One of the most significant conclusions to emerge from this body of work has been that context can affect the judgement of those who use forensic techniques previously thought to be based on objective analysis (for review see; [1]). The key assumption in forensic pattern analysis is that a target pattern, be it a shoe-print, bitemark or fingerprint found at a crime scene, is compared objectively with the pattern (i.e., shoe, teeth, fingerprint) from a suspect. Based on this comparison, the examiner determines whether the suspect is the source of the target pattern. Objective analysis, however, is often only possible for high clarity patterns. Ambiguous comparisons, such as distorted or partial patterns, can be highly complex, with judgement decisions being increasingly susceptible to additional influences (e.g., [2–15]). Several empirical studies have shown that contextual information does indeed influence decisions about forensic evidence; most of these studies have explored this phenomenon in the interpretation of fingerprint evidence [8,9,12,16,17]. Dror and colleagues [8], for example, presented five fingerprint experts with fingerprint pairs that they had – unknowingly – concluded to be a match five years earlier. The experts, however, were told that the prints were the erroneously matched
⁎ Corresponding author at: Department of Psychology, University of Otago, PO Box 56, Dunedin 9054, New Zealand. Tel.: +64 3 479 3989. E-mail address: [email protected] (N.K.P. Osborne).
prints in a high-profile misidentification case, thus creating the expectation that the prints did not match. Four of the five experts changed their original match decision to a non-match or inconclusive decision. Similarly, Dror and Charlton [9] found that experts could be biased towards making both match and non-match decisions when contextual information created these directional expectancies. The studies described above provided participants with particular directional cues. That is, the contextual information attempted to sway them towards a match or non-match decision. Dror and his colleagues [12], however, carried out a more subtle contextual manipulation, by using emotional images as the contextual information. Fingerprint pairs were presented alongside increasing levels of emotional context, beginning with a control condition which had no associated emotional context, and ending with a condition in which print pairs were presented with captioned photographs of high emotion crimes (e.g., rape and murder) and a subliminal prime of the words “same” and “guilty.” The authors found that match decisions increased with increasing levels of emotional context, but only when the print pairs did not contain sufficient information for a clear decision to be made. The authors suggest that emotion only influences decision-making when there is insufficient information for a clear decision to be made [12]. This research has recently been successfully replicated with a larger sample size and a modified method to show that the increase in match decisions is due to the emotional influence per se, rather than merely an increase in match decisions over time [18]. Because fingerprint evidence itself does not contain any inherent contextual information, biased interpretation has the potential to be avoided through the elimination, or at least minimisation, of additional context. For other types of evidence, however, contextual information is more difficult to avoid. One of these is bitemark evidence.
1355-0306/$ – see front matter © 2013 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scijus.2013.12.005
Please cite this article as: N.K.P. Osborne, et al., Does contextual information bias bitemark comparisons?, Sci. Justice (2013), http://dx.doi.org/ 10.1016/j.scijus.2013.12.005
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With a lack of established scientific principles for its use, the interpretation of bitemark evidence continues to be one of the most controversial of the forensic sciences [19–26]. Furthermore, a recent National Research Council (NRC) report on the state of many commonly used forensic science techniques recognises that bitemark analysis can be tainted by examiner bias [27]. Despite this, there is currently no empirical research investigating how context might influence the interpretation of bitemark evidence. There are two main reasons to suspect that bitemark analysis might be vulnerable to contextual bias. The first is that, in the forensic analysis of a bitemark made in skin, a high degree of emotional context is inherent within the evidence. That is, because bitemark impressions are often associated with sexual assault, child abuse, and homicide [28–30], it is unlikely that all emotive context – such as the injuries that the offender has inflicted – can be removed. In cases where there is significant trauma or injury, a forensic odontologist's emotional reaction to the evidence – whether conscious or subconscious – could play a significant role in his or her subsequent forensic decision-making [31]. Second, bitemarks are rarely clear impressions that allow for straightforward analysis. Instead, the appearance of the bite might change over time, or contain bruising, swelling, and broken skin that