Improving Voice Identification Procedures Experimental Data, 2019-2024

Abstract

The Improving Voice Identification Procedures (IVIP) project consisted of three stands: Strand 1 - Parameters, Strand 2 - Voice Distinctiveness and Strand 3 - Social Stereotypes. All data has been archived in this collection. Strand 1: Parameters - Motivation and aims This strand of the project examined aspects of voice identification procedure which were yet to be fully tested within the Home Office approach, such as the length of voice samples, number of foil voices, witness instructions, and parade type. A series of experiments was conducted to identify optimal parameters to increase both the chances that guilty suspects are selected, and that innocent suspects are not selected. Strand 1: Parameters - Key findings Experiments (1 and 2) testing 15s, 30s, and 60s voice samples showed 15s can be used without adversely affecting witness identification performance. Experiments (1 and 2) investigating parade size found no meaningful difference in performance for six- versus nine-person parades. A study considered the impacts of reflecting on a voice after exposure to it (Experiment 3), and shorter (~5 minutes) versus longer (~1 day) retention intervals. No meaningful difference in identification performance was found between reflection/no-reflection conditions, nor shorter/longer retention intervals. A study testing different warning strengths in witness instructions (Experiments 4 and 5) showed stronger warnings improve accuracy in target-absent parades, but at the expense of target-present accuracy. Analysis of data comparing serial and sequential parade formats both online (Experiments 4, 5 and 6) and in the lab (Experiment 6) did not reveal differences in performance across parade types or testing contexts. Experiment 7 addressed the effect of target-foil similarity and the opportunity to relisten to parade voices. Preliminary analyses suggest no difference in performance according to target-foil similarity, but higher accuracy on target-present parades when listening to parade voices once. Strand 2: Voice Distinctiveness - Motivation and aims Voice parade experiments tend to show that certain target speakers are more readily identifiable than others. This strand of the project considered what makes some speakers more distinctive-sounding than others, and whether speakers judged to be more distinctive-sounding are also more memorable. Experiments were conducted to explore correlates of judgements of voice similarity and phonetic features in combination with accent. Further experiments investigated whether the distinctiveness of a voice depends on its position in a population distribution of voices according to long-term f0, a phonetic feature known to be perceptually important for speaker identity, and the relationship between this factor and voice memorability. Strand 2: Voice Distinctiveness - Key findings Varying patterns of correlation were found between listener-judged voice similarity and pitch, formants and articulation rate, within and between different accent groups, with pitch and formants playing important roles (Experiment 1). An experiment exploring the effect of sample duration (3s vs 10s) on listener-judged voice similarity found similar correlations across sample durations (Experiment 2). Further experiments (3, 4, and 5) have examined the role of pitch in listeners’ judgement of voice distinctiveness. Using pitch-manipulated stimuli, the study highlighted that listeners hear the same pairs of speakers as more different in the mid-range than when heard low or high (Experiment 4). Further experimentation showed this was because listeners are more sensitive to pitch differences in the mid-range (Experiment 5). A further experiment (Experiment 6) found that the benefit of mid-pitch on perceived distinctiveness did not extend to memorability, but instead found that voices that are distinctive for reasons other than pitch were more memorable for listeners. Strand 3: Social Stereotypes - Motivation and aims Strand 3 examined the extent to which social perceptions, judgements, attitudes and stereotypes related to voice(s) can motivate witness decision-making during voice parades. Three experiments assessed the relationship between certain aspects of voice and stereotypes about traits and particular criminal and non-criminal behaviours. Developing an understanding of the link between voice stereotypes and voice identification has the potential to reduce the likelihood of an innocent person being selected from a line-up because that person happens to have the most ‘guilty’ sounding voice. Strand 3: Social Stereotypes - Key findings Experiment 1 showed that listeners consistently rated Standard Southern British English (SSBE) highest for all status traits (concerning education, wealth, intelligence) while Belfast, Cardiff and Glasgow rated highest on the solidarity traits (friendliness, trustworthiness, honesty). Belfast and Glasgow also rated well on ‘honourable’ behaviours and low on ‘morally bad’ behaviours. SSBE speakers were thought least likely to commit criminal behaviours and Liverpool speakers most likely. Experiment 2 showed that low-pitched voices were rated lower for solidarity-based traits, whereas high-pitched voices were rated lower for status-based traits. Slower articulation rates resulted in lower ratings for status and solidarity traits and morally good behaviours, but higher ratings for criminal behaviours. Experiment 3 examined whether stereotyping in voice judgements could contribute to (mis)identification of those speakers in voice parades. It found voices most frequently selected from parades were rated more negatively regarding social and behavioural traits (including criminality).In certain crimes, the voice of a perpetrator is heard by a victim or witness, but no recording of the incriminating speech is available. A robber may have been masked, or have attacked from behind, for instance; or contact may only have been over the telephone. If the witness has received sufficient exposure to the perpetrator's voice, earwitness evidence may be collected through a voice parade. A voice parade is conducted using a similar format to a visual identity parade, but using voices rather than faces: the witness is asked whether he or she can pick out the voice of the speaker heard at the crime scene from a line-up of recorded speech samples which includes the suspect's voice along with a number of 'foil' voices. In England and Wales, the current guidelines on how a voice parade should be conducted (published in 2003) were developed as an extension of the police procedure for visual identification parades. However, since the original voice parade guidelines were developed, psychological research has emphasised that although face and voice processing exhibit many parallels, there are also marked differences, and further research is needed to ensure that the details of the voice identification procedure are set up in a way that optimises earwitness performance. The practical and resource requirements of the current procedure are time-consuming and expensive, and the willingness of police forces to engage with the procedure is very variable such that in practice very few parades are undertaken. The IVIP project has four main strands, with the overall aim of improving understanding of earwitness behaviour and improving the interaction of the criminal justice system with the use of earwitness evidence. The first strand will examine aspects of voice identification which are yet to be fully tested within the current procedure (e.g. length of samples, number of foil voices, witness instructions, parade type), with a view to modifying the procedure to optimise earwitness performance. The second strand will investigate from a phonetic perspective why it is that certain speakers are more distinctive-sounding than others and whether speakers judged to be more distinctive are also more memorable. The third strand will entail a study of the degree to which social perceptions, judgements, attitudes and stereotypes related to voice(s) can motivate witness decision-making during voice parades. The final strand of the project will assess and evaluate the extent of police and legal practitioners' awareness and experience of voice parades, beliefs about earwitness memory, attitudes to conducting voice parades and how earwitness evidence is received in court. The IVIP project will achieve significant impact in the criminal justice system, benefitting police and legal practitioners, the judiciary, forensic practitioners and, ultimately, the general public. In addition, its findings will make an important broader contribution to scholarship in phonetics, sociolinguistics, psychology and criminology. Crimes involving the witnessing of a voice are widespread, yet at present this evidence is rarely being adequately capitalised on in the United Kingdom and other countries. Improvements in the understanding of earwitness behaviour and in techniques for collecting earwitness evidence efficiently are essential in addressing these kinds of crimes. This project will offer advances in the implementation of voice parades and improvements in the legal interaction with earwitness evidence which will lead to improved outcomes in the justice system.</p