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British Journal of Psychology (2011)C© 2011 The British Psychological Society

TheBritishPsychologicalSociety

www.wileyonlinelibrary.com

Identification accuracy for single- anddouble-perpetrator crimes: Does accomplicegender matter?

Ahmed M. Megreya1,2∗ and Markus Bindemann3

1Department of Psychology, Menoufia University, Shibin el Kom, Egypt2University College at Qunfudah, Umm Al-Qura University, Mecca, Saudi Arabia3School of Psychology, University of Kent, Canterbury, UK

There is an important mismatch between empirical research on the accuracy ofeyewitness identification and the real world of criminal investigation. Most researchmodels single-perpetrator crimes, but in the real world, most crimes involve multipleperpetrators. This study examined how the number of perpetrators affects eyewitnessidentification by manipulating the gender of accomplices. Observers viewed a videoof a staged crime. The crime was committed by a male or female perpetrator, whowas presented alone or with an accomplice of the same or the opposite gender. Theobservers were then asked to identify the perpetrators from target-present or target-absent line-ups. The results revealed a double-perpetrator disadvantage, which wasmanifested in reduced identification accuracy on target-present line-ups. Importantly,however, the gender of the perpetrator or the accomplice had no effect on thisdisadvantage. This double-perpetrator disadvantage is attributed to the need to divideattention between two concurrent people, compared to single-culprit crimes, ratherthan an impairment of visual encoding processes. The implications of these findings arediscussed.

The reliability of eyewitness identification has been under question for more than acentury. Hugo Munsterberg (1908), for example, already concluded that eyewitnessesare fallible, and Edwin Borchard (1932) found that eyewitness misidentification is thegreatest cause of wrongful conviction of innocents. A large corpus of subsequentresearch has confirmed these observations repeatedly (for extensive reviews see, e.g.,Cutler & Penrod, 1995; Memon, Vrij, & Bull, 2003; Steblay, Dysart, Fulero, & Lindsay,2001; Wells, 1993; Wells, Memon, & Penrod, 2006; Wilcock, Bull, & Milne, 2008). Inaddition, it is now documented that more than 75% of cases of wrongful imprisonment,in which the accused were subsequently exonerated by DNA evidence, involvedeyewitness misidentifications (see Innocence Project, n.d.). These misidentifications not

∗Correspondence should be addressed to Ahmed M. Megreya, Department of Psychology, Faculty of Arts, Menoufia University,Shebin El-Kom, Egypt (e-mail: amegreya@yahoo.com).

DOI:10.1111/j.2044-8295.2011.02084.x

2 Ahmed M. Megreya and Markus Bindemann

only threaten innocent citizens, but also disrupt police investigations by providing falseleads. To avoid, or at least minimize, these consequences, much research has focusedon improving the accuracy of eyewitness identification (e.g., Cutler & Penrod, 1988;Lindsay & Wells, 1985; Macrae & Lewis, 2002; Perfect et al., 2008).

Determining the causes of eyewitness misidentifications provides a fundamentalstarting point for producing such improvements. Wells (1978) distinguished two kindsof variables governing the accuracy of identifications: estimator variables, which includefactors related to both witnesses and perpetrators (such as age and race) as well ascrime situations (such as exposure duration and the presence of a weapon), and systemvariables, which refer to the factors that are directly under the control of the criminaljustice system (such as line-up construction and witness instruction). While hundredsof studies have investigated these categories of variables (for reviews see, e.g., Lindsay& Pozzulo, 1999; Memon et al., 2003; Narby, Cutler & Penrod, 1996; Wells, Wright& Bradfield, 1999; Wilcock et al., 2008), there is an important mismatch betweenthis large literature and the real world of criminal investigations. Almost all previousresearch has examined identification accuracy for single-perpetrator crimes, but onlya few isolated studies have examined multiple-perpetrator crimes. This discrepancy issurprising bearing in mind that many criminal incidents and court cases in real life involvemultiple-perpetrator crimes (see, e.g., Wells & Pozzulo, 2006).

