Classifying Disorders

4 min read Updated Apr 19, 2026

Before diving into specific disorders, we need a framework for classifying them. The MCAT wants you to know the dominant diagnostic manual and the main theoretical lenses used to understand mental illness.

The DSM-5

The Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5), published by the American Psychiatric Association, is the standard reference for diagnosing mental disorders in the United States. It lists hundreds of disorders, each with specific diagnostic criteria.

Cover of the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision (DSM-5-TR), with the title in tan text on a teal background and 'American Psychiatric Association' along the bottom — The DSM-5 (here in its 2022 Text Revision) is the gold-standard diagnostic reference in U.S. psychiatry. MCAT passages routinely quote DSM-5 criteria verbatim, so know the framework: defined symptom clusters plus significant distress or impairment plus a minimum duration. *Credit: American Psychiatric Association via Wikimedia Commons (Public Domain).*

A diagnosis requires:

Presence of specified symptoms.
Symptoms cause significant distress or impairment in daily functioning.
Symptoms are not better explained by another condition (e.g., medical illness, substance use).
For many disorders, symptoms must persist for a specified duration.

The DSM-5 replaced the multi-axial system of DSM-IV with a single-axis approach, integrating psychiatric and medical conditions together.

The Biopsychosocial Model

Mental disorders emerge from interacting factors at three levels:

Biological. Genes, neurotransmitters, brain structure, hormones, prenatal exposures.
Psychological. Cognitions, coping styles, learning history, personality traits.
Sociocultural. Family dynamics, social support, stressors, poverty, discrimination, cultural norms.

No single level tells the whole story. Depression can be triggered by a serotonin system vulnerability (bio), by rumination and hopelessness (psych), or by job loss and isolation (social) - usually all three together.

Three overlapping circles in a Venn diagram labeled Biology (top, listing physical health, genetic vulnerabilities, drug effects), Social (left, listing peers, family circumstances, family relationships), and Psychological (right, listing coping skills, social skills, family relationships, self-esteem, mental health). The circles overlap in the center to indicate that the three domains interact — The biopsychosocial model: mental disorders emerge from the interaction of biological, psychological, and sociocultural factors. The MCAT loves this framework as a contrast to the older, narrower biomedical model. *Credit: Seth Falco via Wikimedia Commons (CC BY-SA 4.0).*

What Counts as “Abnormal”?

The DSM leans on several overlapping criteria:

Statistical deviance. The behavior is far from the norm (rare).
Maladaptive behavior. It interferes with daily functioning.
Personal distress. The person feels significant suffering.
Violation of social norms. The behavior deviates from what the culture considers acceptable.

None alone is sufficient. Genius is statistically deviant but not disordered. Someone in a dangerous job has normal distress. Cultural norms vary. Modern diagnosis requires a cluster of these with documented impairment.

Stigma and the Biomedical Model

Stigma is a major barrier to treatment. People with mental illness face discrimination, self-stigma (internalized shame), and courtesy stigma (affecting family and caregivers). Disclosure risks employment, relationships, and insurance.

The biomedical model frames mental disorders as brain diseases, like diabetes or cancer. This framing can reduce stigma (“it’s not your fault, your brain is sick”) but can also risk reductionism - ignoring the psychological and social dimensions. Modern psychiatry tries to integrate biomedical and psychological perspectives through the biopsychosocial framework.

Comorbidity

Mental disorders often co-occur. A person with major depression may also have anxiety, substance use disorder, or a personality disorder. This comorbidity complicates diagnosis and treatment - the clinician must sort out what’s primary, what’s secondary, and what pattern the patient actually lives with.

What is the DSM-5, and what does a diagnosis require?

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The Diagnostic and Statistical Manual of Mental Disorders, 5th Edition. A diagnosis requires specified symptoms, significant distress or impairment, and exclusion of other explanations. Many disorders require symptoms to persist for a minimum duration.

Name the three levels of the biopsychosocial model.

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Biological (genes, neurotransmitters, brain), Psychological (cognitions, coping, personality), Sociocultural (family, support, stressors, culture). Good clinical care addresses all three levels.