Interrogative Suggestibility among Witnesses with Mild Intellectual Disabilities: the Use of an Adaptation of the GSS

Interrogative Suggestibility amongWitnesses with Mild IntellectualDisabilities: the Use of an Adaptationof the GSS

Rebecca MilneInstitute of Criminal Justice Studies, University of Portsmouth, Portsmouth,

Hampshire, UK

Isabel C. H. ClareDepartment of Psychiatry, Section of Developmental Psychiatry, University of

Cambridge

Ray BullDepartment of Psychology, University of Portsmouth, Portsmouth, Hampshire, UK

Paper accepted November 2001

Background As part of the assessment of witnesses’ ability to provide an account to the policeand the courts, information is sometimes sought concerning their level of interrogative suggest-ibility. The most widely used measure for this is the Gudjonsson Suggestibility Scale (GSS,Gudjonsson 1997), which has two parallel forms (GSS 1 and GSS 2). However, the GSS relates toa verbally presented narrative, not to a visual event, as is more common to witness situations.Methods The present study adapted the scale’s format so that the questions referred to a video-taped incident that had been viewed 24 h earlier by men and women with mild intellectualdisabilities (n¼ 47) and their ‘general population’ counterparts (n¼ 38).Results The pattern of results was identical to that typically obtained using the GSS in that:(1) compared with their general population counterparts, the participants with intellectualdisabilities were more suggestible because of their vulnerability to the ‘misleading questions’;(2) suggestibility scores correlated with the participants’ verbal recall of the incident, and(3) both participants with intellectual disabilities and their general population counterpartswho were misled by questions in the form of two false alternatives were more likely to select thelatter option.Conclusions The implications of these findings for psychological assessments of potentialwitnesses are discussed.

Introduction

There is now, at last, in England and Wales, official acknowledgement that the witness

evidence of people with intellectual disabilities may be crucial for police investigations

and the prosecution of offenders (Sanders et al. 1997; Bull 1999; Kebbell & Hatton 1999;

Gudjonsson et al. 2000; Milne & Bull 2002). Increasingly, therefore, criminal justice

system personnel are seeking advice from health and social care practitioners, and

particularly psychologists, to help these witnesses in providing information to the police

that can, in turn, be used in court. This is especially so since the implementation of the

Journal of Applied Research in Intellectual Disabilities 2002, 15, 8–17

8 # 2002 BILD Publications

Youth Justice & Criminal Evidence Act 1999, which provides, for the first time, for the use

of ‘special measures’ to assist ‘vulnerable’ witnesses or victims, such as men and women

with intellectual disabilities, to give evidence at interview and in the courtroom.

It has been argued (e.g. Kebbell & Wagstaff 1999) that to be credible, a witness must be

both willing and able to provide information that is accurate, reliable, and complete.

Willingness refers to the witness’s motivation to report the truth, whilst ability, which is

the focus of interest here, refers to the capacity to recall and recount accurately the event

in question. In this context, capacity is likely to reflect the interaction of a number of

factors. These include the details of the alleged offence (Kebbell & Wagstaff 1999),

personal characteristics relating to the witness (such as his or her cognitive functioning,

personality, and mental state; see reviews by Gudjonsson 1992, 1993), and the strategies

used by the police and other criminal justice personnel to elicit information (Milne &

Bull 1999).

Among the personal characteristics that have been identified as being of potential

relevance to a witness’s capacity to provide accurate, reliable, and complete information,

the concept of ‘interrogative suggestibility’ has been of particular theoretical and

practical importance (see reviews by Gudjonsson 1992, 1997; Clare & Gudjonsson

1995). Interrogative suggestibility has been defined (Gudjonsson & Clark 1986) as

‘the extent to which, within a closed social interaction, people come to accept messages

communicated during formal questioning, as a result of which their subsequent beha-

vioural response is affected’ (p. 84). It has been conceptualized as comprising two

distinct vulnerabilities; (i) to ‘Yield’ to questions that are leading because they indicate

the answer that is required; and (ii) to ‘Shift’ initial answers in response to ‘negative

feedback’ from the interviewer.

