Psychosis, Credibility and Polygraph Testing: When Belief Is Not the Same as Truth

When a person makes a false statement, the natural forensic assumption is that they are lying. In most contexts, that assumption has reasonable validity. But in cases involving psychosis, it carries a fundamental conceptual error — one with serious implications for credibility assessment, expert evidence, and the administration of justice.

A person experiencing active psychosis may report events, perceptions, or experiences that are objectively untrue, while holding those reports with complete sincerity. They are not deceiving. They are describing their experienced reality. Understanding why this distinction matters — and what it means for physiological credibility assessment — is the central concern of this article.

When a False Statement May Not Be a Lie

The framework underpinning most forensic credibility assessment — including psychophysiological methods — rests on a distinction between what a person believes and what they consciously choose to communicate. Deception, in the forensic sense, requires a deliberate attempt to create a false belief in the mind of another. Where that deliberate intent is absent, the conceptual basis for interpreting physiological arousal as an indicator of deception becomes considerably more complicated.

Psychosis fractures this model in several ways simultaneously. Consider the forensic categories that must be distinguished in any case involving a potentially psychotic individual:

• Objective factual accuracy — whether an event occurred as described in external, verifiable reality.
• Subjective belief — what the individual genuinely believes to be true at the time of reporting.
• Conscious deception — a deliberate and knowing attempt to create a false impression.
• Delusional belief — a fixed, false belief maintained with conviction in the absence of, or contrary to, evidence, arising from a recognised psychiatric condition.
• Hallucinated experience — a perceptual experience occurring in the absence of an external stimulus, experienced as real by the individual.
• Malingering or deliberate symptom fabrication — the conscious and volitional production or exaggeration of psychiatric symptoms for external gain.

These are separate forensic questions. A person may be objectively incorrect without being deceptive. They may hold a firmly delusional belief while also, in a quite separate matter, choosing to lie. They may report a genuine hallucination truthfully, even though the hallucination itself has no objective correlate. Or they may fabricate the report of a hallucination entirely. None of these possibilities excludes any other. They must each be evaluated on the available evidence.

Objective falsity is not necessarily deception. In forensic credibility assessment, that distinction is not a technicality — it is the foundation on which the entire analysis must rest.

Psychosis Does Not Automatically Make Credibility Assessment Impossible

The question of whether physiological credibility assessment can be meaningfully applied in individuals with significant psychiatric illness is one that has attracted limited but informative empirical attention. A paper published in European Psychiatry in 2012 by Hirschmann and Guzner examined the feasibility of polygraph examination in patients with schizophrenia or schizoaffective disorder who were actively experiencing delusions.¹

The study examined 23 inpatients with DSM-IV diagnoses of schizophrenia or schizoaffective disorder who held identified delusional beliefs. Participating patients were asked both trivial true/false questions and questions relating directly to the content of their established delusions, as identified by a senior psychiatrist. Results were correlated with the Brief Psychiatric Rating Scale (BPRS). Of the 23 examinations conducted, 17 were judged to have produced valid physiological data.

This is a small sample, and the authors themselves characterised the findings as preliminary. The results should not be cited as proof that polygraph examination is validated for psychotic populations. They are not. What the data do suggest, cautiously, is that:

• In domains unrelated to their specific delusional content, the physiological responses of psychotic inpatients appeared broadly similar to those recorded in the general population.
• Where questions addressed content that fell within the individual's established delusional system, patients appeared physiologically to respond as though the delusional belief were true — which, from their subjective perspective, it was.

The latter finding has direct and important implications for forensic interpretation. A physiological credibility assessment records responses to propositions as the examinee experiences them, not as external reality determines them. If a person genuinely believes that a persecutor is following them, or that they have received a divine instruction, or that they witnessed an event that did not occur, their physiological response to truthfully affirming that belief will likely be indistinguishable from any other truthful response. The instrument records subjective belief states, not objective facts.

