Psychological disorders—also called mental disorders or mental illnesses—are clinically significant patterns of thinking, feeling, or behaving that cause personal distress, impair daily functioning, or both. These patterns often represent a serious departure from cultural norms and must reflect dysfunction in psychological, biological, or developmental processes.(Macmillan USA)

How Are They Classified?

The DSM-5 (Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, and its text revision) is the primary system used by mental health professionals to diagnose mental disorders. The WHO’s ICD-11 is another widely used international standard.(Macmillan USA)

Major DSM-5/-TR categories include:(Open Text WSU)

• Neurodevelopmental Disorders (e.g. ADHD, autism spectrum disorder, intellectual disability)
• Schizophrenia Spectrum & Other Psychotic Disorders (characterized by delusions, hallucinations)
• Bipolar & Related Disorders (alternating manic and depressive episodes)
• Depressive Disorders (e.g. major depressive disorder, persistent depressive disorder)
• Anxiety Disorders (e.g. panic disorder, phobias, social anxiety, GAD)
• Obsessive-Compulsive & Related Disorders (e.g. OCD, hoarding, body dysmorphic disorder)
• Trauma- & Stressor-Related Disorders (e.g. PTSD, acute stress disorder)
• Dissociative Disorders (e.g. dissociative identity disorder)
• Somatic Symptom & Related Disorders (e.g. somatic symptom disorder, illness anxiety disorder)
• Feeding & Eating Disorders (e.g. anorexia nervosa, bulimia, binge-eating)
• Sleep-Wake Disorders (e.g. insomnia, narcolepsy)
• Personality Disorders (Cluster A, B, C types—paranoid, borderline, avoidant, etc.)
• Substance-Related & Addictive Disorders (e.g. alcohol, drug, or behavioral addictions)
• Disruptive, Impulse-Control & Conduct Disorders (e.g. conduct disorder, oppositional defiant)
• Neurocognitive Disorders (e.g. dementia, delirium)(Open Text WSU, Healio)

Common Examples & Prevalence

Some of the most frequently observed disorders include:

• Anxiety Disorders: Affect roughly 19–30% of adults in their lifetime.(Verywell Health)
• Depressive Disorders: ~8% of adults annually experience major depression.(Verywell Health)
• Bipolar Disorders: Affect about 2–3% of adults.(Verywell Health)
• Obsessive-Compulsive Disorder (OCD): Around 2–3% of people experience OCD.(Verywell Health, Open Text WSU)
• Schizophrenia & Psychotic Disorders: Under 1% prevalence, serious chronic impact.(Verywell Health)
• Trauma-Related Disorders (e.g. PTSD): Lifetime risk about 8–9%.(Verywell Health)
• Personality Disorders: Up to ~9% of adults diagnosed during their lifetime.(Verywell Health, Open Text WSU)
• Eating Disorders: ~1–5% of population; includes anorexia, bulimia, binge-eating.(Healthline, Verywell Mind)
• Substance Use Disorders: Cover alcohol, drugs, gambling, with wide-ranging prevalence.(Verywell Health, Healthline)

Causes & Risk Factors

Mental disorders typically arise from a complex interplay of:

• Genetic predisposition
• Brain chemistry and structure
• Traumatic life experiences or chronic stress
• Environmental and social adversity, such as poverty or discrimination(World Health Organization)

Culture also shapes how behaviors are interpreted and whether they meet diagnostic criteria.(Macmillan USA)

Diagnosis & Treatment

Diagnosis usually involves a clinical evaluation—including questionnaires, interviews, and ruling out medical causes. Conditions must cause significant distress or functional impairment to meet diagnostic thresholds.(Verywell Mind, Wikipedia, Healio)

Treatment may combine:

• Psychotherapy: cognitive-behavioral therapy (CBT), dialectical behavior therapy (DBT), exposure therapy, supportive counseling
• Medications: SSRIs, mood stabilizers, antipsychotics, anxiolytics, stimulants
• Lifestyle modifications: diet, exercise, sleep hygiene
• Social support, peer groups, rehabilitation programs
• In some cases, neurotherapeutic procedures like ECT, rTMS, or EMDR are used.(World Health Organization, Verywell Health, Wikipedia)

Early detection and intervention are key to improving outcomes and reducing long-term impact.(Verywell Health, Verywell Mind)

In Summary

Concept	Details
Definition	Disturbance in cognition, emotion, or behavior causing distress or impairment
Categories	Major DSM-5 groups such as mood disorders, anxiety disorders, psychoses, neurodevelopmental, etc.
Common Disorders	Anxiety, depression, bipolar, OCD, PTSD, schizophrenia, eating & substance use disorders
Causes	Genetic, biological, psychological, social, environmental
Treatment Options	Psychotherapy, medication, lifestyle change, support resources

If you’d like a more focused summary on a particular disorder—such as PTSD, OCD, ADHD, or personality disorders—or want practical guidance on recognizing warning signs or seeking help, feel free to ask!