Intuitively, one might expect that as the number of any to-be-remembered itemsincreases, identification accuracy for these items from memory decreases. However, inthe field of eyewitness identification, it is imperative to precisely quantify these effectsto assess the likelihood of correct person identifications. Moreover, if it is possible todelineate any additional processes underlying the identification of multiple perpetrators,then this might provide vital clues to the cause of any multiple-perpetrator effects. Toillustrate, it is possible, for example, that it is difficult to store identity information frommultiple persons separately. This could lead to a reduction in eyewitness accuracy inmultiple-perpetrator crimes. From another perspective, however, one could also predictthat the presence of multiple perpetrators can improve subsequent identification, by en-couraging witnesses to compare different identities and extract some distinctive features.At present, the little existing evidence appears to support the former perspective.

In one notable study, Clifford and Hollin (1981) presented subjects with a videotapedepicting either a violent robbery or a non-violent interaction. In the violent condition,a man tore a handbag from a woman by force, whereas he simply approached thewoman to ask for directions in the non-violent condition. In both scenarios, this targetman was either acting alone or was accompanied by two or four other men, whoalso appeared in close proximity to the female actor. Eyewitnesses were then asked toidentify the principal perpetrator using a target-present or target-absent photographicline-up consisting of 10 persons. The results showed that accuracy was reduced by thepresence of the additional male actors, especially in the non-violent condition. Thus, thetarget man was identified by 40% of observers when acting alone but accuracy decreasedto 30% and 20% when he was accompanied by two or four men, respectively.

In a subsequent field experiment, Shepherd (1983) presented a class of studentswith two young men, who entered the classroom and asked for directions in an eventlasting for 20 s. After long retention intervals (1 month or 4 months), all witnesses werepresented with video-taped line-ups, which contained neither of the targets, only oneof the targets, or both. A correct identification rate of approximately 20% was recordedwhen the line-up contained one of the two targets, but, even more strikingly, only oneof 41 witnesses could identify both targets.

Double perpetrator crimes and accomplice gender 3

More recently, Fahsing, Ask, and Granhag (2004) conducted an archival study, whichexamined offender descriptions from 236 witnesses of 48 real bank and post officerobberies in Oslo (Norway). The results showed that the number of perpetrators wasthe best predictor of the quality and quantity of offender descriptions compared to otherevent factors such as duration, type of weapon, and retention interval. Specifically,both the accuracy and completeness of the descriptions decreased significantly in themultiple-perpetrator crimes as compared to the single-perpetrator crimes.

Taken together, all of these studies suggest that incidents involving more than one per-petrator reduce eyewitness identification accuracy. In our own work, we have recentlyreplicated these findings under strictly controlled laboratory settings (Megreya & Burton,2006). In this study, observers viewed a single face or two simultaneously presentedtarget faces until they felt confident that they could identify them subsequently. Thislearning phase was followed by a simultaneous 10-face identification line-up, in whichjust one or neither of the targets could appear. In this immediate memory test, correctidentification rates dropped from 59% for the single-target condition to just 34% fortwo-target displays. Interestingly, however, this study also showed that the basis of thistwo-face disadvantage is not wholly due to memory constraints, but lies in the difficultyof encoding unfamiliar faces in the first place. This is borne out by the observation thatthe same two-face disadvantage is also found when observers had to match a singletarget or two concurrent targets to a simultaneously presented line-up of 10 faces.These conditions reduce any memory requirement to a possible minimum, but, despitethis, correct identification rates again fell from 70% for single-target displays to only 54%when two-face targets were shown above the line-up.

Intriguingly, this two-face disadvantage was greatest when the two-face targets wereplaced close together. This indicates that identification accuracy might decrease inmultiple-perpetrator crimes because the concurrent target persons exert an influenceon each other during identity encoding. However, the means by which encoding isimpaired in the presence of multiple faces remains uncertain. One possibility is that faceencoding is disrupted by a difficulty to separate the defining characteristics of each facialidentity and, therefore, to store each identity accurately. Accordingly, there is evidencethat the integration of individual facial features (e.g., eyes, nose, mouth) into a coherentholistic percept is impaired, and identification accuracy declines, when observers areattempting to encode several faces simultaneously (Palermo & Rhodes, 2002; see alsoReinitz, Morrissey, & Demb, 1994).