will make for ambiguous patterns [27,32]. As we know from previous research, contextual influences have the greatest effect when the information to be interpreted is ambiguous (e.g., [2–15]). Despite the potential for contextual bias in bitemark analysis, most recently noted by Page and colleagues [31], researchers have yet to empirically evaluate this possibility. In the present study, we provide an exploratory investigation of this issue. We had two primary objectives. First, we set out to examine whether there would be an influence of an additional emotional context when the evidence itself presupposes an emotional context. Second, we wanted to know whether we would observe similar results for forensically-trained dental students, compared with non-dental students (undergraduate psychology students). In fingerprint analysis, we know that both experts and non-experts can be influenced by contextual information [8,9,12,16]. However, research suggests that people with at least some training and experience will process information differently to those with no training and experience; the latter tend to make more intuitive-like judgements, especially in cases of uncertainty [33–38]. Therefore, familiarity with dental features and a forensic knowledge base could lead our forensically trained dental sample to be more influenced by context than the nondental sample. The undergraduate dental curriculum at the University of Otago is unique in that it provides an in depth grounding in forensic odontology. Hence it was felt that, although they were not experts in bitemark interpretation, these students have considerable knowledge and experience with the analysis of teeth and dental structures for forensic purposes. In addition, a dentist with little to no forensic training is able to present expert testimony in actual bitemark casework in New Zealand. By comparing forensically-trained dental students to non-dental students, we aim to explore whether the level of expertise will lead to differences in vulnerability to contextual bias. To address these two objectives, we employed a modified version of Dror and colleagues' [12] paradigm. 2. Method 2.1. Participants Participants were recruited from one of two populations; dental and non-dental students. Our dental sample (n = 178; M age = 22.4 years, SD = 3.07; range = 19.1 to 46.4 years; 68 males) comprised students enrolled in the Bachelor of Dental Surgery, Bachelor of Dental Technology, and Bachelor of Oral Health programmes at the School of Dentistry, University of Otago, New Zealand. While Dental Surgery students receive a full forensic odontology course interwoven throughout their four clinical years of study, Dental Technology and Oral Health students
receive specific instruction in bitemark analysis at the start of their programme. These participants were reimbursed with the choice of petrol voucher, supermarket voucher or a movie ticket. Our non-dental sample (n = 182; M age = 20.3 years, SD = 2.82, range = 17.8 to 49.9 years; 60 males) comprised undergraduate Psychology students, also from the University of Otago. These participants were given the opportunity to earn a small amount of course credit following the completion of an online worksheet based on the experiment. All potential participants were warned that they might be presented with graphic crime scene photographs as part of the experiment; this warning was reiterated immediately prior to informed consent procedures. 2.2. Materials 2.2.1. Bitemark impressions Bitemark impressions were collected from 15 volunteers. To maximise the visibility of the bite impression in the skin, all volunteers were Caucasoid. They were instructed to press a maxillary orthodontic dental cast firmly on their forearm for approximately 60 s, producing a clear impression of the dental cast on the skin. The impression was then photographed with a Cannon Powershot G11 camera, which was positioned on a stationary Cullmann 40180 Induro Alloyflex 6 m AX114 tripod, 32 cm above the table on which the volunteer's arm was placed. The bitemark impression was photographed at 90° with an ABFO No. 2 scale positioned through the first premolars of each bitemark. This process was repeated ten times on each volunteer to build a database of 150 photographed bitemark impressions. 2.2.2. Dental overlays We utilised Adobe Illustrator to create digital dental overlays of each dental cast used to form the impressions. Each dental cast was photographed in colour with the ABFO No. 2 scale. The photograph was inverted along its vertical axis. Using the pen tool, a Bezier curve was drawn around the cutting surface of the teeth on the photograph of the dental cast. The Bezier curves were placed over the bitemark photographs on skin and repositioned to match the positioning of the teeth on the bitemark; this was done to account for the fact that the skin is a relatively poor impression material [39–41]. Both the dental cast and bitemark photographs were removed to reveal a transparent trace outline of the cutting edge of the teeth. This process was repeated for all 150 dental casts. 