Forensic Credibility Assessment for Alleged Victims and Witnesses

The question of whether a forensic credibility assessment should ever be offered to an alleged victim or witness is one of the most sensitive issues in forensic psychophysiology. It requires careful thought, professional caution, trauma-informed practice and a clear understanding of what such an examination can and cannot achieve.

Public discussion of polygraph examinations is often shaped by misconceptions. The phrase “lie detector” implies a simple mechanical test that reveals truth or deception. In practice, a polygraph examination is a structured psychophysiological assessment that records cardiovascular, respiratory and electrodermal activity during carefully formulated questioning. It does not read minds, detect lies directly, or produce infallible results.

When the person being examined is an alleged victim or witness rather than a suspect, additional professional, ethical and safeguarding considerations arise. The examination must be voluntary, appropriately screened, carefully structured and interpreted within a wider investigative and evidential context. The purpose is strictly limited to providing one source of investigative information—not proof, not verdict, not diagnosis.

This article explains the carefully limited role that a forensic credibility assessment may play in cases involving alleged victims and witnesses. It addresses the statement validity assessment methodology, suitability screening, trauma-informed practice, Bayesian analysis, investigative opinions, corroboration and professional limitations. It is written for solicitors, safeguarding professionals, investigators, therapists and informed members of the public.

What This Examination Cannot Determine

Before discussing the potential investigative role of a forensic credibility assessment in victim and witness cases, it is essential to state clearly what such an examination cannot achieve. These limitations apply to all examinations of this kind, but they are especially important in sensitive cases involving alleged victims and witnesses.

These are not merely regulatory disclaimers. They reflect the inherent nature of the examination. A polygraph records physiological activity that may be associated with attention, emotional salience, threat appraisal and autonomic arousal. The interpretation of that activity provides investigative information that may support or question the credibility and consistency of a specific account. It does not and cannot replace independent evidence, clinical assessment, legal process or safeguarding review.

Why Consider a Credibility Assessment in Victim or Witness Cases?

Given these limitations, why might a carefully conducted forensic credibility assessment be considered in cases involving an alleged victim or witness? The answer is that in certain circumstances, such an assessment may contribute one additional source of investigative information that can be considered alongside other material. It is not a substitute for evidence; it is a supplement to the evidential picture.

Situations where this may be considered include:

• Disputed disclosures — where a specific disclosure or account has been questioned and additional assessment of the credibility and consistency of the account may assist investigation or decision-making.
• Historical allegations — where the passage of time, the absence of physical evidence and the complexity of the case create particular challenges for investigation.
• Safeguarding concerns — where a safeguarding decision requires the widest possible range of information, including structured assessment of the consistency of a specific disclosure.
• Witness credibility questions — where the credibility or consistency of a witness account is a material issue in investigation, family proceedings, or professional decision-making.
• Legal consultations — where a solicitor or barrister seeks additional investigative assessment to inform case strategy, defence preparation or private instruction.
• Investigative support — where other forms of evidence are limited and a statement verification assessment may provide structured additional information.

In every case, the emphasis is on additional investigative information rather than proof. The examination does not determine whether an allegation is true or false. It provides structured information about the credibility and consistency of a specific disclosure or account, which must then be interpreted alongside all other relevant evidence and professional advice.

The Statement Verification Approach

This is a central element of the examination methodology used by The Centre for Forensic Neuroscience in cases involving alleged victims and witnesses.

Rather than repeatedly revisiting the details of a traumatic event through direct questioning, the examination is structured around a defined written disclosure or witness statement. The examinee is asked whether they maintain the credibility and accuracy of that specific account. This approach has several advantages in trauma-informed practice.

Trauma-Informed Practice

Polygraph examinations involving alleged victims and witnesses must be conducted with a clear understanding of the potential impact of trauma on the examination process and its interpretation. Trauma, distress, anxiety, dissociation and vulnerability can all affect physiological responses in ways that are not related to deception.

An alleged victim may experience heightened autonomic arousal simply because the examination process itself triggers distress, anxiety or traumatic memory. This does not mean the person is being deceptive. It means that physiological responses must be interpreted with particular care and professional judgement in cases where trauma is a factor.

Trauma-informed practice in the context of polygraph examinations involving alleged victims includes:

• Recognising that trauma-related symptoms may affect both suitability and interpretation.
• Understanding that dissociation, hyperarousal, avoidance and emotional dysregulation may produce physiological responses unrelated to deception.
• Ensuring that the examination does not require repeated detailed recall of traumatic events.
• Providing clear informed consent, including the voluntary nature of the examination and the right to withdraw at any point.
• Ensuring that appropriate psychological, legal or professional support is available before, during and after the examination.

Not every case is suitable for a polygraph examination. Some referrals should be declined on the basis of trauma, psychological vulnerability, safeguarding concerns or welfare considerations. The decision to proceed should always prioritise the wellbeing of the person being examined.

Suitability Screening

Before any examination is accepted, a structured suitability screening process is undertaken. This is not a formality; it is a professional and ethical requirement that directly affects the quality and defensibility of the examination.

