Psychosis arises from a combination of chemical imbalances, structural changes, and disrupted communication between brain regions. There is no single “broken switch” that causes it. Instead, several systems go wrong at once, often involving too much of one chemical messenger in some areas, too little in others, faulty wiring between key regions, and in many cases, a brain that has been slowly changing for months or years before symptoms appear.
Too Much Dopamine in One Area, Too Little in Another
The most established explanation centers on dopamine, a chemical messenger involved in motivation, pleasure, and how the brain assigns importance to experiences. In psychosis, dopamine doesn’t simply go “too high.” The pattern is more specific: deep brain regions involved in emotion and reward release too much dopamine, while the prefrontal cortex, the area responsible for planning, reasoning, and organized thinking, gets too little.
This split explains why psychosis produces two very different sets of symptoms. The flood of dopamine in deeper brain structures overstimulates receptors that help the brain decide what’s meaningful. When these receptors are overactivated, ordinary sensory input can feel loaded with significance, contributing to hallucinations and delusions. Meanwhile, the dopamine shortage in the prefrontal cortex starves the circuits responsible for motivation, clear speech, and emotional range, producing the flat affect and withdrawal that often accompany psychotic disorders. Brain imaging studies using PET scans have confirmed measurable differences in dopamine levels across the prefrontal cortex, the cingulate cortex, and the hippocampus between people with schizophrenia and healthy controls.
A Second Chemical System Goes Wrong
Dopamine doesn’t act alone. A second chemical messenger called glutamate, the brain’s primary excitatory signal, also malfunctions in psychosis. Specifically, certain receptors that respond to glutamate (called NMDA receptors) become underactive. This matters because of which cells are affected first: a type of inhibitory neuron that normally keeps excitatory brain cells in check.
Think of it like a braking system. These inhibitory neurons are supposed to fire rapidly and prevent other neurons from becoming too active. When their NMDA receptors don’t work properly, the brakes fail, and excitatory neurons fire without restraint. The result is a kind of neural chaos, with cortical and hippocampal circuits generating unregulated signals. This mechanism is part of the reason drugs like ketamine and PCP, which block NMDA receptors, can produce psychosis-like symptoms in otherwise healthy people. Research shows that ketamine suppresses activity in inhibitory neurons far more than in excitatory ones, triggering the same disinhibition pattern seen in schizophrenia.
The Brain’s Relay Station Misfires
The thalamus sits near the center of the brain and functions as a relay station, filtering and routing sensory information to the cortex for processing. In psychosis, communication between the thalamus and the prefrontal cortex breaks down. Brain imaging studies using functional MRI show weakened connections between specific thalamic regions and the prefrontal cortex, which normally provides feedback to help the thalamus decide what sensory information to let through and what to suppress.
When this filtering system fails, the brain may be flooded with unfiltered sensory data. Irrelevant sights, sounds, and internal signals that would normally be screened out reach conscious awareness and get processed as if they were real and important. This disrupted feedback loop is thought to contribute to hallucinations, attentional deficits, and the sense of being overwhelmed that many people with psychosis describe. Reduced connectivity between the thalamus and visual and auditory processing areas may specifically underlie the visual and auditory disturbances common in schizophrenia.
Brain Tissue Loss Begins Early
Psychosis isn’t only a chemical problem. Structural changes are visible on brain scans. People experiencing their first psychotic episode show progressive gray matter loss in the superior temporal gyrus, a region critical for processing sound and language. In one study, the rate of tissue loss in parts of this region reached 3.0% to 3.8% per year, and the severity of that loss correlated with how intense a person’s delusions were at follow-up.
These changes don’t start overnight. The prodromal phase, a period of subtle psychological and behavioral shifts that precedes full psychosis, can last weeks to several years. During this time, people often experience mild disturbances in perception, concentration, memory, stress tolerance, and social functioning. Neurocognitive decline during this phase is thought to influence how severe the disability becomes later. The prodrome most commonly affects adolescents and young adults, which aligns with one of the more striking genetic findings in psychosis research.