2.2.3. Bitemark pairs From the database of 150 bitemark impressions and dental overlays, 96 bitemark pairs were created. Each pair comprised one bitemark impression and one dental overlay. The bitemark impression was labelled “crime scene” and the digital overlay was labelled “suspect dental overlay”. Forty-eight of the pairs contained clear bottom-up information. Twenty-four of these were unambiguous (i.e., clear) matches, in which the bitemark impression and the dental overlay had been created using the same dental cast (see Fig. 1A). Twenty-four of the pairs were unambiguous mismatches, in which the bitemark impression was paired with a dental overlay from a set of teeth that were clearly different from those that created the impression (see Fig. 1B). The remaining 48 pairs were ambiguous pairs; these did not contain enough information to make a clear decision as to whether or not the bitemark impression and suspect dental overlay were from the same source. The ambiguous bitemark pairs were designed to be representative of a real world situation, where bitemark impressions might be photographed in poor light, the ABFO No. 2 scale may be missing, or some of the teeth impressions may be missing or distorted due to movement on skin during the biting process ([39–41]; see Fig. 1C). The distortions in the ambiguous pairs were made or enhanced using tools in Adobe Photoshop.
Please cite this article as: N.K.P. Osborne, et al., Does contextual information bias bitemark comparisons?, Sci. Justice (2013), http://dx.doi.org/ 10.1016/j.scijus.2013.12.005
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(A)
(B)
(C) Fig. 1. Example of an unambiguous match (A), an unambiguous mismatch (B) and an ambiguous (C) bitemark stimulus pair.
2.2.4. Contextual information Captioned crime-related colour photographs were used to induce high and low emotional states. The photographs depicted victims of – or crime scenes involved in – violent crimes (e.g., rape or murder; see Fig. 2). All victims in the photographs were Caucasoid to match the
colour of the skin on which bitemark impressions were produced. The photographs used in this experiment were obtained from the International Affective Picture System (IAPS) – a database of emotion inducing photographs used in emotion research [42] – and by conducting internet image searches using words such as “crime scene” and “murder.”
Please cite this article as: N.K.P. Osborne, et al., Does contextual information bias bitemark comparisons?, Sci. Justice (2013), http://dx.doi.org/ 10.1016/j.scijus.2013.12.005
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Fig. 2. Example of a photograph intended to induce a high level of emotion. The caption presented alongside this photograph was “A woman is found dead at the [hotel name], Lubbock. Police believe the attack may have been random.”.
2.3. Experimental procedure
3. Results
Participants were given brief background information about the ‘physical comparison method’ of bitemark analysis; this background information comprised seven Powerpoint slides describing the process of photographing, inverting, and tracing the dental cast to create the bitemark overlays. Participants then completed six practice trials. On each trial, participants saw one bitemark impression on skin and one suspect dental overlay (from this point, these will be referred to as stimulus pairs) and were asked to indicate, using the computer keyboard, whether the bitemark impression and suspect dental overlay were from the same source (match; by pressing the S key) or a different source (mismatch; by pressing the D key). After each practice trial, participants received feedback as to whether their decision was correct or incorrect before the next trial was presented. Stimulus pairs presented during the practice trials were always clear matches or clear mismatches (i.e., unambiguous stimuli). Participants were then presented with the 96 test trials. One stimulus pair was presented at a time. Once the participant made a decision on a trial, the next stimulus pair was presented; no feedback was given on response accuracy. The 96 stimulus pairs were presented in three blocks of 32 pairs. Within each expertise group, participants were randomly assigned to one of two conditions. In the no-context condition (dental n = 90; non-dental n = 91), the 96 stimulus pairs were presented with no accompanying information on all three blocks. In the context condition (dental n = 88; non-dental n = 91), emotional contextual information was introduced on two of the trial blocks. The first block comprised stimulus pairs with no accompanying information. In the second block, the pairs were preceded with a high emotion captioned photograph. In the final block, high emotion captioned photographs were accompanied by a subliminal prime, in which the words “same” and “guilty” were presented for 88 ms immediately before the presentation of each stimulus pair. The three blocks were always presented in the same order, so that there were no carryover effects of emotional context across the blocks. In both conditions, the order of presentation of the stimulus pairs was counterbalanced both within and across the blocks, such that each participant saw all 96 pairs and no pair was presented twice. Presentation of the 16 ambiguous and 16 unambiguous pairs (eight clear matches and eight clear mismatches) was also randomised within each block.