These vulnerabilities can be measured using either of the parallel versions of the

Gudjonsson Suggestibility Scale (GSS 1 and GSS 2, Gudjonsson 1984, 1987, 1997), a

behavioural test constructed particularly for the assessment of suggestibility that has

been validated extensively (see Gudjonsson 1997). Both scales comprise a narrative

passage, which is read to the participant to obtain measures of free recall. Subsequently,

20 questions, 15 of which are misleading (i.e. they infer the incorrect response, including

five false alternative questions), are presented. The interviewee’s ‘Yield’ score (i.e. the

number of questions to which he or she gives an incorrect response by ‘going along’ with

the premise of the question) is determined. These items are interspersed with five non-

leading (true) questions. Regardless of the accuracy of the responses given by the

interviewee, the interviewer then provides ‘negative feedback’ (i.e. the interviewer

informs the interviewee that they have provided a number of incorrect responses

and that they must be more accurate in answering the questions when repeated) and

insists that it is necessary to present the questions a second time. The extent to which the

interviewee subsequently changes their initial answers to these 20 questions in response

to this ‘negative feedback’ from the interviewer results in their ‘Shift’ score. ‘Total

suggestibility’ comprises the sum of the ‘Yield’ and ‘Shift’ scores.

A number of studies have found that, compared with their counterparts of average

intellectual ability, people with intellectual disabilities have more limited memories and

are more vulnerable to questions that are ‘misleading’ (Tully & Cahill 1984; Clare &

Gudjonsson 1993; Gudjonsson & Clare 1995; Cardone & Dent 1996; Gudjonsson 1997;

Everington & Fulero 1999). Consistent with their vulnerability to acquiescence (answer-

ing ‘yes’ to closed ‘yes’/‘no’ questions, Clare & Gudjonsson 1993; Heal & Sigelman 1995),

to which the misleading questions are conceptually related (Gudjonsson 1997), people

Journal of Applied Research in Intellectual Disabilities 9

# 2002 BILD Publications, Journal of Applied Research in Intellectual Disabilities, 15, 8–17

with mild intellectual disabilities are more likely to ‘Yield’ than their general population

counterparts (Clare & Gudjonsson 1993; Cardone & Dent 1996). However, there is no

significant difference between the two groups in their response to ‘negative feedback’

(Clare & Gudjonsson 1993; Cardone & Dent 1996; Gudjonsson et al. 2000). Whilst, in part,

this probably reflects a ‘ceiling’ effect resulting from the very high initial ‘Yield’

(Gudjonsson 1990), from clinical experience, it sometimes appears that, having been

misled initially, people with intellectual disabilities try to cope with the feedback by

simply repeating the response they gave initially.

Using the GSS (Form 1), Tully & Cahill (1984) found that suggestibility scores

predicted the accuracy of the accounts of witnesses with intellectual disabilities during

police interviewing. Though they only examined ‘Total suggestibility’, rather than

‘Yield’ and ‘Shift’ separately, ‘Total suggestibility’ scores (i.e. ‘Yield’ plus ‘Shift’ scores)

obtained before a staged incident correlated negatively with the number of items of

correct information provided by the participants and positively with the amount of

erroneous information. This implies that suggestibility is, at least in part, a cognitive

phenomenon.

The GSS has been criticised (Baxter 1990; Cardone & Dent 1996), because it is based on

a verbally presented story that is quite unlike most real criminal incidents, perhaps

limiting its validity. In addition, it has been suggested (Cardone & Dent 1996) that the use

of the GSS may overestimate the suggestibility of men and women with intellectual

disabilities because multimodal input (e.g. visual and auditory such as in a real life

event) as opposed to unimodal input (used in the GSS) may aid the encoding and recall of

memory traces (Kosslyn & Koenig 1992), leading to more complete and accurate recall.

Since vulnerability to misleading questions is normally (negatively) related to recall (e.g.

Tully & Cahill 1984), it would be expected that material presented in more than one

modality would increase participants’ resistance to misleading questions (i.e. lead to

lower ‘Yield’ scores). In contrast, ‘Shift’, which is thought to be more closely related to

anxiety and coping mechanisms (Sharrock & Gudjonsson 1993), is less likely to be

affected by presentation in more than one modality. In support of this suggestion,

Cardone & Dent (1996) found that presenting the GSS visually as well as verbally

improved the completeness and accuracy of recall and reduced ‘Yield’ among people

with mild intellectual disabilities, but it had no effect on ‘Shift’ scores. However, the

visual presentation in this study merely comprised 17 slides highlighting key points in

the verbal story; therefore, it has limited ecological validity.

The aim of the present study was to examine the impact on suggestibility among

adults with mild intellectual disabilities and their counterparts in the general population

of a multimodal (visual and verbal) presentation with improved ecological validity. It

was expected that, notwithstanding the change in format, the intellectually disadvan-

taged participants would be more suggestible than their peers and that there would be a

negative relationship between recall and susceptibility to misleading questions.

