Schizophrenia has no single cause. It develops from a combination of genetic vulnerability, brain chemistry differences, prenatal exposures, and environmental stressors that interact over the course of a person’s development. The general population has roughly a 1% lifetime risk, but specific factors can raise that risk significantly.
Genetic Risk and Heritability
Genes are the strongest known risk factor for schizophrenia. If one identical twin has it, the other has about a 33% chance of developing it too, compared to 7% for non-identical twins. That gap tells researchers a lot: identical twins share nearly all their DNA, so the higher concordance points directly to genetics. But the fact that it’s 33% and not 100% also proves that genes alone don’t determine who gets schizophrenia. Something else has to go wrong.
More than 100 regions of the genome have been linked to schizophrenia risk, but one stands out. A gene called C4, which helps regulate how the brain prunes its own connections, carries the strongest known genetic risk. During late adolescence and early adulthood, the brain naturally eliminates synapses it no longer needs, streamlining itself for adult function. People with certain versions of the C4 gene produce more of a protein that tags synapses for elimination. The more of this protein the brain makes, the more synapses get pruned, and the higher the risk of schizophrenia. This timing lines up precisely with when symptoms typically first appear, in the late teens and early twenties, and may explain why the disorder emerges at that particular stage of life.
Dopamine, Glutamate, and Brain Signaling
Two chemical signaling systems in the brain are consistently disrupted in schizophrenia: dopamine and glutamate.
The dopamine connection is the oldest and most established. Every first-line medication for schizophrenia works by blocking a specific type of dopamine receptor in the brain. When researchers genetically engineer mice to overproduce these receptors, the animals display behaviors that mirror schizophrenia. In people with the disorder, there’s evidence that these receptors may be more sensitive or exist in a higher-activity state than normal, which could amplify dopamine signals in parts of the brain involved in motivation, reward, and the filtering of sensory information. This overactivity is thought to drive hallucinations and delusions.
The glutamate story adds another layer. Drugs that block a specific glutamate receptor (the same receptor targeted by ketamine and PCP) produce symptoms that look remarkably like schizophrenia in healthy people, including not just hallucinations but also the social withdrawal, flat emotional expression, and cognitive problems that dopamine models don’t fully explain. These drugs appear to preferentially shut down a type of brain cell that normally keeps other neurons in check. Without that braking system, neural activity becomes noisy and uncoordinated, impairing the brain’s ability to process information clearly. Animal models with reduced glutamate signaling show physical brain changes similar to those seen in schizophrenia, including loss of the tiny connection points between neurons and reduced volume in memory-related brain regions.
Prenatal Exposures
What happens in the womb matters. When a pregnant person’s immune system mounts a strong inflammatory response, whether from infection, severe stress, or nutritional deficiency, it can alter fetal brain development in ways that increase schizophrenia risk decades later.
Specific infections during pregnancy have been linked to higher rates of schizophrenia in offspring. After Finland’s 1957 influenza epidemic, researchers tracked children born to mothers who were infected and found elevated rates of schizophrenia per 1,000 live births. Maternal infection with influenza during the first trimester, exposure to the parasite Toxoplasma gondii, and herpes simplex virus type 1 have all been associated with increased risk. It’s not the infection itself reaching the fetus that seems to matter most. Rather, it’s the mother’s immune response: the flood of inflammatory signaling molecules that cross the placenta and disrupt normal brain wiring during critical developmental windows.
Severe psychological stress during pregnancy also qualifies as a priming event. Documented effects in offspring include both physiological and behavioral changes, with increased risk for schizophrenia and other psychotic disorders among them.
Cannabis Use During Adolescence
Cannabis use, particularly during teenage years, is one of the more actionable risk factors. A meta-analysis of six major long-term studies found that people who had ever used cannabis had a 40% higher risk of developing a psychotic disorder. For frequent users, the risk roughly doubled. One of the earliest and largest studies followed over 50,000 Swedish military conscripts and found that those who had tried cannabis by age 18 were 2.4 times more likely to receive a schizophrenia diagnosis, with risk increasing alongside frequency of use.
This doesn’t mean cannabis causes schizophrenia in everyone who uses it. Most users never develop a psychotic disorder. But for people who carry genetic vulnerability, adolescent cannabis use appears to act as a trigger during a period when the brain is still maturing and pruning its connections.
Urban Upbringing and Social Stress
Growing up in a city increases schizophrenia risk compared to growing up in a rural area. The explanation isn’t pollution or noise, at least not primarily. A leading theory, known as the social defeat hypothesis, proposes that the chronic experience of feeling excluded, outcompeted, or subordinated sensitizes the brain’s dopamine system over time. According to this framework, repeated social stress gradually trains the dopamine system to overreact, eventually crossing into the territory of psychosis.
The hypothesis ties together five major risk factors under one mechanism: urban upbringing, migration, childhood trauma, low IQ, and drug abuse. All involve prolonged exposure to situations where a person may feel excluded from a dominant group. Brain imaging studies have found that people raised in cities show altered stress responses in brain regions involved in processing threats and regulating emotions, and notably, this effect is linked to where someone grew up rather than where they currently live. The evidence for social defeat as a mechanism is strongest for migration and childhood trauma, while the link to urban upbringing is suggestive but still being studied.
Structural Brain Changes
People with schizophrenia have measurable physical differences in brain structure. The most consistent finding, documented since the earliest brain imaging studies, is enlarged ventricles: the fluid-filled spaces in the center of the brain. In schizophrenia, these spaces are roughly 130% the size of those in unaffected people, reflecting a loss of surrounding brain tissue rather than an increase in fluid production.
That tissue loss is widespread. Gray matter volume, the outer layer of the brain where most processing happens, is reduced across large areas of the frontal, temporal, and parietal lobes. These are regions responsible for decision-making, language comprehension, social cognition, and sensory integration. The thalamus, which acts as a relay station for sensory information, is also smaller, particularly on the right side. Even deep brain structures involved in movement and motivation show reduced volume. These changes are globally interrelated with ventricular enlargement, meaning the ventricles grow as the surrounding tissue shrinks.
Whether these structural differences are a cause of schizophrenia, a consequence of it, or both remains an active question. Some changes appear to be present before the first psychotic episode, suggesting they play a role in the disorder’s development rather than just reflecting its progression.
Inflammation and Immune System Activity
A growing body of evidence points to chronic, low-grade inflammation as a contributing factor. People with schizophrenia consistently show elevated levels of inflammatory signaling molecules in their blood, including several that are elevated from the very first episode and remain high throughout the course of the illness.
These inflammatory markers are not just slightly elevated. Multiple meta-analyses have confirmed that molecules involved in triggering and sustaining immune responses are raised in people at every stage of schizophrenia, from first episode to chronic illness to acute relapse. This pattern suggests that the immune system is not simply reacting to the stress of being ill but may be actively contributing to brain dysfunction. Inflammation can impair the blood-brain barrier, alter neurotransmitter signaling, and accelerate the loss of neural connections, all of which overlap with the brain changes seen in schizophrenia.
This immune component connects back to prenatal risk. If a mother’s immune activation during pregnancy can prime the fetal brain for later vulnerability, and if that vulnerability includes an overactive inflammatory response, the immune system may be involved in schizophrenia from its very earliest origins through its eventual onset.