A later feasibility study by the same authors, examining patients with schizophrenia or schizoaffective disorder and delusional beliefs, reinforced this pattern.² Again working with small samples, the researchers reported that valid examinations could be produced in a proportion of cases, and again observed that delusional content appeared to be processed physiologically as truth. The consistent finding across both studies is conceptually coherent: the physiological signature of a lie requires a subjective awareness of saying something the speaker knows to be false. Where that awareness does not exist — as it does not in genuine delusional belief — the usual mechanism of differential physiological responding may not operate in the expected direction.

The Problem of Delusional Belief

The clinical definition of a delusion is important here. Delusions are not simply unusual or mistaken beliefs. They are fixed false beliefs arising from a psychiatric process, maintained with conviction, not amenable to correction by evidence or argument, and not explicable by the individual's cultural context. They are, in the neurological and phenomenological sense, the individual's reality.

This creates a specific interpretive problem for any forensic evaluator considering credibility. If an examiner asks a deluded individual “Did you hear a voice instructing you to do X?” and that individual answers “Yes” — believing it entirely — the physiological correlates of that response may well reflect genuine belief rather than deception, regardless of the objective absence of any external voice. Conversely, if a non-deluded individual fabricates the claim of having heard a voice, they are lying, and the physiological correlates of that lie may be recoverable within a structured examination.

The fundamental forensic question, then, is not “did this person say something that is objectively false?” but rather: does what this person is saying fall within or outside the boundary of their genuinely held altered perception of reality?

That boundary may be difficult to determine from the examination alone. It requires collateral clinical information, psychiatric assessment, review of historical records, and careful analysis of whether the specific proposition being examined is consistent with the individual's established symptom pattern or constitutes a separate and potentially fabricated account.

A Topical Illustration: The Lindsay Clancy Proceedings

The criminal trial of Lindsay Clancy in Massachusetts — at the time of writing, with the jury still in deliberations at Plymouth Superior Court — illustrates the forensic complexity that arises when courts must evaluate claims that simultaneously engage questions of psychosis, hallucination, and potential malingering.

Ms Clancy pleaded not guilty by reason of lack of criminal responsibility to charges arising from the deaths of her three children in January 2023. Her defence team argued that she was experiencing severe postpartum psychosis at the time of the offences. Court reporting indicates that evidence at trial addressed the possibility of command hallucinations — voices experienced as instructing conduct. The prosecution maintained that she retained the capacity to understand the nature and criminality of her actions. Forensic psychologist Kirk Heilbrun, called in connection with the prosecution case, is reported to have testified that the specific command hallucinations described by Ms Clancy had not been documented in any clinical record before or after the incident in question (CBS News; PBS NewsHour; Courthouse News Service, August–September 2026).

No opinion is offered here on Ms Clancy's guilt or criminal responsibility. Those questions are properly for the jury. What the case does illustrate — with unusual clarity — is the forensic problem created when the central disputed question involves a mental phenomenon that is inaccessible to external verification.

If a person genuinely experienced a command hallucination, then their report “I heard a voice telling me to do this” is subjectively truthful. The voice was real to them, in the only sense in which a hallucination can be said to be real. No polygraph examination, no psychiatric assessment, and no forensic tool currently available can determine whether an internal perceptual experience objectively occurred. That question is, by its nature, beyond external verification.

What forensic assessment can do — carefully and with appropriate caveats — is evaluate whether the description of the experience is consistent with the known phenomenology of command hallucinations; whether it is consistent with contemporaneous clinical presentations; whether the individual's account has remained stable across multiple interviews; and whether the temporal relationship between reported symptoms and behaviour is plausible given the known course of the disorder. These are questions of inference and convergent evidence, not direct detection.

Mental Illness Does Not Exclude Selective Fabrication

One of the more important conceptual errors in this area is the assumption that mental illness and malingering are mutually exclusive categories. They are not.