Method

Participants

There were two groups of participants, one of which comprised an administrative

sample of 34 men and 13 women (mean age: 35 years; range: 19–59 years) attending six

designated day-services for people with mild intellectual disabilities. Whilst it was not

10 Journal of Applied Research in Intellectual Disabilities


possible to assess the intellectual functioning of the participants, all of them were able to

express themselves verbally, using sentences. As far as was known, none of them lived

entirely independently. Since administrative samples may differ according to local

criteria for service eligibility, the centres from which the participants were recruited

were based in two distant geographical locations (Portsmouth and London).

The other group comprised 13 men and 25 women (mean age: 39 years; range: 19–

62 years) from Cambridge, recruited from a local job centre and through a newspaper

advertisement. Many of the participants were employed and the majority had academic

qualifications.

In both groups, most participants were of white, British ethnic origin.

Event

A 3 min video recording, edited from an educational film about road safety, was used.

The recording showed a boy, Jamie, knocked down whilst crossing a road on his way to

school by a man driving a yellow car. The event was chosen because it was dramatic,

realistic without being upsetting, and suitable for viewers from the two groups. It

contained visual and auditory information.

Adaptation of the GSS (see Appendix 1)

A suggestibility measure, based on the Gudjonsson Suggestibility Scale, was devised.

The adapted scale contained 12 questions about the video-recording. Four of these were

misleading, in that they created an expectation of an affirmative answer that would be an

incorrect response (e.g. ‘Was the man in the yellow car driving faster than the speed

limit?’ when the viewer had no information about either the driver’s speed or the speed

limit for the road), another four were false alternatives (or forced-choice questions), where

both the alternatives were wrong (for example, ‘Did the boy have a bandage on his head

or his arm?’ when no bandage was shown). These questions were interspersed with four

non-leading questions relating to information which was shown in the film (e.g. ‘Was the

boy’s name Jamie?’).

Procedure

The video-taped event was shown on a television set to small groups of participants.

The participants were asked to watch and listen carefully because they would be

asked later about what they thought of the film. Approximately, 24 h later, each

participant was interviewed individually as part of a larger study comparing the effect

on the recall of witnesses of different types of questioning (see Milne et al. 1999). At

the end of the interview phase of the study, the adaptation of the GSS scale was

presented, following the instructions provided by Gudjonsson (1997, p. 11). Each

question was read out clearly, in turn, and a response sought. At the end, regardless

of the participant’s responses, he or she was given ‘negative feedback’ by being told

firmly: ‘You have made a number of errors. It is therefore, necessary to go through the

questions once more, and this time try to be more accurate.’ The questions were repeated.

Each person was told that they had done well, and they were paid a standard fee for

participating. The responses to the questions prior to, and following the negative

feedback, were scored.



Scoring

Adaptation of the GSS

The responses to this measure were scored in two ways. Firstly, the scoring criteria

devised for the GSS (Forms 1 and 2, Gudjonsson 1997) were used to provide ‘Yields 1 and

2’, ‘Shift’, and ‘Total suggestibility’ scores, as follows:

1 ‘Yield 1’. Each of the eight misleading questions which was answered affirmatively, or

for which a false alternative was chosen, prior to the negative feedback, was scored.

Scores on ‘Yield 1’ could range from zero to eight.

2 ‘Yield 2’. This was similar to ‘Yield 1’, but it referred to the answers given after the

negative feedback. It indicated the impact of the feedback on each partic-

ipant’s vulnerability to misleading questions. Scores on ‘Yield 2’ could range from zero

to eight.

3 ‘Shift’. A distinct change in response to any of the repeated 12 questions follow-

ing the negative feedback was scored. The range of possible scores was, therefore,

between zero and 12.

4 ‘Total suggestibility’. The range of possible scores for ‘Total suggestibility’ (i.e. the sum

of ‘Yield 1’ and ‘Shift’) was between zero and 20.

Secondly, each participant’s response to each misleading and false alternative ques-

tion was scored as ‘misled’ (as an incorrect response, as for ‘Yield 1’; range: 0–8); as

‘resistant’ (a definite negative response, i.e. a correct response; range: 0–8), or as ‘don’t

know’ (range: 0–8).

It must be noted that for each participant raw scores are expressed as a percentage of

the total possible score (see Tables 1 and 3).

Interview data

The reported information elicited during the interview phase of the study was

coded as either ‘correct’, ‘incorrect’ or a ‘confabulation’. A piece of information

was coded as incorrect (or ‘distortion’) if the ‘recalled’ detail was discrepant with the

relevant detail in the film (e.g. red jumper instead of black jumper; an error concerning

colour). If a detail was mentioned that was not present in the film, it was coded

as a confabulation (or ‘fabrication’; for further details of the scoring see Milne

et al. 1999).

