FM11.1-2 | Mental Illness: Classification & Symptomatology — Summary & Reflection

KEY TAKEAWAYS

Mental illness classification in forensic practice rests on two internationally recognised systems — ICD-11 (WHO) and DSM-5 (APA). The major ICD-11 categories relevant to forensic work include schizophrenia spectrum disorders, mood disorders, stress-related disorders (including PTSD, now in its own ICD-11 block separate from anxiety disorders), OCD-related disorders, substance use disorders, and neurocognitive disorders.

The core symptom vocabulary centres on five entities: hallucinations (percepts without external stimulus — classified by modality: auditory most common in psychosis, visual in organic conditions, tactile in substance withdrawal); illusions (misperceptions of real stimuli, not in themselves diagnostic of mental illness); delusions (fixed, false, unshakeable beliefs — classified as persecutory, grandiose, Othello/jealousy, nihilistic, somatic, erotomanic); obsessions (ego-dystonic, intrusive, unwanted thoughts the patient recognises as their own and resists — distinguished from delusions by ego-dystonicity); and lucid interval (period of restored mental clarity during which testamentary capacity, civil capacity, and criminal responsibility are intact).

Forensically, these distinctions matter because courts require precise, ICD-11-anchored descriptions linked to the patient's mental state at the specific time of the legally relevant act. The BNS Section 22 (formerly IPC Section 84) defence requires showing that unsoundness of mind at the time of the act rendered the accused incapable of knowing the nature of the act or that it was wrong — not merely that a diagnosis exists.

REFLECT

Consider a case where a person with a known diagnosis of schizophrenia is charged with assaulting a neighbour. The accused claims he heard voices commanding the attack and believed the neighbour was sent to kill him. His lawyer argues insanity. The prosecution argues that the accused appeared calm and purposeful immediately after the assault. As the forensic physician preparing the medico-legal opinion, what specific symptoms would you look for, what documents would you review, and how would you structure your court report to address both the classification of the disorder and the mental state at the exact time of the act? What is the distinction between a psychiatric diagnosis and the McNaughten test, and why does that distinction matter for your opinion?