A person can suffer genuine and severe psychiatric illness — including psychosis — while simultaneously choosing to exaggerate, embellish, or fabricate a particular symptom. The presence of a real diagnosis does not immunise against volitional misrepresentation. Equally, the identification of malingering indicators does not establish that every reported symptom is fabricated. These are independent analytical questions that must be evaluated separately.

Consider the range of competing forensic hypotheses that may need to be weighed in any given case:

• Genuine psychosis, accurate symptom reports — the individual is ill and is describing their experience faithfully.
• Genuine psychosis, partially fabricated symptom reports — the individual is genuinely ill but is embellishing or reconstructing specific experiences for strategic reasons.
• Genuine psychosis in remission, retrospective reconstruction — the individual had a real episode but their retrospective account has been shaped by subsequent knowledge, suggestion, or motivated reasoning.
• No genuine psychosis, fabricated disorder — the individual is malingering a condition they do not have.

Response validity assessment — the psychometric evaluation of effort, symptom validity, and response consistency — can contribute meaningfully to distinguishing between these hypotheses. Instruments such as the Structured Inventory of Malingered Symptomatology (SIMS) and the Miller Forensic Assessment of Symptoms Test (M-FAST) may identify patterns inconsistent with genuine cognitive or psychiatric presentation. But a negative malingering indicator does not prove that all reported symptoms are genuine; it reduces the probability of wholesale fabrication within the tested domain.

Atypical symptom phenomenology, conversely, should not automatically be treated as evidence of fabrication. Genuine psychotic illness often presents atypically, and the phenomenology of postpartum psychosis in particular can involve rapid fluctuation, mixed features, and presentations that differ markedly from textbook descriptions. The absence of prior documentation of a specific symptom is notable and requires explanation; it is not, by itself, conclusive proof of fabrication.

Mental illness and malingering are not mutually exclusive. A person can suffer genuine psychiatric illness while still deliberately misrepresenting one specific symptom. Both hypotheses must be evaluated independently, against independent evidence.

Forensic evaluators working in this area must therefore construct and test a range of competing hypotheses, weighting each against the available evidence rather than gravitating towards a single binary conclusion.

What This Means for Forensic Polygraph Practice

Several practical conclusions follow from the above analysis for any practitioner considering physiological credibility assessment in a case involving psychiatric illness.

First, the proposition being examined matters enormously. A question that falls within the individual's established delusional framework will not function in the same way as a question about an event that the individual knows objectively occurred or did not occur. Pre-examination psychiatric consultation is therefore not optional in these cases; it is methodologically necessary.

Second, suitability must be assessed individually and carefully before any examination is accepted. The relevant considerations include:

• Current level of psychotic symptoms — including agitation, disorganised thinking, and paranoid ideation that might compromise the integrity of the examination.
• Cognitive functioning — whether the individual has sufficient capacity to understand, retain, and respond meaningfully to structured test questions.
• Ability to distinguish between external events and internal experiences — a fundamental requirement for any examination that aims to assess responses to propositions about the external world.
• Medication effects — antipsychotic medications, mood stabilisers, and anxiolytics may affect autonomic arousal in ways that influence physiological recording and complicate standard interpretation.
• Attention and comprehension — whether the individual can sustain adequate attention throughout a structured examination format.
• Whether the specific issue under examination falls inside or outside the individual's delusional framework — central, as discussed above, to any meaningful interpretation.
• The quality of available collateral information — psychiatric records, clinical notes, and contemporaneous accounts against which the examinee's self-report can be evaluated.

Third, the output of any examination in this context must be interpreted with explicit acknowledgement of its limitations. Physiological credibility assessment can provide a data point within a broader evidential framework; it cannot provide a determination of whether a hallucination occurred, whether psychosis was present at a material time, or whether a person bears criminal or civil responsibility. Those questions require integrated assessment from multiple sources, over time, by qualified clinicians and forensic evaluators.