Results

Comparison of suggestibility between the two groups

Table 1 shows the mean percentage scores for the two groups on all six suggestibility

variables. The mean percentage scores were compared statistically using analysis of

variance.

As expected, compared with their ‘general population’ counterparts, the group of

participants with mild intellectual disabilities were more suggestible. The difference in

‘Total suggestibility’ between the two groups arose from the greater susceptibility to

‘misleading questions’ of the ‘intellectual disabilities’ group. Initially, they were sig-

nificantly less likely to provide ‘resistant’ or ‘don’t know’ responses to the misleading

questions. However, there was no significant difference in their ‘Shift’ scores, indicating

that they did not respond differently to ‘negative feedback’.



The effect of recall on suggestibility

Correlation analyses (Pearson’s Product Moment) were conducted to see if recall in the

preceding interview was related to the responses to the suggestive questions. The

analysis took the form of correlating each type of response to the suggestibility scale

(i.e. Yield 1 and 2, etc.) with the interview recall scores for total correct, total incorrect and

total confabulation (where mean correct recall for intellectual disabilities¼ 87, and

general population (GP)¼ 181; where mean number of incorrect details for intellectual

disabilities¼ 14 and GP¼ 17; and mean confabulations for intellectual disabilities¼ 18,

and GP¼ 8; see Milne et al. 1999 for full interview data and analysis).

Table 2 shows that whilst there was no significant relationship between the number of

incorrect items recalled and any of the components of suggestibility, participants who

recalled more correct information in the preceding interview were less susceptible to

misleading questions both prior to, and following, negative feedback. They were also

much more likely to give ‘don’t know’ responses. In addition, the number of confabula-

tions during the preceding interview correlated with susceptibility to misleading ques-

tions both prior to, and following, feedback and difficulty in providing a ‘don’t know’

response. A further analysis was then carried out.

Position bias in incorrect responses to the false alternatives

Chi-squared analysis was conducted to examine which of the two options (i.e. first or

last), were chosen by the participants who were misled (as opposed to resisting or giving

Table 1 Mean percentage suggestibility scores for the two groups of participants

Suggestibility score

Population type

Intellectual disabilities General population

Yield 1 61�� 30��

Yield 2 64�� 32��

Shift 22 19Total suggestibility score 40�� 25��

Resistance 17�� 31��

Don’t know 10�� 27��

��P < 0.0001.

Table 2 Correlation between recall and responses to the suggestibility scale

Suggestibility

Recall

Correct Incorrect Confabulations

Yield 1 �0.417�� ns 0.318��

Yield 2 �0.444�� ns 0.272��

TSS �0.323�� ns nsNot misled 0.353�� ns �0.236�

Don’t know 0.291�� ns �0.246�

TSS is the total suggestibility score; �P < 0.05, ��P < 0.01, ��P < 0.001, ��P < 0.0001.



a ‘don’t know’ response). Participants (both adults with intellectual disabilities and those

from the general population) were significantly more likely to pick the last option

(w2¼ 24, df¼ 1, P < 0.001).

Discussion

The main aim of the present study was to compare suggestibility among adults with mild

intellectual disabilities and their counterparts in the general population, using an

adaptation of an established measure (the GSS, Gudjonsson et al. 1997) with a visual

and auditory event. In addition, it was expected that there would be a negative

relationship between recall and susceptibility to misleading questions.

As expected, adults with intellectual disabilities were more suggestible on all mea-

sures, and less resistant to misleading questions than their counterparts from the general

population. This is similar to previous research which has found that, compared with

their peers, men and women who are intellectually disadvantaged are more susceptible

to misleading questions (e.g. Clare & Gudjonsson 1993; Gudjonsson & Clare 1995;

Everington & Fulero 1999). However, consistent with previous findings (Clare &

Gudjonsson 1993; Cardone & Dent 1996), there was no difference between the two

groups in the extent to which they changed their responses following ‘negative feedback’

(‘Shift’).

