Schizophrenia has no single cause. It develops from a combination of genetic vulnerability, brain chemistry imbalances, and environmental triggers that interact across a person’s life, often from before birth through early adulthood. The condition affects roughly 1 in 345 people worldwide, and understanding what drives it means looking at several interconnected factors rather than any one explanation.
Genetics Play the Largest Role
Of all the contributing factors, inherited genes carry the most weight. A landmark Finnish twin study found that 83% of the variation in schizophrenia risk comes from genetic factors, with the remaining 17% from environmental influences unique to each individual. That doesn’t mean a single “schizophrenia gene” exists. Hundreds of small genetic variations each contribute a tiny increase in risk, and their effects add up.
Having a first-degree relative with schizophrenia raises your risk substantially compared to the general population. But genetics alone don’t seal your fate. Identical twins share virtually all their DNA, yet when one twin develops schizophrenia, the other develops it only about half the time. Something beyond genes has to push a vulnerable brain over the edge.
Too Much Dopamine in the Wrong Places
The most established chemical explanation involves dopamine, a brain signaling molecule tied to motivation, pleasure, and perception. In schizophrenia, dopamine doesn’t malfunction everywhere equally. Deep brain structures involved in emotion and reward become flooded with excess dopamine, while the prefrontal cortex, the area responsible for planning and reasoning, gets too little. This pattern is sometimes called the “revised dopamine hypothesis.”
The dopamine excess in deeper brain regions drives what clinicians call positive symptoms: hallucinations, delusions, and paranoia. These symptoms occur because overactive dopamine signaling in the brain’s reward circuitry distorts how incoming information gets tagged as important or meaningful. Ordinary events start to feel loaded with hidden significance. Meanwhile, the dopamine shortage in the prefrontal cortex contributes to difficulty concentrating, flat emotional expression, and loss of motivation.
Most antipsychotic medications work by blocking dopamine receptors, which is why they tend to reduce hallucinations and delusions more effectively than they improve cognitive or motivational symptoms.
Glutamate and the Other Chemical Story
Dopamine isn’t the whole picture. For over two decades, researchers have known that drugs blocking a different type of brain receptor, the NMDA glutamate receptor, can produce the full range of schizophrenia-like symptoms in healthy people: hallucinations, social withdrawal, and cognitive problems. Ketamine and PCP are two well-known examples. When these same drugs are given to people who already have schizophrenia, their symptoms get dramatically worse and can stay worse for extended periods.
Glutamate is the brain’s primary excitatory signal, meaning it helps neurons fire and communicate. When NMDA receptors don’t function properly, the downstream effects include reduced connections between neurons, shrinkage in brain areas like the hippocampus, and impaired memory. This glutamate dysfunction likely explains the cognitive difficulties and emotional blunting that dopamine-focused treatments struggle to address.
The Brain Looks Physically Different
Brain imaging consistently shows structural differences in people with schizophrenia. The fluid-filled spaces inside the brain, called ventricles, are about 127 to 130% the size of those in healthy individuals. That might sound dramatic, but the actual volume difference is modest, around 1.5 cubic centimeters in younger patients and closer to 9.6 cubic centimeters in older ones.
What matters more is what the enlargement reflects. It’s a sign of lost gray matter, the tissue containing the brain’s processing cells. In older patients with schizophrenia, gray matter volume is measurably reduced, and the total loss of brain tissue is roughly 3.5 times greater than what the ventricular enlargement alone would suggest. Much of this tissue loss shows up as widened grooves on the brain’s surface, where cortex has thinned. Progressive cortical shrinkage over time has been estimated at approximately 37 cubic centimeters compared to healthy individuals, enough to fully account for the ventricular changes.
A Hidden Vulnerability Unmasked in Adolescence
One of schizophrenia’s most distinctive features is its timing. Symptoms typically emerge in late adolescence or early adulthood, even though the underlying vulnerability is present from much earlier. The leading explanation centers on synaptic pruning, a normal developmental process where the brain eliminates excess connections to become more efficient.
During adolescence, everyone’s brain prunes synapses, particularly in the prefrontal cortex. In people predisposed to schizophrenia, this pruning appears to be excessively aggressive. Specific genetic risk variants increase production of a signaling molecule called C4A, which tags neural connections for removal by the brain’s immune cells (microglia). Laboratory studies show that when microglia or neurons are derived from people with schizophrenia, the engulfment of synapses is measurably more aggressive than in cells from healthy controls, and the degree of excess pruning correlates with C4 expression levels.
