Schizophrenia doesn’t have a single cause. It develops from a combination of genetic vulnerability, brain chemistry changes, structural brain differences, environmental stressors, and substance use. The condition affects roughly 1 in 300 people worldwide, with symptoms typically appearing in the late teens to early 20s for men and the late 20s to early 30s for women. Each of these five contributing factors can raise risk on its own, but they most often interact with one another.
1. Genetic Vulnerability
Genetics play the largest single role in schizophrenia risk. Heritability estimates from twin and family studies consistently land around 80 to 85 percent, meaning that most of the variation in who develops the condition can be traced to inherited factors. But “heritable” doesn’t mean “determined by one gene.” Researchers have spent roughly $250 million searching for specific genes of major effect, examining nearly 800 candidates including two of the most talked-about, COMT and DISC1. The results have been largely contradictory, with no single gene standing out as a clear driver.
Genome-wide studies have instead revealed that schizophrenia risk is spread across a staggering number of common genetic variants, with one estimate putting the count at over 1,500. These variants are individually tiny in their effect. Some sit near genes involved in the dopamine system, others in the complement system (part of the immune response that also shapes brain development during early life). The picture that has emerged is one of broad genetic susceptibility rather than a few broken genes. Having a close relative with schizophrenia raises your risk substantially, but most people with that family history never develop the condition, which is why the other causes on this list matter so much.
2. Brain Chemistry Imbalances
For decades, the leading explanation for schizophrenia symptoms centered on dopamine, a chemical messenger involved in motivation, reward, and how the brain filters incoming signals. The original theory proposed that overactive dopamine signaling in certain brain pathways caused the hallucinations, delusions, and disorganized thinking known as positive symptoms. Brain imaging in people with schizophrenia has confirmed that dopamine function is disrupted, though the strongest changes appear in movement-related pathways rather than the limbic (emotional) pathways researchers originally suspected.
Dopamine isn’t acting alone. Glutamate, the brain’s primary excitatory messenger, also plays a role. Some dopamine-releasing neurons co-release glutamate or another signaling molecule called GABA, and this co-transmission varies across different brain regions. Disruptions in the balance between these systems can cascade into the dopamine abnormalities seen on brain scans. This is why newer treatment approaches look beyond dopamine alone. The chemistry of schizophrenia is less like a single broken switch and more like a network of signals that have fallen out of sync.
3. Structural Brain Differences
People with schizophrenia show measurable physical differences in brain structure. Compared to people without the condition, total brain volume is about 2.2 percent smaller. Gray matter, the tissue that contains most of the brain’s nerve cell bodies, is reduced by roughly 3.3 percent overall and 4.4 percent in the prefrontal cortex, the region responsible for planning, decision-making, and working memory. At the same time, the fluid-filled ventricles deep inside the brain are enlarged: lateral ventricles are about 27 percent larger and third ventricles about 30 percent larger.
These differences aren’t static. Gray matter loss in schizophrenia appears to accelerate with age. In one study, people with schizophrenia lost about 3.4 milliliters of gray matter per year compared to 2.7 milliliters per year in healthy individuals. That faster rate of tissue loss may help explain why cognitive symptoms like memory problems and difficulty concentrating can worsen over time, even when other symptoms are being managed. It’s still unclear whether these brain changes are a cause of schizophrenia, a consequence of it, or both, but they are consistently present and likely contribute to the severity of symptoms.
4. Environmental Stressors
Genes set the stage, but environmental factors can push a vulnerable person toward developing the condition. Several categories of environmental risk have strong research support.
Prenatal and Birth Complications
Oxygen deprivation during birth is one of the most well-documented early risk factors. Perinatal asphyxia is associated with more than a fourfold increase in odds of developing schizophrenia, even after accounting for other complications and family history of psychiatric illness. Having three or more oxygen-related birth complications raises risk more than fivefold. Infections also matter: childhood viral infections affecting the central nervous system are linked to roughly double the risk of adult schizophrenia. Infection with the parasite Toxoplasma gondii raises odds by about 80 percent.
Maternal infection during pregnancy tells a more nuanced story. On its own, it does not significantly raise the risk of psychosis in the child. But when a mother has both an infection during pregnancy and a history of psychiatric illness, the two factors interact and the child’s risk increases meaningfully. Maternal infection also makes childhood infections about 50 percent more likely, and those childhood infections carry their own independent risk. It’s a chain of vulnerability rather than a single event.
Childhood Adversity and Urban Living
Severe childhood experiences, particularly the death of a close family member, are associated with higher rates of schizophrenia. The link is strongest for the most extreme stressors rather than everyday hardships. Growing up in an urban environment also raises risk compared to rural upbringing, and migration, especially to a country where you are part of an ethnic minority, has been identified as an independent risk factor across multiple studies. Even the season you’re born in plays a small role: people born in winter or spring months have a slightly higher incidence, possibly because of increased exposure to infections during fetal development.
5. Substance Use, Especially Cannabis
Cannabis is the substance most clearly linked to schizophrenia risk. Any cannabis use is associated with a 40 percent increased risk of developing psychosis. Daily use raises that figure to two to three times the risk of non-users. Researchers estimate that cannabis accounts for 8 to 14 percent of all schizophrenia cases, a meaningful share of a condition that affects millions of people globally.
Age of first use matters enormously. Starting cannabis before age 16 is tied to a higher likelihood of earlier-onset psychosis. Among people who develop a psychotic disorder, those who used cannabis experience their first episode roughly three years sooner than those who didn’t. This timing effect suggests cannabis isn’t just correlated with schizophrenia but actively accelerates the process in people who are already vulnerable, likely by disrupting the dopamine and glutamate systems during a critical window of brain development.
How These Causes Interact
No single factor on this list is sufficient to cause schizophrenia on its own. A person can carry extensive genetic risk and never develop symptoms. Another person can experience severe birth complications and childhood trauma yet remain unaffected. The condition emerges when enough of these factors converge in the same individual, each one slightly tipping the balance of brain development and chemistry.
This is why schizophrenia runs in families without following a simple inheritance pattern, why it appears more often in certain environments, and why it can seem to come out of nowhere in people with no obvious risk factors. The five causes listed here represent the best-supported categories of risk that researchers have identified, and understanding them helps explain both who is most vulnerable and why prevention efforts need to address more than just one piece of the puzzle.