PSYCHOLOGY 1: Understanding Mental Disorder Classification Systems

Understanding Mental Disorder Classification Systems (DSM-5 & ICD-11)

Advanced Psychology ESL – Methodology & Clinical Terminology

1. Introduction

Modern psychology uses diagnostic systems to classify mental disorders consistently and scientifically.
The two most widely used systems are:

  • DSM-5 – Diagnostic and Statistical Manual of Mental Disorders

  • ICD-11 – International Classification of Diseases

These systems provide a shared professional language and guide assessment, differential diagnosis, treatment planning, and research.

2. Why Diagnostic Systems Exist

Diagnostic manuals bring structure, clarity, and reliability to mental-health assessment.

Clear, measurable definitions
Disorders are defined by observable, measurable symptoms rather than assumptions.

Diagnostic criteria
Each diagnosis typically requires:

  • a minimum number of symptoms

  • a minimum duration

  • functional impairment or distress

Differentiating between disorders
Clinicians must determine whether symptoms are better explained by another disorder, a medical condition, or substance effects.

Standardized communication
Shared terminology improves consistency across clinicians, countries, and disciplines.

Support for research
Reliable definitions allow systematic study of disorders and development of treatments.

3. How the DSM-5 Organizes Disorders

DSM-5 uses a descriptive, symptom-based approach.

Symptom clusters
Disorders are defined by groups of symptoms (e.g., positive, negative, and cognitive symptoms in schizophrenia).

Duration requirements
Many diagnoses specify a required timeframe.
Example: schizophrenia requires 6 months of disturbance, with 1 month of active symptoms.

Functional impairment
Symptoms must meaningfully disrupt daily functioning.

Rule-outs
Clinicians must exclude substance effects, medical conditions, and other psychiatric diagnoses.

Clinical significance
Symptoms must cause distress or dysfunction, not simply be unusual.

4. How the ICD-11 Organizes Disorders

ICD-11 is designed for global use across diverse healthcare systems.
It emphasizes:

  • cultural variation

  • practical clinical descriptions

  • flexibility and clinician judgment

  • worldwide applicability

ICD-11 relies less on strict checklists and more on concise descriptions supported by clinical expertise.

5. DSM-5 Examples: Applying Methodology in Practice

A. Schizophrenia

Positive symptoms
hallucinations, delusions, disorganized speech, disorganized or catatonic behaviour

Negative symptoms
reduced emotional expression, avolition, social withdrawal, anhedonia, alogia

Cognitive symptoms
impaired attention, working-memory deficits, executive-function problems

Diagnostic requirements

  • disturbance for 6 months

  • at least 1 month of active symptoms

Exclusions
Rule out bipolar disorder, schizoaffective disorder, substance effects, and medical conditions.

B. Dissociative Identity Disorder (DID)

Core features
two or more identity states, discontinuity in sense of self, recurrent memory gaps, significant distress or impairment

Etiology
strongly associated with severe, chronic childhood trauma

Differential diagnosis
must be distinguished from schizophrenia, bipolar disorder, PTSD, and borderline personality disorder

6. Key Academic Vocabulary

  • etiology – cause or origin of a disorder

  • pathophysiology – biological mechanisms involved

  • symptom cluster – group of symptoms that occur together

  • diagnostic criteria – required features for diagnosis

  • operational definition – measurable definition

  • phenomenology – subjective experience of symptoms

  • differential diagnosis – determining which disorder best explains symptoms

  • functional impairment – reduced ability to function normally

  • clinical significance – meaningful distress or impairment

  • comorbidity – presence of two or more disorders simultaneously

  • validity – accuracy of a diagnosis

  • reliability – consistency across clinicians

  • case conceptualization – clinician’s formulation of the patient’s difficulties

7. Discussion Questions

  1. Why are standardized diagnostic manuals essential in psychology?

  2. What strengths and limitations does DSM-5 have?

  3. How does ICD-11 differ from DSM-5 in methodology and purpose?

  4. Compare schizophrenia and DID in terms of symptoms and etiology.

  5. Why is differential diagnosis critical for treatment planning?

  6. How might diagnostic labels help or harm patients?

  7. Should mental disorders be classified mainly by biology, behaviour, environment, or all three?