These findings can be explained within Gudjonsson & Clark’s (1986) theoretical model

of interrogative suggestibility. This is based on a socio-cognitive perspective, where

suggestibility is dependant upon the coping strategies that people use when faced

with the ‘uncertainty’ and ‘expectations’ of the interrogative situation. In their dichot-

omous model, interrogative suggestibility comprises two distinct susceptibilities (i) to

‘Yield’ to leading questions, which relates to the reliability of testimony and closely

reflects memory processes; and (ii) to ‘Shift’, the change from the initial answer in

response to ‘negative feedback’, where ‘Shift’ is related to coping processes which are

particularly affected by certain personality characteristics and experience (e.g. Gudjons-

son 1992). As would be predicted from their model, in the present study ‘Yield 1’ and

‘Yield 2’ and the ‘Total suggestibility’ scores were negatively related to correct recall

and positively related to confabulations reported in the preceding interview, and

seemed to reflect cognitive factors. However, in contrast, ‘Shift’ scores were not,

supporting the suggestion that ‘Shift’ primarily reflects personality characteristics

and social experience.

Interestingly, it was also found that when adults with intellectual disabilities

responded to false alternatives (i.e. questions that do not contain the correct alternative)

Table 3 Percentage forced-choice question responses for the two groups of participants

Response

Population group

Intellectual disabilities General population

First 57 19Last 96 54Resistance 29 42Don’t know 22 45



they chose the last option, i.e. have a last-option bias. This is to some extent consistent

with Prosser & Bromley’s (1998) suggestion that this reflects the difficulties of adults with

intellectual disabilities in remembering the reply choices. However, in the present study,

the general population also showed this last-option bias. The implication is that ques-

tions of this type should be avoided when interviewing any interviewee, regardless of

intellectual disability.

It would have been useful to have included a greater number of different formats of the

to-be-remembered material. At the same time, it would have been helpful to involve

‘general population’ participants who were more representative of this group. Despite

recruiting primarily through a job centre, some of our participants had undergraduate

and even postgraduate degrees. Nevertheless, the similarity between the pattern of our

findings and those obtained by Gudjonsson (1984 et seq.; see Gudjonsson 1997 for review)

indicates that, despite its apparent lack of ecological validity, the Gudjonsson Suggest-

ibility Scale seems a useful measure of vulnerability to misleading questions and

interrogative pressure. It should be maintained as part of the psychological assessment

of potential witnesses with intellectual disabilities. Both sets of findings suggest that

compared with their counterparts in the general population, adults with intellectual

disabilities are much more susceptible than their peers to misleading questions about

material that they do not recall.

The findings indicate that interviewers need, firstly, to make great efforts to assist

people with intellectual disabilities to recall freely and spontaneously as much correct

information as possible about alleged offences (for example, using the cognitive inter-

view, Milne et al. 1999) and, secondly, to try to avoid questions that imply the desired

response. Psychological assessments of potential witnesses to criminal offences should

stress that recent studies (Bull 1999; Milne 1999; Kebbell et al. 2002) suggest that, when

interviewed appropriately, people with intellectual disabilities can provide accurate

accounts of events. The onus must rest with the interviewer not to ask inappropriate

questions. However, there may be a long way to go before such methods are routinely

used by the police (Milne & Bull 1999) or the courts (Kebbell et al. 2002). Only rigorous

training models that span an interviewer’s career and adequate supervision in the

workplace will help the transference of appropriate questioning skills into the field

(Clarke & Milne 2001).

Acknowledgments

The present authors are grateful to all the participants for their help, to the staff at the

centres, to our interviewers, and to Professor Gisli Gudjonsson for his support for

this study. The study was carried out in part fulfilment of the first author’s PhD, which

was funded by a bursary from the University of Portsmouth. Additional funding was

provided by the Mental Health Foundation – the present authors are also grateful to

them for their support. In addition, the present authors would like to thank two

anonymous reviewers for their helpful comments on a prior draft of this paper.

Correspondence

Any correspondence should be directed to Dr Rebecca Milne, Institute of Criminal

Justice Studies, University of Portsmouth, Museum Road, Portsmouth, Hampshire PO1

2QQ, UK. E-mail: [email protected]; Tel.: þ44 2392843927.



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Appendix 1

Adaptation of the GSS

(1) Did the boy have a sister?

(2) Was the front door blue or white?

(3) Did the children cross at the traffic lights or the zebra crossing?

(4) Was the man in the yellow car driving faster than the speed limit?

(5) Was the van driver delivering boxes?

(6) Did the lollipop man give the boy the time?

(7) Was the boy’s name Jamie?

(8) Did the car’s right or left headlight smash in the accident?

(9) Was the driver of the yellow car a man?

(10) Did the boy drop his lunch box during the accident?

(11) Did the boy have a bandage on his head or his arm?

(12) Was the boy put on a stretcher?



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Interrogative Suggestibility among Witnesses with Mild Intellectual Disabilities: the Use of an Adaptation of the GSS