Alternatively, the multiple-perpetrator disadvantage could arise from capacity limitsin face processing, whereby only one face can be encoded at a time (Bindemann, Burton,& Jenkins, 2005; Bindemann, Jenkins, & Burton, 2007; Jenkins, Lavie, & Driver, 2003).Observers would therefore need to divide attention between two or more concurrentfaces, which would leave less time available to encode each identity than when onlya single person is present. These competing explanations therefore differ in termsof the encoding that is possible when multiple perpetrators are observed. The firstexplanation implies that typical encoding processes are impaired when more than oneperson is observed, so that it becomes challenging to encode both identities as separableand integrated face percepts. The second explanation, on the other hand, impliesthat faces are encoded in a typical manner, but in insufficient depth for subsequentrecognition.

To dissociate these possibilities, the gender of a perpetrator and an accomplicewas manipulated selectively in the current study in a video-based eyewitness paradigm.This manipulation is based on evidence that face gender is extracted early during face

4 Ahmed M. Megreya and Markus Bindemann

processing and with great accuracy (Baudouin & Tiberghien, 2002; Bruce et al., 1993;O’Toole, Vetter, Troje, & Bulthoff, 1997; Rossion, 2002). Pairing male and female facestherefore provides a useful tool for examining the perceptual basis of the identificationdeficit in multiple-perpetrator crimes, by allowing observers to distinguish multipleperpetrators along this dimension. Using this manipulation, observers were presentedwith video footage depicting a male or a female perpetrator stealing a laptop froman office. The perpetrator was shown either alone or with an accomplice of thesame or the opposite gender. After a short delay, the observers were then asked toidentify the principle perpetrator from a target-present or target-absent photographicline-up.

In this paradigm, if the multiple-perpetrator effect arises from a difficulty in encodingconcurrent faces into discrete percepts, then such encoding should be facilitated whena male and female perpetrator are presented compared to the two-male or two-femalecondition, by providing separable pre-defined perceptual categories. If, on the otherhand, recognition accuracy is comparable across all conditions, then this would implythat the multiple-perpetrator effect arises from divided attention between concurrentpeople, by reducing the depth to which each person is encoded, rather than the natureof the encoding process itself.

Note that the focus of this design on male and female perpetrators also lends itselfto looking at gender effects in eyewitness identification. Sex differences have beena topic of intense interest in face identification, but previous studies have yieldedlargely inconsistent findings, including, for example, a general advantage for femaleover male observers (McBain, Norton, & Chen, 2009; Rehnman & Herlitz, 2007), aselective advantage for female observers looking at female faces (Cross, Cross, & Daly,1971; Lewin & Herlitz, 2002; Rehnman & Herlitz, 2006), but also a similar own-sex biasin men (Ellis, Shepherd, & Bruce, 1973; Wright & Sladden, 2003; for a brief review, seeMegreya, Bindemann, & Havard, 2011). These previous studies are, however, largelyconfined to face recognition paradigms (see Wilcock et al., 2008). The current studytherefore also provides an opportunity to explore these sex differences in an eyewitnesstask.

MethodParticipantsA total of 534 undergraduate students (191 males and 343 females) from MenoufiaUniversity (Egypt) volunteered to participate in the study. Ages ranged from 18 to 23years, with a mean of 19.4 years (SD = 0.7). All participants had normal or corrected-to-normal vision.

Materials

Crime videosThe stimulus material consisted of six video clips. Each video showed a young manor a young woman completing the staged theft of a laptop computer. In two of thevideos, this principal character was presented alone. In the remaining four videos, anaccomplice of the same or the opposite gender accompanied this principal character.The single-person videos began with the principal character entering an unoccupieduniversity office. At this moment, their faces were captured clearly in a frontal view.They then searched shelves, drawers, and a filing cabinet, until a laptop was found. Theperpetrator picked up the laptop and took a final look around the office before leaving.

Double perpetrator crimes and accomplice gender 5

The double-target videos included the same sequence of events, but with assistancefrom an accomplice, who helped to search the room. All videos lasted for approximately90 s and provided several opportunities to study the face of the principal perpetrator infrontal and profile views.