Fourth, where the research of Hirschmann, Guzner and colleagues is invoked in a forensic context, it must be presented accurately. The studies involved small samples, an inpatient psychiatric setting, and yielded preliminary findings that the authors themselves did not characterise as definitive validation. Seventeen valid examinations from a sample of 23 is a meaningful preliminary signal, but it is not a basis for unreserved generalisation to all psychotic presentations in all forensic contexts. The honest position is that this research opens a question it does not fully answer.

Conclusion

The presence of mental illness should neither automatically exclude a person from credibility assessment nor, by its mere presence, be treated as evidence that whatever they say must be deceptive. Those are opposite errors, and both are analytically indefensible.

The more important forensic question is a narrower one: does the proposition being examined sit inside or outside the boundary of the individual's altered perception of reality? A person who has incorporated a false event into their psychotic worldview will likely respond physiologically as though it is true — because, to them, it is. A person who is genuinely ill but deliberately misrepresenting one specific aspect of their account is engaged in conscious deception and may respond accordingly. These are not equivalent situations, and forensic assessment that fails to distinguish between them risks producing conclusions that are worse than uninformative.

Credibility assessment in cases involving psychosis requires the integration of psychometric evidence, behavioural observation, psychiatric history, collateral information, account consistency, temporal plausibility, and a willingness to hold multiple competing hypotheses simultaneously without prematurely collapsing them into a single narrative. No single tool resolves the question. The practitioner's task is to narrow the range of plausible interpretations through evidence, not to reach a verdict through any one method.

A Note on Suitability Assessment

Cases involving significant psychiatric history require careful individual review before an examination appointment is accepted. At the Centre for Forensic Neuroscience, referrals involving reported mental health conditions — including psychotic disorders, affective psychosis, personality disorder with dissociative features, or any condition that may affect cognitive functioning, attention, or the capacity to distinguish internal experience from external events — are assessed on a case-by-case basis.

Where a referring solicitor, clinician, or safeguarding professional is uncertain whether a particular individual is suitable for physiological credibility assessment, an initial pre-screening discussion is available before any appointment is arranged. This process is designed to protect the integrity of the examination, safeguard the examinee, and ensure that any resulting data can be responsibly interpreted within the clinical and forensic context of the case.

Enquiries regarding suitability in complex psychiatric cases may be directed through our pre-examination suitability assessment process.

References

1. Hirschmann, S. & Guzner, I. — The feasibility of the polygraph examination in psychotic patients. European Psychiatry, 2012; 27(Suppl 1). doi: 10.1016/S0924-9338(12)74825-3
2. Hirschmann, S., Guzner, I. & Lev-Ari, L. — The feasibility of the polygraph examination in psychotic patients. WebmedCentral Psychiatry, 2014. Article ID: WMC004663.
3. American Psychiatric Association — Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5). Washington, DC: American Psychiatric Association, 2013.
4. Rogers, R. (Ed.) — Clinical Assessment of Malingering and Deception, 4th edition. New York: Guilford Press, 2018.
5. Rosenfeld, J. P. — Detecting Deception: Current Challenges and Cognitive Approaches. Chichester: Wiley-Blackwell, 2018.
6. Vrij, A. — Detecting Lies and Deceit: Pitfalls and Opportunities, 2nd edition. Chichester: Wiley, 2008.
7. CBS News; PBS NewsHour; Courthouse News Service — Trial reporting, Commonwealth v. Clancy, Plymouth Superior Court, Massachusetts, August–September 2026.

Dr Keith Ashcroft is a Chartered Investigative Psychologist and Polygraph Examiner at the Centre for Forensic Neuroscience. He provides physiological credibility assessments, investigative psychology consultancy, and expert forensic opinion for solicitors, clinical teams, and safeguarding professionals. The views expressed in this article are analytical and educational in nature and do not constitute legal or clinical advice.