Synaptic Pruning Gone Too Far
During adolescence, the brain naturally eliminates unused connections between neurons, a process called synaptic pruning. This is normal and essential for efficient brain function. But in people at risk for psychosis, this process appears to go overboard, stripping away connections that should have been kept.
One of the strongest genetic findings in schizophrenia research, published by a team at Harvard’s Broad Institute, links the disorder to variants of a gene called C4. Higher-risk versions of C4 lead to increased production of a protein that tags the connection points between neurons for destruction. Immune cells in the brain called microglia then engulf those tagged connections. The result is excessive loss of synapses in the prefrontal cortex during a developmental window that coincides exactly with the typical age of psychosis onset: late adolescence and early adulthood. This helps explain why a disorder with roots in early brain development doesn’t produce obvious symptoms until the late teens or twenties.
Inflammation Inside the Brain
Neuroinflammation is increasingly recognized as a contributor to psychosis rather than just a byproduct. During active psychotic episodes, the brain’s resident immune cells, microglia, become abnormally activated. This activation drives up levels of inflammatory signaling molecules, particularly IL-6 and TNF-alpha, both in brain tissue and in the bloodstream. Elevated levels of these molecules have been measured repeatedly in people with schizophrenia compared to healthy controls.
This chronic inflammation damages the blood-brain barrier (the protective lining that normally keeps harmful substances out of the brain), increases oxidative stress that harms neurons, and may worsen the dopamine and glutamate imbalances described above. The relationship likely runs in both directions: chemical imbalances trigger inflammation, and inflammation worsens chemical imbalances, creating a self-reinforcing cycle.
Genetics Set the Stage
Psychosis has a strong genetic component. Schizophrenia’s heritability is estimated at roughly 80%, based on a large meta-analysis of twin studies. That figure means most of the variation in who develops the disorder is attributable to genetic differences rather than environment alone. But the genetics are extraordinarily complex. No single gene causes psychosis. Instead, hundreds of small-effect genetic variants combine with a few rarer, higher-impact mutations.
Beyond C4, two other genes illustrate how genetics influence the brain in psychosis. The DISC1 gene is involved in how neurons migrate during brain development, how they extend their branches, and how they transport chemical receptors to the right locations. Variants of this gene have been linked to altered hippocampal structure in healthy people and to impaired working memory. The COMT gene codes for an enzyme that breaks down dopamine in the prefrontal cortex. One version of COMT produces a highly active enzyme that clears dopamine quickly, potentially contributing to the prefrontal dopamine deficit seen in psychosis. Despite the 80% heritability estimate, very little of the specific genetic architecture has been pinpointed precisely enough for targeted treatment.
Stress and Substances as Triggers
Genetics load the gun, but environment often pulls the trigger. Chronic stress activates the body’s stress-response system, flooding the brain with cortisol. The hippocampus and prefrontal cortex are especially vulnerable to sustained cortisol exposure because they have a high density of cortisol receptors. Over time, elevated cortisol can shrink dendritic branches (the tree-like extensions neurons use to communicate), reduce neuronal survival, and contribute to measurable reductions in brain volume. In people already at high genetic risk, this stress-driven tissue loss may be enough to tip the brain into psychosis.
Cannabis, particularly high-potency strains rich in THC, represents another well-documented trigger. THC activates cannabinoid receptors in the brain that normally regulate the release of other chemical messengers. When THC floods these receptors, it boosts dopamine levels in the striatum and mesocorticolimbic pathways, the same deep brain regions already implicated in psychosis. The relationship is dose-dependent: higher-potency cannabis and more frequent use carry greater risk. Animal research confirms that THC also affects the maturation of the dopamine system during development, which may explain why adolescent cannabis use carries a particularly elevated risk for later psychosis. The incidence of a first psychotic episode across the general population is about 50 per 100,000 people, but that rate rises substantially in populations with heavy cannabis exposure and other environmental risk factors.