Note that the male and female actors who were the principal perpetrators (whopicked up the laptops) in half of these videos were used as accomplices in the otherversions of the video. Moreover, in an effort to control for distinctiveness effects in facerecognition (see, e.g., Dewhurst, Hay, & Wickham, 2005; Valentine, 1991), the principalperpetrators and the accomplices were also rated according to their distinctivenessprior to the experiment. For this purpose, 20 students judged how easy it would beto ‘pick each face from a crowd’ using a seven-point scale ranging from ‘very difficult’(typical) to ‘very easy’ (distinctive). A one-factor analysis of variance (ANOVA) showedthat these distinctiveness ratings were comparable for all faces (female target 5.6, femaleaccomplice 5.7, male target 5.5, male accomplice 5.3; F(3,57) = 1.03, p = .38). The samesubjects also rated the similarity of the female faces and the male faces to each otherusing a similar seven-point rating scale ranging from ‘very different’ to ‘very similar’. Apaired-samples t-test showed that these similarity ratings, between the male (1.9) andfemale faces (1.5), also did not differ, t(19) = 1.58, p = .13. All of the perpetratorsand accomplices therefore appeared evenly matched in terms of distinctiveness andsimilarity.

Line-upsFor the line-up recognition test, four simultaneous line-ups were created that consistedof 10 unfamiliar faces. In two of these line-ups, the face of the principal perpetrator fromthe video was present among the line-up but in the other two line-ups, this target personwas absent. The line-up photographs of the target faces were captured immediately afterthe crime videos were recorded, using a high-quality digital camera and good lightingconditions. Note that all female actors (the principal target and the accomplices) werewearing headscarves in the videos and photographs that covered the hair and ears but leftthe central area of the face on display. This represents the typical cultural headdress ofEgyptian females. Egyptian observers process these faces quantitatively and qualitativelysimilarly to male faces, in which all facial features are visible (Megreya & Bindemann,2009).

In addition to these faces, 20 foil images were taken from Egyptian databases ofunfamiliar male faces (Megreya & Burton, 2008) and unfamiliar female faces (Megreya& Bindemann, 2009) for the construction of the line-up arrays. To concord with policeregulations for constructing fair identity parades (see, e.g., Police and Criminal EvidenceAct 1984, Code D), these foils were chosen by the primary investigator to resemble thesubjects in age and general appearance. Notably, all of the foil faces were captured by thesame digital camera, by which the target photographs were taken and under the samelighting conditions. This manipulation is important to minimize any superficial pictorialdiscrepancies between the line-up members. In addition, all face images were shown inthe same full-face view, with a neutral expression, in greyscale colour tones, and at a sizeof approximately 5 × 7 cm. Note that the line-up images of the female faces (includingthe principal target) were also wearing a standard headscarf, which was different to theheadscarves seen on the videos. Once again, the purpose of this procedure was to avoidrecognition by pictorial cues that are unrelated to facial information. Video stills of theactors (targets and accomplices) and the target-present line-ups are shown in Figures 1and 2.

6 Ahmed M. Megreya and Markus Bindemann

Figure 1. A demonstration of the stimuli used in the study. The top images are video stills showingonly the principal male perpetrator (A), and the principal perpetrator with an accomplice of the same(B) or the opposite (C) gender. The target-present line-up shown below consisted of digital imagestaken at the same time as the videos.

DesignThe study employed a 2 (participant gender: female vs. male) × 2 (target gender:female vs. male) × 3 (conditions: single perpetrator vs. double perpetrator withaccomplice of the same gender vs. double perpetrator with accomplice of the oppositegender) × 2 (line-up type: target present vs. target absent) between-participant design.The line-up identification decisions were the dependent variables. Specifically, therewere three separate measures for target-present line-ups: hits (identification of thecorrect target from the line-up), misses (a target-absent response when the target isin fact present), and misidentifications (identification of an incorrect face from the line-up). In addition, two measures were calculated from the target-absent line-ups: correctrejections (correctly responding that the target is absent from the line-up) and falsepositives (FPs: identification of an incorrect face from the line-up). Note that performanceon target-present trials is dissociable from target-absent trials, so the data from themeasures were analysed separately for these line-up categories (see Megreya & Burton,2007).