All analyses described are based on the number of match (i.e., “same”) decisions that participants made. Initial analyses were planned comparisons based on our hypothesis that context would only affect decisionmaking when the stimuli were ambiguous. We submitted the data to a 2 (expert level; dental, non-dental) × 2 (context condition; nocontext, context) × 3 (block; 1, 2, 3) ANOVA with repeated measures across block, separately for the ambiguous and unambiguous stimulus types. For the unambiguous stimulus pairs, the results were in line with our expectation that there would be no interactions between expert level and context condition. The only significant result in this analysis was a main effect of Block, F(2) = 6.85, p b .05, with a small increase in match decisions over progressive blocks (Block 1: M = 8.01, SD = 0.71; Block 2: M = 8.05, SD = 0.91; Block 3: M = 8.21, SD = 0.95). For the ambiguous stimulus pairs, however, the ANOVA resulted in a significant Block × Expert Level interaction, F(2) = 3.81, p b .05, and a Block × Context condition interaction, F(2) = 4.09, p b .05. The remaining analyses investigate the interaction effects observed when decisions were made about ambiguous stimulus pairs.
3.1. Block × Context condition interaction In the absence of context, match decisions increased as the experiment progressed, while the rate of making match decisions remained consistent throughout all blocks of trials for those participants who received contextual information. Independent-samples t-tests for each block, with context as the between-subjects factor, revealed that participants in the no context condition made significantly more match decisions than those in the context condition on Block 3, t(358) = 2.13, p b .05, but not Blocks 1 and 2 (ps N .05; see Fig. 3).
3.2. Block × Expert Level interaction Relative to non-dental students, dental students made an increasing number of match decisions as the experiment progressed. Independentsamples t-tests for each block, with expert level as the between-subjects factor, revealed that dental students made significantly more match decisions than non-dental students on Block 2, t(358) = 3.13, p b .05 and Block 3, t(358) = 4.13, p b .05, but not Block 1 (p N .05; see Fig. 4).
Please cite this article as: N.K.P. Osborne, et al., Does contextual information bias bitemark comparisons?, Sci. Justice (2013), http://dx.doi.org/ 10.1016/j.scijus.2013.12.005
Mean number of match decisions
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14 13 12 11
No Context Context
10 9 8 1
2
3
Block Fig. 3. Mean number of match decisions (±1SE) made on ambiguous stimulus pairs, shown as a function of context condition (no context, context) and block (1, 2, 3).