The result is that prefrontal circuits, which were already subtly impaired from early development, lose enough connections during adolescent pruning to cross a critical threshold. Below that threshold, the circuits can no longer maintain the synchronized firing patterns needed for working memory, planning, and reality monitoring. This collapse in prefrontal function also disrupts the brain’s control over dopamine release in deeper structures, which helps explain why psychosis emerges at the same time cognitive problems worsen.
Infections and Nutrition Before Birth
What happens during pregnancy can significantly shape schizophrenia risk. Prenatal exposure to certain infections is one of the strongest non-genetic risk factors identified. Rubella is the most dramatic example: 20% of children exposed to rubella in the womb went on to develop schizophrenia in adulthood, representing a 10 to 20-fold increase in risk.
Influenza during pregnancy carries a smaller but still meaningful effect. Exposure during early to mid-pregnancy is associated with a 3-fold increased risk, and first-trimester exposure specifically confers a 7-fold increase. Toxoplasmosis, a parasitic infection often contracted from undercooked meat or cat litter, is linked to a 2.5-fold increase when detected in maternal blood during pregnancy. Herpes simplex virus type 2 has also been associated with psychotic disorders in offspring.
The common thread appears to be inflammation rather than the specific pathogen. Mothers whose pregnancies gave rise to children who later developed schizophrenia had nearly twice the normal levels of certain inflammatory signaling molecules during the second trimester. The developing fetal brain is exquisitely sensitive to immune activation, and sustained inflammation during critical periods of brain formation can alter how neural circuits are wired, creating vulnerabilities that remain hidden until adolescence.
Cannabis and Schizophrenia Risk
Cannabis use is consistently linked to higher schizophrenia risk, though the relationship is more modest than some headlines suggest. A large genetic analysis found that cannabis users had 1.37 times the odds of developing schizophrenia compared to non-users. Observational studies put the figure at 1.43 times, and adjusting for tobacco use didn’t change the association.
These numbers represent population-level averages. For people who already carry significant genetic risk, cannabis use during adolescence, when the brain is undergoing synaptic pruning, likely poses a greater threat than it does for someone with low genetic vulnerability. Cannabis affects the same dopamine and glutamate systems implicated in schizophrenia, and using it during a period of active brain remodeling may accelerate the loss of prefrontal connections in predisposed individuals.
Childhood Trauma and Urban Environments
Growing up in a city and experiencing childhood trauma both independently raise the risk of psychotic disorders, but their combination is particularly potent. Research comparing people who grew up in different environments found that the link between childhood trauma and psychotic disorder grew stronger at each level of urban density. In the least urban settings, people with high trauma scores had 2.76 times the odds of developing a psychotic disorder. In the most urban settings, that figure climbed to 5.66 times.
The interaction suggests these factors don’t simply stack on top of each other. Urban environments may amplify the biological impact of trauma, possibly through chronic social stress, noise, pollution, or reduced access to the natural environments that buffer stress responses. For someone with genetic vulnerability, the combination of a stressful upbringing in a dense urban setting could provide enough environmental pressure to tip brain development toward illness.
The Prodromal Phase: Warning Signs Before Psychosis
Schizophrenia rarely arrives without warning. About 75% of people who develop the condition pass through a prodromal phase, a period of gradual change that precedes the first psychotic episode. Nonspecific symptoms like anxiety and mood disturbances often appear well before any hint of psychosis, while subtle psychotic-like experiences, such as feeling that ordinary events carry personal significance or perceiving things slightly differently, typically emerge about a year before a full episode.
The behavioral changes during this phase can include social withdrawal, declining performance at school or work, neglect of personal hygiene, flattened emotions, vague or disorganized speech, odd beliefs, unusual perceptual experiences, and a noticeable loss of motivation or energy. Sleep disturbances, irritability, and depression are also common. In adolescents, these signs are easy to dismiss as typical teenage behavior, which is part of why the average delay between symptom onset and treatment remains so long. Recognizing this pattern, especially in someone with a family history, creates an opportunity to intervene earlier, when treatment tends to be most effective.