Double perpetrator crimes and accomplice gender 7

Figure 2. A demonstration of the stimuli used in the study. The top images are video stills showingonly the principal female perpetrator (A), and the principal perpetrator with an accomplice of the same(B) or the opposite (C) gender. The target-present line-up shown below consisted of digital imagestaken at the same time as the videos.

ProcedureThe experiment was run in a teaching room at Menoufia University that was equippedwith data projection facilities. Testing was conducted in groups of 10–15 participants,and seating was arranged so that each participant had a clear view of the screen. Thetesting sessions consisted of three phases. First, participants viewed one of the six crimevideos. They were asked to watch these videos carefully and to expect a subsequentrecognition test. They were then asked to complete a questionnaire as a filler task, whichlasted for approximately 10 min. Finally, all participants were given an identification testconsisting of a single 10-face identity line-up. In this test, a target-present or a target-absentphotographic line-up was projected onto the screen. Participants were given unbiasedtask instructions concerning that the perpetrators on the previously seen videos mightor might not be present in the line-up. Their task was to decide whether they werepresent, and if so, to identify them by recording the number of the appropriate facefrom the line-up in a response sheet. To avoid confusion regarding which person had tobe identified following double-perpetrator videos, participants were instructed clearly

8 Ahmed M. Megreya and Markus Bindemann

to identify the main perpetrator committing the laptop theft, and not the accomplicethat was also depicted in the videos.

ResultsPerformance on target-present line-upsTable 1 shows the participants’ responses for target-present line-ups. Performance inthis task is expressed as a percentage of the number of observers who selected eachof the response categories (N = 296). A hierarchical log-linear (HILOG) analysis wasconducted with participant gender (females vs. males), target gender (females vs. males),the number of perpetrators (single vs. double with same gender accomplice vs. doublewith opposite gender accomplice), and the identification decisions (hits, misses, andmisidentifications) as factors. This analysis revealed that identification decisions werenot affected by the gender of the participants [� 2 (2) = 0.10, p = .95] or the gender ofthe target faces [� 2 (2) = 4.37, p = .11], and there was no interaction between thesethree factors [� 2 (2) = 0.08, p = .96]. Importantly, however, an interaction between thenumber of perpetrators and the identification decisions was found [� 2 (4) = 19.22, p =.001]. All other interactions were not significant.

The follow-up chi-square test revealed that participants were less accurate in thedouble-perpetrator condition compared to the single-perpetrator condition [� 2 (4) =19.52, p = .001]. However, as the chi-square test is additive (Castellan, 1965), thecontingency table of the target-present data was partitioned further for providing astronger analysis. For this purpose, the data from male and female targets were collapsed,and the data from misses and misidentifications were also combined, to produce a 2(single vs. double perpetrator) × 2 (correct vs. incorrect identifications) contingencytable. This data shows that fewer correct identifications (29%) and more incorrectdecisions (71%) were made in the double-perpetrator condition compared with thesingle-perpetrator condition (54% and 46%, respectively). Yates chi-square test confirmedthat these differences are significant [� 2 (1) = 16.85, p < .001].

Performance on target-absent line-upsTable 2 shows the percentage responses for target-absent line-ups (N = 238). A similarHILOG analyses did not reveal any significant main effect or interactions. This indicatesthat the gender of the participants [� 2 (1) = 0.01, p = .93] and the gender of the targets[� 2 (1) = 0.40, p = .52] did not affect identification decisions, and these factors alsodid not interact with each other [� 2 (1) = 0.08, p = .77]. In addition, an interaction

Table 1. The percentages of responses in the target-present line-ups (frequencies in parentheses)

Hits Misses Misidentifications

Male targetSingle 48 (24) 30 (15) 22 (11)Male accomplice 20 (10) 50 (25) 30 (15)Female accomplice 28.6 (16) 37.5 (21) 33.9 (19)

Female targetSingle 59.6 (31) 21.1 (11) 19.2 (10)Female accomplice 37.8 (17) 42.2 (19) 20 (9)Male accomplice 30.2 (13) 41.9 (18) 27.9 (12)

Double perpetrator crimes and accomplice gender 9

Table 2. The percentages of responses in the target-absent line-ups (frequencies in parentheses)

Correct rejections False positives

Male targetSingle 67.5 (27) 32.5 (13)Male accomplice 59.5 (25) 40.5 (17)Female accomplice 68.4 (26) 31.6 (12)

Female targetSingle 61.4 (27) 38.6 (17)Female accomplice 57.9 (22) 42.1 (16)Male accomplice 63.9 (23) 36.1 (13)

between identification decisions and the number of perpetrators was also not found [� 2

(2) = 1.0, p = .60].