4. Discussion Our findings fall into two main categories: the effect of context and the effect of dental experience.
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is that our participants were suspicious about whether the bitemarks were recovered from the crimes depicted in the photographs. After all, the bitemark impressions displayed to participants were photographed under ideal conditions, inflicted on similarly coloured skin, and never broke the skin's surface. In support of this hypothesis, similar results to ours have been obtained in at least one other study of contextual bias [16]. In that study, experts and non-experts were required to judge fingerprint pairs, with the knowledge of a previous examiner's decision. The supposed previous examiner was well-known and prominent (high bias condition), or unknown (low bias condition). Experts in the high bias condition were actually less biased and more accurate than their control counterparts — a finding that the authors attributed to experts becoming aware of the experimental manipulation and the reasons behind it. It is possible that similar factors were at play in our study. We must emphasise here that significant effects of context only occurred when the bitemark pairs were ambiguous. When the bitemark pairs contained clear bottom-up information, context did not exert any effect on matching decisions. This finding is highly consistent with previous literature (e.g., [2–15]). Ambiguity, however, is a hallmark of forensic evidence. 4.2. Effect of dental experience
4.1. Effect of context
Mean number of match decisions
Providing contextual information clearly influenced participants' decisions. As a result, this is the first empirical study to show that bitemark decisions are susceptible to factors outside of the bitemarks themselves. The direction in which this effect occurred, however, was somewhat unexpected. In the context condition, the only difference between Block 2 (where there was no group effect) and Block 3 (where there was a significant group effect) was the addition of the subliminal presentation of the words ‘same’ and ‘guilty’ — words that we might have expected to bias participants towards making a match. Why, then, did our contextual information decrease matching decisions relative to the control group? One explanation for this finding could be that participants who were exposed to contextual information might have felt more accountable for the decisions that they made. Accountability refers to implicit or explicit expectations that our decisions or beliefs will need to be justified to others [43–45]. When people feel that they need to justify their decisions, they become more cautious [46,47], less complacent [48], use more information to inform their decisions [49,50], and use less top-down information [49]. Accountability research therefore provides a highly plausible explanation for our findings. Why might participants in this experiment have felt more accountable when exposed to context, when it appears that those in previous fingerprint research did not (e.g., [12,18])? The most likely explanation
14 13 12 11
Dental
The dental students increased their rate of match decisions as they progressed through the trials. There is more than one way to explain this finding. For example, we know from previous literature that experts are inclined to apply intuitive-like judgements that can rely on topdown processing [51–53]. Although our dental sample did not have extensive experience in analysing bitemarks, their knowledge and experience with the structure of teeth could have led them to apply judgements of this kind. These types of judgments are more likely to occur as a person's experience with a given task increases [54], as would have occurred after multiple exposures to the bitemark stimuli. Alternatively, multiple exposures combined with previous dental experience could have led dental students to gain confidence in their ability over time. Empirical data suggest that experts are inclined to be overconfident in their decisions, more so than non-experts [55]. We also know that, as people become more confident, they tend to use less of the available information to inform their decision-making [56–58], thereby discounting potentially relevant information or its importance [56,59]. In the current study, if a dental overlay had six teeth and the bitemark impression had seven, but both patterns contained highly similar information, a dental student might explain this discrepancy by inferring that the tooth of the suspect had been removed after the bite took place. In contrast, non-dental students might be less likely to make this type of inference. Although we did not directly examine participants' confidence or the inferences that they made, this would be a relatively simple task for future research. Another potential explanation for dental students' increase in match decisions over time is that dental students were becoming complacent in their decision-making. Complacency can occur when a task is repetitive (as our procedure undoubtedly was) and becomes automated [60]. Although both experts and non-experts are vulnerable to complacency [61], experts appear to be especially vulnerable [62,63], possibly because they are more automatic in their decision-making [63,64].
Non-Dental
10
4.3. General discussion
9 8 1
2
3
Block Fig. 4. Mean number of match decisions (±1SE) made on ambiguous stimulus pairs, shown as a function of expert level (dental, non-dental) and block (1, 2, 3).
