Schizophrenia - MCAT Behavioral Sciences

Schizophrenia

6 min read Updated Apr 19, 2026

Schizophrenia is a severe psychotic disorder characterized by distorted thinking, perception, and emotion. It affects about 1% of the population worldwide, usually emerging in late adolescence or early adulthood (earlier in men, later in women). Despite stereotypes, schizophrenia is not “split personality” - that’s dissociative identity disorder.

Medical illustration showing a 3D brain on the left with the lateral ventricles highlighted in blue, and on the right two ghosted brain outlines comparing the lateral ventricles in a non-schizophrenic versus a schizophrenic brain. The schizophrenic brain shows visibly enlarged lateral ventricles compared to the normal brain — One of the most reproducible structural findings in schizophrenia: enlarged lateral ventricles, which reflect underlying brain tissue loss. *Credit: BruceBlaus via Wikimedia Commons (CC BY-SA 4.0).*

Symptoms

DSM-5 requires at least two of the following for a significant portion of a month, with at least one being hallucinations, delusions, or disorganized speech:

Positive Symptoms (Excess of Normal Function)

Things that are “added” compared to healthy function.

Hallucinations. Sensory experiences without external stimuli. Auditory (voices) are most common; visual, tactile, olfactory also occur. Auditory hallucinations often comment on the person’s behavior or issue commands.
Delusions. Fixed false beliefs held despite contrary evidence. Categories:
- Persecutory (being followed, harassed, poisoned).
- Grandeur (being a famous person, having special powers).
- Reference (innocuous events refer specifically to oneself - the TV anchor is sending me messages).
- Control (an external force is controlling one’s thoughts or behavior).
Disorganized speech. Loose associations, word salad, neologisms (made-up words), derailment (drifting between topics).
Disorganized or catatonic behavior. Grossly inappropriate behavior, rigid postures, repetitive movements, or lack of movement.

Negative Symptoms (Reduction of Normal Function)

Things that are “taken away” compared to healthy function.

Flat/blunted affect. Reduced emotional expression in face and voice.
Alogia. Poverty of speech; few words.
Avolition. Lack of motivation; difficulty initiating goal-directed activities.
Anhedonia. Inability to experience pleasure.
Social withdrawal. Retreat from relationships.

Negative symptoms are often more disabling over time and are harder to treat with medication.

Cognitive Symptoms

Impairments in attention, working memory, and executive function - less dramatic than positive symptoms but major contributors to long-term disability.

Course

The disorder often progresses through:

Prodromal phase. Social withdrawal, odd beliefs, declining function, lasting months to years before full onset.
Active phase. Full psychotic symptoms emerge - delusions, hallucinations, disorganization.
Residual phase. Positive symptoms attenuate; negative and cognitive symptoms persist. Relapses can send the patient back to active phase.

Early treatment during the prodromal or first-episode phase significantly improves long-term outcomes.

Biological Basis

Dopamine Hypothesis

Schizophrenia involves dysregulation of dopamine signaling.

Mesolimbic pathway (VTA → nucleus accumbens): hyperactive → produces positive symptoms (hallucinations, delusions).
Mesocortical pathway (VTA → prefrontal cortex): hypoactive → produces negative symptoms (flat affect, avolition).

Sagittal diagram of the human brain showing four major dopamine pathways. Black arrows trace projections from the ventral tegmental area (VTA) to the nucleus accumbens (mesolimbic) and from the VTA to the prefrontal cortex (mesocortical, shown in purple). Additional arrows show projections from the substantia nigra to the striatum (nigrostriatal) and a tuberoinfundibular pathway. The hippocampus is also labeled — The four major dopamine pathways. The dopamine hypothesis of schizophrenia centers on two of them: a hyperactive mesolimbic pathway (VTA → nucleus accumbens) drives positive symptoms; a hypoactive mesocortical pathway (VTA → prefrontal cortex) drives negative symptoms. The nigrostriatal pathway (substantia nigra → striatum) is the one degenerated in Parkinson's disease - and the one D2 antagonists hit to cause movement side effects. *Credit: NIDA / Quasihuman via Wikimedia Commons (Public Domain).*

Evidence:

Amphetamines (which boost dopamine) can induce psychosis in healthy people.
First-generation antipsychotics block D2 dopamine receptors and reduce positive symptoms.
Levodopa (dopamine precursor, used for Parkinson’s) can trigger psychotic symptoms.

Glutamate Hypothesis

Evidence for glutamate dysfunction, especially at NMDA receptors. PCP and ketamine (NMDA antagonists) can produce both positive and negative symptoms.

Other Findings

Enlarged cerebral ventricles - indicating brain tissue loss.
Reduced hippocampal volume.
Genetic. Heritability around 80%. Concordance ~50% in monozygotic twins, ~10% in dizygotic twins.
Environmental triggers. Prenatal viral infections, birth complications, cannabis use (especially heavy adolescent use), and severe stress in genetically vulnerable individuals.

Two coronal MRI brain slices side by side from a pair of 44-year-old monozygotic twins, labeled UNAFFECTED on the left and AFFECTED on the right. Red arrows point to the lateral ventricles, which are visibly larger and darker in the affected twin compared to the unaffected twin — Coronal MRI slices from monozygotic twins discordant for schizophrenia: the affected twin (right) shows enlarged lateral ventricles compared to the genetically identical unaffected twin (left). Because their genomes are essentially identical, the structural difference highlights the role of environmental and developmental factors on top of inherited risk. *Credit: Daniel Weinberger, NIMH via Wikimedia Commons (Public Domain).*

Treatment

Antipsychotic Medications

First-generation (typical) antipsychotics (haloperidol, chlorpromazine) - block D2 receptors. Effective for positive symptoms, less effective for negative symptoms, and cause movement-related side effects (tardive dyskinesia - involuntary repetitive movements).

Second-generation (atypical) antipsychotics (risperidone, olanzapine, clozapine) - also block D2 but act on serotonin receptors. Better for negative symptoms, fewer movement side effects, but carry metabolic risks (weight gain, diabetes). Clozapine is the most effective but requires blood monitoring due to risk of agranulocytosis.

Psychosocial Treatment

Family therapy - reducing high expressed emotion (harsh criticism, over-involvement) in family reduces relapse.
Cognitive behavioral therapy - helping patients challenge delusional beliefs and cope with hallucinations.
Supported employment, social skills training - helping patients maintain function.

Distinguish positive from negative symptoms of schizophrenia, with examples.

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Positive symptoms are ADDITIONS to normal functioning (hallucinations, delusions, disorganized speech). Negative symptoms are SUBTRACTIONS from normal functioning (flat affect, avolition, alogia, anhedonia).

According to the dopamine hypothesis, which pathway is hyperactive in schizophrenia's positive symptoms?

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The mesolimbic pathway (VTA → nucleus accumbens). Hypoactivity in the mesocortical pathway (VTA → prefrontal cortex) is associated with negative symptoms. First-generation antipsychotics reduce positive symptoms by blocking D2 receptors.

What is the typical course of schizophrenia in terms of phases?

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Prodromal (subtle decline, social withdrawal, odd beliefs) → Active (full psychotic symptoms) → Residual (positive symptoms attenuated, negative/cognitive persist). Early treatment during prodromal or first-episode phase improves outcomes.