Choosers versus non-choosersFollowing Havard and Memon (2009), the overall choosing behaviour of the observerswas also examined by collapsing data from target-present and target-absent line-ups andfor male and female target trials. A chooser was defined as someone who selected aface from a target-present or a target-absent line-up, regardless of whether this decisionwas correct (hits) or incorrect (misidentifications and FPs). A non-chooser, on the otherhand, was defined as someone who did not choose a line-up face, regardless of whetherthis rejection was correct (CR) or incorrect (misses). The numbers of choosers (38.5%,30.6%, and 30.9%) and non-choosers (30.9%, 35.1%, and 34%) appeared evenly matchedacross the three experimental conditions: single, double with same gender accomplice,and double with opposite gender accomplice. This observation was confirmed by a3 × 2 chi-square test, which found no differences in choosing behaviour between theconditions [� 2 (1, N = 534) = 1.63, p = .20].

General DiscussionThe main aim of this study was to examine the effect of the number of perpetrators oneyewitness identification accuracy by systematically varying the gender of a principalperpetrator and of an accomplice. Participants witnessed a staged crime, which wascommitted either by a single perpetrator (male or female), or by the perpetrator andan accomplice of the same gender, or by the perpetrator and an accomplice of theopposite gender. After a brief filler task, all eyewitnesses were then asked to identifythe perpetrators from photographic 10-face identification line-ups, in which the targetscould be present or absent. The results showed that double-perpetrator crimes lead tofewer correct person identifications than single-perpetrator crimes. Importantly, thiswas found regardless of whether the accomplice was of the same or the opposite genderof the principal perpetrator. These effects were confined to target-present line-ups; thenumber of perpetrators had no effect on identification accuracy for line-ups in whichthe target was not present.

Considering first the baselines of identification accuracy for single-perpetrator crimes,the Egyptian male and female perpetrators were correctly identified by only 48% and 60%of witnesses in target-present line-ups, and the same faces were missed by 30% and 21%

10 Ahmed M. Megreya and Markus Bindemann

of witnesses and misidentified by 22% and 19%, respectively. In target-absent line-ups,approximately 30% and 40% of witnesses also falsely identified a male or female foil.This low level of performance converges with the well-documented identification ratesin many Western countries (for reviews see, e.g., Cutler & Penrod, 1995; Memon et al.,2003; Wells & Olson, 2003; Wilcock et al., 2008) and suggests that the unreliability ofeyewitness identification is an international phenomenon.

This low accuracy is even more problematic considering that eyewitness performancedeteriorated further when the crime involved more than one perpetrator, especiallyin the target-present line-ups. Under these conditions, the principal male and femaletargets in the double-perpetrator crimes were correctly identified only by a quarter anda third of eyewitnesses, respectively. This double-perpetrator disadvantage replicatesprevious laboratory-based research, which employed a variety of face processing tasks(Megreya & Burton, 2006), as well as eyewitness identification paradigms (Clifford &Hollin, 1981). However, the present study adds a novel finding to this literature, whichis that the double-perpetrator disadvantage persisted regardless of whether the genderof a perpetrator and an accomplice is the same or different.

This is an important finding. Previous research demonstrates that the encoding ofgender information from faces is an easy and robust process (for a review, see, e.g.,Bruce & Young, 1986). It follows that pairing male and female faces in the currentvideo displays should not be subject to identity interference, because of the structuraldifferences between these faces, so that faces of opposite gender should be encoded asseparate units (Brown & Perrett, 1993; Burton, Bruce & Dench, 1993). This suggests thatthe double-perpetrator disadvantage is not simply due to a difficulty in separating thefacial characteristics of two people that are encountered simultaneously, which thereforecannot be encoded and stored accurately (see Palermo & Rhodes, 2002). Instead, wesuggest that this effect might reflect the need to divide attention between two concurrentculprits, which limits the depth or detail of the stored descriptions that can be formedfor these persons (see Fahsing et al., 2004).