Why Alleged Victims Are Assessed Differently

It is important to understand that an alleged victim or witness is not being examined in the same way as a suspect. The purpose of the assessment is fundamentally different. An alleged victim is not being examined to establish guilt or innocence. The purpose is to assess the credibility and consistency of the examined account within a defined and carefully limited scope.

Historical memory is influenced by a range of factors including trauma, the passage of time, emotional state at the time of encoding, post-event information and natural processes of memory reconstruction. These factors do not necessarily indicate deception. A person may provide an account that contains inconsistencies, gaps or shifts in detail without being untruthful. Equally, an account that is internally consistent is not automatically accurate.

The methodology used in these assessments is designed to take these factors into consideration. It does not assume that physiological responses have the same significance in a victim or witness context as they might in a suspect examination. The interpretation of findings is informed by an understanding of trauma, memory science and the particular circumstances of the case.

Understanding Investigative Opinions

Any opinion arising from a forensic credibility assessment involving an alleged victim or witness is investigative rather than evidential. It is based on the structured interpretation of psychophysiological data, statement analysis and professional judgement. It is not a verdict, a clinical diagnosis, a legal finding or proof of any event.

Where appropriate, findings may be communicated using an Investigative Concern Band framework. This approach assists the instructing professional in understanding the degree to which the examination findings support the credibility and consistency of the examined account.

How Are the Findings Reported?

The final opinion is produced by integrating the available findings using Bayesian analysis and professional judgement. The examination does not simply produce a binary “truthful” or “deceptive” opinion. Instead, findings are expressed as an Investigative Concern Band, representing the degree to which the examination supports the credibility and consistency of the examined account.

This approach reflects the reality that forensic assessments operate within a framework of uncertainty. Rather than claiming certainty, the Investigative Concern Band communicates a graduated assessment that assists professional decision-making. It is not proof that alleged events occurred or did not occur. It is a structured forensic opinion designed to be considered alongside all other available evidence.

Bayesian Credibility Assessment

Bayesian analysis is a structured method for evaluating how strongly the available evidence supports one interpretation compared with another. In the context of a forensic credibility assessment, it provides a principled framework for combining multiple sources of information into an overall assessment of evidential support.

The analysis combines:

• Examination findings — the psychophysiological data recorded during the polygraph examination
• Statement analysis — the structure, consistency and content of the examined account
• Psychophysiological data — cardiovascular, respiratory and electrodermal activity patterns
• Case information — relevant collateral information where available and appropriate
• Examiner judgement — the professional assessment of the examining practitioner

The result is expressed as a Bayes Factor. A Bayes Factor is a measure of evidential support. It describes how much more likely the observed evidence would be under one hypothesis (for example, that the examined account reflects genuine autobiographical memory) compared with an alternative hypothesis (for example, that the account does not reflect genuine autobiographical memory).

A Bayes Factor does not produce the probability that abuse occurred. It measures the relative strength of evidential support provided by the examination findings. A Bayes Factor of 10, for example, means that the examination findings are 10 times more likely to be observed if the examined account is credible and consistent than if it is not. This is a statement about evidential support, not about the probability of a past event.

This distinction is important. The Bayesian approach increases confidence in the assessment by quantifying uncertainty, but it does not eliminate uncertainty. The analysis is one component within a broader structured forensic opinion.

Illustrative Example

The following is a hypothetical example illustrating how examination findings might be reported. It is provided for illustrative purposes only and does not represent any individual case.

The Role of Professional Judgement

The Bayesian model supports—but does not replace—the examiner’s professional judgement. Statistical analysis provides a structured framework for combining and weighing evidence, but the final opinion is a structured forensic opinion rather than an automated statistical output.

The examining practitioner considers the full range of available information, including factors that may not be fully captured by a statistical model. These include the quality of the examination, the behaviour and presentation of the examinee, the context in which the assessment was requested, and any recognised limitations that may affect the reliability of the findings.

The overall opinion should be understood as a carefully considered professional assessment, informed by data and analysis but ultimately reflecting the judgement of a qualified practitioner operating within a defined forensic framework.

The Importance of Corroboration

A forensic credibility assessment is only one source of investigative information. Its findings should never be viewed in isolation and should always be considered alongside other relevant evidence, information and professional advice.

Corroborating information may include:

Examination findings should never stand alone. They should be interpreted as one component within a wider evidential, safeguarding and investigative context.

A Carefully Limited Role

The Centre for Forensic Neuroscience approaches forensic credibility assessments involving alleged victims and witnesses with particular professional caution. The examination has a carefully limited role and its findings must be understood within that context.

The examination is offered only where a voluntary, appropriately screened and carefully structured process is possible. Every case is assessed individually. Where safeguarding, welfare, trauma-related or professional concerns are identified during screening, the examination may be declined.

This careful and conservative approach is essential to maintaining the professional defensibility and ethical integrity of the process. The Centre for Forensic Neuroscience will not accept a referral where the examination could cause harm, where the person is not able to give genuine informed consent, or where the result is likely to be used in a manner inconsistent with the investigative limitations of the process.

This article is provided for general information and professional guidance. It is not legal advice, clinical advice or a substitute for professional consultation. Examination findings should be interpreted cautiously and in context. Where safeguarding, legal, coercion or mental health concerns are present, appropriate professional advice should be sought.