This study is the first to investigate whether contextual information can influence the interpretation of bitemark evidence, and represents an exploratory investigation of this issue using a paradigm previously tested with fingerprint evidence [12,18]. We observed a significant effect of context, but one that differed in direction to that observed in fingerprint research using the same methodology. In light of this
Please cite this article as: N.K.P. Osborne, et al., Does contextual information bias bitemark comparisons?, Sci. Justice (2013), http://dx.doi.org/ 10.1016/j.scijus.2013.12.005
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difference, it is important to consider two key differences between fingerprint and bitemark evidence. First, fingerprint evidence is obtained by comparing whether two patterns made in the same way share a common source. That is, a pattern is compared directly with another pattern. In contrast, bitemark evidence is obtained by comparing an impression made in skin or other soft substance (e.g., food) with a set of teeth. Regardless of how a suspect's pattern of dentition is extracted, it will not be a direct comparison to the skin or food impression [65]. For this reason, bitemark evidence is considered impression evidence, rather than pattern evidence [27]. Research into other forms of impression evidence – such as shoeprints [2] and bullet comparisons [4] – have failed to find evidence for contextual bias. Future research should aim to uncover why some types of evidence might be more – or differentially – vulnerable to contextual bias than others. Perhaps, for example, the process of directly comparing patterns is cognitively different to that used when two patterns represent an inverse of each other or, as in the current study, an indirect comparison. The other key difference between bitemark evidence and fingerprint evidence is that bitemark evidence on skin presupposes an emotional context. That is, because bitemarks on skin are generally the result of significant force, it is impossible to view them without some contextual knowledge that pain has been inflicted. Consequently, we might expect that additional emotional information would make little difference to decisions about the stimuli. Our findings provide some support for this proposition. In contrast to findings from the fingerprint literature [12,18], we only observed an effect of context when we added the subliminal prime to the emotional stimuli; on their own, the emotional stimuli did not exert a significant effect on participants' decisions. Researchers could further investigate this issue by having participants compare a suspect's dentition to a bitemark recovered from a nonskin surface (i.e., a food item). As well as differences between fingerprint and bitemark evidence per se, it is also possible that we failed to make our ambiguous bitemark stimuli ambiguous enough to see the effect we expected. It is well documented that contextual bias increases as task difficulty increases (e.g., [2–15]). Although we manipulated various factors to increase ambiguity (e.g., poor lighting, the absence of a scale bar, and low quality images), it was impossible to make the actual bite impressions appear as distorted as those that would be encountered in field work [39–41]. That is, the factors that make bitemark stimuli ambiguous – such as wounding or severe bruising – could not be replicated in our stimuli. Furthermore, the impressions were always made on the flat hard surface of the arm, and therefore are not likely to reflect how a bite would look on the buttocks or breast – where bites are most likely to occur [39,41,66]. It is important to acknowledge that participants in our study did not have the option of making an ‘inconclusive’ decision. Although this was done to amplify any effects of context, it is possible that it might have influenced participants' decision-making. We know from the fingerprint literature, however, that analysts rarely utilise this response option [8,9]. Furthermore, it was suggested in the NRC Report [27] that the pressure to solve the types of crimes in which a bitemark is found – usually child abuse, sexual assault and homicide – could render experts even less likely to display uncertainty. 5. Conclusions Bitemark analysis has been criticised for not having its basis firmly within the scientific community [22,24,27,67–70]. However, its continued use in a forensic capacity means that it is imperative to conduct scientific research to understand the cognitive processes involved in bitemark decisions. Our study is the first to show that bitemark comparisons – whether they are made by people with or without dental experience – are susceptible to contextual influences, although the mechanisms by which this occurred are by no means clear. While