This suggestion receives some support from the literature on divided attention, whichshows that the explicit (Baddeley, Lewis, Eldrige, & Thomson, 1984; Craik, Govoni,Naveh-Benjamin, & Anderson, 1996; Dewhurst, Barry, Swannell, Holmes, & Bathurst,2007) and implicit (Light, Prull, & Kennison, 2000; Mulligan, 1998) recognition of anystudied items is greatly impaired if subjects perform a distractor task during the initiallearning phase. Theorists have explained this damaging effect by arguing that attentionis imperative for visual encoding and for elaborating the semantic properties of stimulithat are used for subsequent recognition, but dividing attention reduces the availableresources for these processes (see, e.g., Craik et al., 1996; Mulligan, 1998). There isincreasing evidence that similar constraints apply to face identification. This includesthe discovery of capacity limits in face processing, such that only a single face can beaccommodated at a time (Bindemann et al., 2005, 2007; Burton, Bindemann, Langton,Schweinberger, & Jenkins, 2009; Jenkins et al., 2003), as well as the finding that dividedattention reduces explicit memory for faces (Jenkins, Burton, & Ellis, 2002; Jenkins,Lavie, & Driver, 2005). These findings provide a potential explanation for our results,even if the current study still falls short of delineating the exact cause of these effects.However, by drawing on research on capacity limits in face processing (Bindemannet al., 2005, 2007; Jenkins et al., 2003) and divided attention (e.g., Dewhurst et al.,2007; Mulligan, 1998), we propose that the resolution to this phenomenon might lie inthe domain of cognitive psychology.

Double perpetrator crimes and accomplice gender 11

To this point, another issue for future research is that the double-perpetratordisadvantage was noticed only on target-present line-ups, while FP rates were similarin the single- and double-perpetrator crimes. This is consistent with a variety of face-processing tasks including immediate memory and perceptual-matching paradigms(Megreya & Burton, 2006), in which the two-face disadvantage was observed onlyin target-present line-ups. By contrast, the FP rates were very high in the single-faceconditions (roughly 25% across those memory and matching tasks), but accuracy wasnot reduced further by the addition of a second target face. Importantly, however,Megreya and Burton (2006) also found that the sequential presentation of two-facetargets produced a significant increase in FPs compared to the simultaneous presentation.Accordingly, Megreya and Burton (2006) suggest that observers exhibit a general biasto identify a face, regardless of the range of targets currently available, but that thesequential presentation of targets lowers the criterion for choosing a face. The analysisof choosers versus non-choosers in the present study partially supports this suggestion,as the rate of choices was equivalent for the single- and double-perpetrator crimes, wherethe two perpetrators were presented simultaneously.

It is of course conceivable that a range of factors determines the rate of FPs on target-absent trials. Akin to the target-present conditions, one might have expected also thatdouble-perpetrator crimes increase rates of FPs on target-absent trials, as more line-upfaces could provide potential matches to the target identities compared to when only asingle person was observed. On the other hand, multiple-perpetrator crimes might alsoincrease an observer’s reluctance to choose a line-up face by amplifying the perceiveddifficulty, and therefore the fallibility, of this task. If accuracy in target-absent conditionsis determined by such conflicting factors, then this might counteract a double-perpetratoreffect for these line-ups here. At present, these explanations are clearly speculative buthighlight that many aspects of eyewitness identification, and of the multiple-perpetratoreffect, still need to be further understood.

Another potential issue for future research concerns the use of female headscarvesin the current study. It is conceivable that these headscarves facilitate the distinc-tion between male and female perpetrators, which may have affected the outcomeof this study. However, this seems unlikely as line-up identification accuracy wascomparable regardless of whether two males (no headscarves), two females (twoheadscarves), or a male and a female perpetrator (one headscarf) were observed.To investigate this possibility further, a control study was also conducted in whichobservers were asked to classify the sex of male and female faces that were shownwith and without headscarves. This showed that headscarves do not improve theaccuracy of sex classifications and, if anything, reduce the speed with which thesedecisions are made.1 Our previous research also indicates that this cultural dressexerts a broad effect, which increases sensitivity to internal facial features in fe-male models with headscarves and male models without (Megreya & Bindemann,2009). However, the use of female models with headscarves might also reflect a

