Avoiding Anchoring Bias in Psychiatric Expert Evidence

The Clinical Mechanism of Cognitive Anchoring

In forensic psychiatric practice, anchoring bias occurs when an expert relies excessively on the first information encountered—such as an initial police summary, the narrative of the alleged offence, or an early provisional diagnosis—when evaluating a defendant. For instructing solicitors, recognising how this heuristic can influence the trajectory of a clinical assessment is essential for ensuring that evidence remains robust and objective, satisfying the duties imposed by the Criminal Procedure Rules (CrimPR) Part 19.

Anchoring bias often manifests when an initial ‘anchor’, such as a specific prosecution narrative, constrains the subsequent clinical inquiry. If an expert is instructed with emotionally charged accounts before reviewing longitudinal medical records or conducting an independent interview, there is a risk that the assessment becomes a process of confirming the initial narrative rather than a neutral evaluation. This is particularly relevant in cases involving complex diagnoses. In assessments of diminished responsibility under the Homicide Act 1957, the focus on the actus reus may inadvertently lead an expert to work backward to justify a diagnosis of an ‘abnormality of mental functioning’, rather than objectively determining whether such an abnormality existed at the time of the offence. Maintaining an independent psychiatric assessment framework is necessary to avoid the premature narrowing of clinical hypotheses.

Procedural Safeguards under CrimPR Part 19

The expert’s overriding duty to the court, set out in CrimPR 19.2, requires them to provide an opinion that is not influenced by the pressures of litigation. When instructing an expert, solicitors should ensure the expert has access to the full spectrum of evidence—including defence statements, independent witness accounts, and historical clinical records—rather than relying solely on the prosecution’s bundle. This breadth of source material allows the expert to triangulate data points against the clinical presentation.

Where two experts are instructed, the joint statement procedure under CrimPR 19.6 provides a formal opportunity to test the basis of conclusions. It is within this forum that a forensic psychiatrist may identify where cognitive biases have led to divergent conclusions and justify their opinion, acknowledging where the evidence does not support their initial premise. This is distinct from the instruction of a single joint expert; where the court directs a single joint expert under CrimPR 19.7, the expert acts for the court rather than the parties, and the mechanism for challenging the report is governed by the rules on written questions.

Common Pitfalls in Symptom Validity

A frequent source of anchoring is the reliance on historical psychiatric labels which may no longer be relevant. A defendant with a past diagnosis of a personality disorder might be viewed through that ‘lens’ in subsequent criminal matters, obscuring a new or distinct clinical presentation. To mitigate this, practitioners may employ assessments of symptom validity. The use of structured tools such as the SIMS (Structured Inventory of Malingered Symptomatology) or the Rey 15-item memory test provides an objective data point, though these must be interpreted within the context of the clinical evaluation rather than as definitive proof of malingering or credibility.

Furthermore, when using structured professional judgement tools such as the HCR-20v3, experts should ensure they do not weigh static factors so heavily that they fail to account for dynamic elements. Risk assessment is a clinical process that must remain sensitive to the defendant’s evolving circumstances within the custodial or community environment.

Practical Strategies for Legal Professionals

• Sequencing of Instructions: Provide the expert with raw clinical data alongside legal papers to ensure the clinical history is established before the expert is asked to comment on specific legal tests, such as the Pritchard criteria for fitness to plead.
• Challenging the Narrative: When reviewing draft reports, solicitors should scrutinise the rationale. A robust report should address competing diagnoses and discuss evidence that may contradict the expert’s primary conclusion.
• Clarifying Causation: Ensure the expert differentiates between psychiatric disorder and the acute effects of voluntary intoxication. Anchoring on an offender’s ‘chaotic history’ can lead to an underestimation of the specific impact of substance use on criminal conduct.
• Expert Independence: Always instruct the expert to form an independent view. If the expert’s opinion shifts during the evidence exchange, this should be viewed as evidence of an objective, iterative process.

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