it looks likely that we increased people's threshold for making a match decision, it is unclear whether this was a direct result of the contextual information per se, an indirect effect mediated by accountability, or merely a result of suspicion about our experimental manipulation. We encourage researchers to address the questions raised by these findings to gain further insight into the mechanisms that underlie the interpretation of bitemark evidence. Funding This study was funded by grants awarded to Rachel Zajac by the University of Otago and by the Marsden Fund Council (from the Government funding administered by the Royal Society of New Zealand). Support for the preparation of this manuscript was granted to Nikola Osborne by the Graduate Research Committee, by means of the University of Otago Postgraduate Publishing Bursary (Doctoral). Acknowledgements The authors would like to acknowledge Matt Blair for his contribution to stimulus construction, Hadyn Youens for his assistance in computer programming, and Dr. Brian Niven for the statistical advice. References [1] I.E. Dror, S.A. Cole, The vision in blind justice: expert perception, judgement, and visual cognition in forensic pattern recognition, Psychon. Bull. Rev. 17 (2010) 161–167. [2] J.H. Kerstholt, R. Paashuis, M. Sjerps, Shoe print examinations: effects of expectation, complexity and experience, Forensic Sci. Int. 165 (2007) 30–34. [3] J.H. Kerstholt, A.R. Eikelboom, Effects of prior interpretation on situation assessment in crime analysis, J. Behav. Decis. Mak. 20 (2007) 455–465. [4] J.H. Kerstholt, A. Eikelboom, T. Dijkman, R. Stoel, R. Hermsen, B. van Leuven, Does suggestive information cause a confirmation bias in bullet comparisons? Forensic Sci. Int. 198 (2010) 138–142. [5] W.C. Thompson, Interpretation: Observer Effects, Wiley Encyclopedia of Forensic Science, John Wiley & Sons, Ltd., 2009 [6] W.C. Thompson, S. Ford, T. Doom, M. Raymer, D. Krane, Evaluating forensic DNA evidence: essential elements of a competent defense review, The Champion 27 (2003) 16–25. [7] W.C. Thompson, S.A. Cole, Psychological aspects of forensic identification evidence, in: M. Costanzo, D. Krauss, K. Pezdek (Eds.), Expert Psychological Testimony for the Courts, Lawrence Erlbaum & Associates, New York, 2006, pp. 31–68. [8] I.E. Dror, D. Charlton, A.E. Péron, Contextual information renders experts vulnerable to making erroneous identifications, Forensic Sci. Int. 156 (2006) 74–78. [9] I.E. Dror, D. Charlton, Why experts make errors, J. Forensic Identif. 56 (2006) 600–616. [10] I.E. Dror, G. Hampikian, Subjectivity and bias in forensic DNA mixture interpretation, Sci. Justice 51 (2011) 204–208. [11] T.A. Busey, I.E. Dror, Special abilities and vulnerabilities in forensic expertise, in: A. McRoberts (Ed.), The Fingerprint Sourcebook, National Institute of Justice, Washington, DC, 2011, pp. 1–23. [12] I.E. Dror, A.E. Péron, S.-L. Hind, D. Charlton, When emotions get the better of us: the effect of contextual top-down processing on matching fingerprints, Appl. Cogn. Psychol. 19 (2005) 799–809. [13] P.C. Giannelli, Independent crime laboratories: the problem of motivational and cognitive bias, Utah L. Rev. 2 (2010) 247–266. [14] P.C. Giannelli, Cognitive bias in forensic science, Crim. Just. 25 (2010) 61–68. [15] I. Blanchette, A. Richards, The influence of affect on higher level cognition: a review of research on interpretation, judgement, decision making and reasoning, Cogn. Emot. 24 (2009) 561–595. [16] G. Langenburg, C. Champod, P. Wertheim, Testing for potential contextual bias effects during the verification stage of the ACE-V methodology when conducting fingerprint comparisons, J. Forensic Sci. 54 (2009) 571–582. [17] I.E. Dror, C. Champod, G. Langenburg, D. Charlton, H. Hunt, R. Rosenthal, Cognitive issues in fingerprint analysis: inter and intra-expert consistency and the effect of a ‘target’ comparison, Forensic Sci. Int. 208 (2011) 10–17. [18] N. Osborne, R. Zajac, An imperfect match? Emotional context influences fingerprint decisions, Appl. Cogn. Psychol. (2013) (submitted for publication). [19] J.C. Dailey, C.M. Bowers, Aging of bitemarks: a literature review, J. Forensic Sci. 42 (1997) 792. [20] M. Bowers, I.A. Pretty, Expert disagreement in bitemark casework, J. Forensic Sci. 54 (2009) 915–918. [21] A. Deitch, An inconvenient tooth: forensic odontology is an inadmissible junk science when it is used to“ match” teeth to bitemarks in skin, Wis. Libr. Rev. 1205 (2009) 1205–1236. [22] E. Beecher-Monas, Reality bites: the illusion of science in bite-mark evidence, Cardozo L. Rev. 1369 (2009) 1369–1410.
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