1In this control study, 78 participants (36 males) classified the sex of 15 male and 15 female faces, which were all depictedwith and without female headscarves. Across conditions, accuracy ranged from 91–96% (mean SD = 6%). A 2 (male vs.female observers) × 2 (male vs. female faces) × 2 (headscarf vs. no headscarf) ANOVA showed an effect of face sex,F(1,76) = 9.49, p � .01, due to higher accuracy for male faces (95%) than female faces (92%). The remaining main effectsand all interactions were not significant. Analogous analysis of mean correct reaction times showed an interaction of facesex and headscarf, F(1,76) = 8.22, p � .01. For faces without headscarves, responses were faster for male (707 ms) thanfemale faces (764 ms), F(1,152) = 13.87, p � .001, while performance was similar for male (781 ms) and female faces(787 ms) with headscarves, F(1,152) � 1. The remaining interactions were not significant.

12 Ahmed M. Megreya and Markus Bindemann

potential strength of this research, by extending the multiple-perpetrator effect to anauthentic sample of Middle Eastern participants (see Henrich, Heine, & Norenzayan,2010).

Finally, this study also lent itself to studying sex differences in eyewitness identifi-cation. Previous studies provide a mixed view of sex differences in face recognition,as a wide range of patterns has been observed (e.g., Cross et al., 1971; Ellis et al.,1973; Lewin & Herlitz, 2002; McBain et al., 2009; Rehnman & Herlitz, 2006, 2007;Wright & Sladden, 2003). Overall, this evidence is pointing to a recognition advantagein female observers and especially for viewing female faces (see Megreya et al., 2011),but it is less clear if this persists in eyewitness identification (Shapiro & Penrod, 1986;Wilcock et al., 2008). In this study, we failed to find any gender effects, despite a largesample size, confirming the results of a recent study by Megreya, Memon, and Havard(2011). A possible explanation for this discrepancy is that any gender effects are simplyoutweighed by the large individual differences that exist between observers, beyonda person’s sex (see, e.g., Burton, White, & McNeill, 2010; Darling, Martin, Hellman, &Memon, 2009; Russell, Duchaine, & Nakayama, 2009). If so, this would suggest that theseindividual differences provide a much more important theme for future research than sexdifferences.

In addition to these theoretical considerations, the double-perpetrator disadvantagehas important implications for the legal system. The current findings suggest, for instance,that the criminal justice system must exercise great caution when the reliability ofeyewitness identification is evaluated in multiple-perpetrator crimes. This disadvantagealso presents serious challenges for the line-up identification procedures that shouldbe used in double-perpetrator crimes. A meta-analysis by Steblay et al. (2001) suggeststhat sequential line-ups are not the answer, and some researchers are developing newidentification techniques for crimes involving more than one perpetrator (Dempsey &Pozzulo, 2008; Wells & Pozzulo, 2006), but the success of these new techniques isstill limited. Which perpetrator should be involved in the line-up is another importantquestion, and studies examining identification accuracy for the principal perpetratoras compared to the accomplice are largely mixed (Dempsey & Pozzulo, 2008; Wells& Pozzulo, 2006). In addition, it also remains difficult to involve the two perpetratorsin the same identity line-up, as this manipulation raises statistical concerns. These arechallenges that await innovative breakthroughs.

ConclusionsThis study investigated the number of perpetrators encountered in a crime and theirgender as a possible source of eyewitness identification errors. The results revealed adouble-perpetrator disadvantage (see Clifford & Hollin, 1981; Megreya & Burton, 2006),with no effect for perpetrators’ gender. We therefore suggest that the double-perpetratordisadvantage does not arise from a difficulty in encoding and storing the identities oftwo perpetrators separately. Rather, this effect might originate from a need to divideattention between two concurrent people, compared to single-culprit crimes, whichreduces the depth or detail to which faces are encoded.

AcknowledgementWe would like to thank Catriona Havard for her advice for analyzing this study.

Double perpetrator crimes and accomplice gender 13

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Received 8 November 2010; revised version received 18 August 2011