Causes and risk factors for schizophrenia

Many people ask, "Why did this happen to me, or to my family member?" There is no single answer. Researchers have not found one cause of schizophrenia. It seems to come from a mix of inherited risk, early brain development and life events.^1,2

This page explains what raises the risk and what does not. A risk factor is something that makes an illness more likely. It does not mean the illness will happen, and it is never a reason for blame.

How common is it, and who is affected?

The World Health Organization estimates that about 1 in 300 people worldwide live with schizophrenia.⁵ Some estimates of lifetime risk are closer to 1 in 100.² People are usually first diagnosed between ages 16 and 30.¹⁵

Nobody chooses to have it. It happens to people from every background.

Genes and family history

Schizophrenia sometimes runs in families, but no single gene causes it. Many genes each add a small amount of risk.¹ A large 2022 study found common gene differences at 287 places in our DNA that are linked to schizophrenia. The researchers also estimate that 60 to 80 out of 100 of the differences between people in who develops it are tied to genes.³

Here is what that means in real life:

• If one identical twin has schizophrenia, the other twin has about a 1 in 2 chance of developing it. For non-identical twins, the chance is about 1 in 8.²
• A parent or sibling with schizophrenia raises your risk more than sixfold. Even so, most people with an affected parent or sibling never develop it.⁴
• Because identical twins share the same genes and still do not always both develop it, genes cannot be the whole story.²

Brain development

People with schizophrenia may have small differences in the size of some brain areas and in how those areas connect. Some of these differences may begin before birth. Scientists are still studying how they lead to symptoms.¹

Chemicals that carry messages between brain cells may also play a role. These include dopamine and glutamate.⁴ This is one reason antipsychotic medicines, which act on these messenger systems, can help. You can read more in Antipsychotic medicines.

Problems around pregnancy and birth are somewhat more common in people who later develop schizophrenia. Examples are low birth weight, early labor and too little oxygen at birth.² A mother's viral illness or poor nutrition during pregnancy has also been linked to higher risk.^1,4 These are risk factors only. Most babies born after such problems do not develop schizophrenia, and a mother should never be blamed.

Cannabis (marijuana)

Cannabis is one of the best-studied risk factors you can do something about. The WHO says heavy cannabis use is linked to a higher risk of schizophrenia.⁵ The younger a person starts, and the more often they use, the greater the risk.⁴

Strength and daily use matter. A large study in 11 places in Europe and Brazil compared people having a first episode of psychosis with people from the same areas. People who used cannabis every day had higher odds of psychosis than people who never used it. The odds were higher still for daily use of strong cannabis, with 10% or more THC (the ingredient that causes the "high"). The researchers estimated that if strong cannabis were taken out of the picture, about 1 in 8 first-episode cases across the study sites might not have happened. In London the estimate was about 3 in 10, and in Amsterdam about 5 in 10, where strong cannabis was most common.⁷

What science can and cannot say. Most people who use cannabis never develop psychosis. Experts still debate how much cannabis causes the illness and how much it brings out an illness that was already waiting in a person with high genetic risk.^8,9 Because no one can tell in advance who is most vulnerable, teens and young adults, and anyone with a close relative with psychosis, are safest not using it.

Cannabis can also make symptoms worse in people who already have schizophrenia. If you use cannabis and want to cut back, see the page on therapy and recovery for help with substance use.

Other drugs, sleep and medical causes

Psychosis can also come from other things, and these do not always mean schizophrenia. Misusing alcohol or drugs, going without sleep for a long time, some prescription medicines and some medical conditions can all cause psychosis.⁶ In older adults, brain diseases such as Alzheimer's disease or Parkinson's disease can cause it.⁶ See psychosis in later life.

Stimulant drugs deserve a special mention. Cocaine and amphetamines, along with LSD, can raise the risk of psychosis.² Doctors check for drug and medical causes before they diagnose schizophrenia. Read how in getting diagnosed.

Stress, trauma and hard life events

Stress alone does not cause schizophrenia. But in a person who is already vulnerable, stressful events can help bring on the first episode of psychosis. Examples include losing a loved one, losing a job or home, the end of a relationship, and physical, sexual or emotional abuse.² Stress can also make symptoms worse for a person who already has the illness.⁴

Researchers have looked closely at childhood trauma. A review of 41 studies found that people who had hard childhoods, such as abuse or neglect, had about 2.8 times the odds of psychosis as people who did not.¹⁰ Most people who go through a painful childhood never develop psychosis.

City life and moving to a new country

Studies from several countries show that schizophrenia is more common in cities. A review of the research found that people living in the most urban places had about 2.4 times the risk of people in the most rural places.¹¹ Experts do not yet know exactly why. Stress and isolation are among the ideas being studied.

People who move to a new country, and their children, also have higher rates. A large review found that first-generation migrants had about 2.7 times the risk, and second-generation migrants (born in the new country) about 4.5 times. The second number is less certain because fewer studies measured it.¹² The risk was higher still for people who came from places where most people are Black.¹²

This does not mean any group of people is more "prone" to mental illness. The reasons are not fully known. Ideas include the stress of leaving home, hardship and unfair treatment. Even with a higher risk, most migrants do not develop schizophrenia. If you or a family member are new to this country, getting care in your own language with a trained interpreter can make a real difference.

What does not cause schizophrenia

Some old ideas still cause pain for families, so it helps to set them straight.

If you are a parent, you did not cause your child's illness. You can be an important part of their recovery. Learn how in helping a family member with psychosis.

Can it be prevented?

Not with certainty, because no one cause is known and genes cannot be changed. But some steps may lower the risk or catch problems sooner:

• Teens and young adults can avoid cannabis and other mind-altering drugs, especially if a parent or sibling has psychosis.
• Good prenatal care helps a pregnancy go well.
• Gradual changes in thinking, mood and social life often show up before a first episode.¹⁵ New withdrawal, odd thoughts or a drop in school or work are worth a check-up, so care can start sooner.

None of these guarantees anything. If someone develops schizophrenia despite doing everything right, that is not their fault.

When to get help

See a doctor soon if you or someone you love has new, strange beliefs, hears voices others do not hear, or pulls away from people and normal life. These can be early signs. The page on signs of schizophrenia lists what to look for.

If a person is in danger, may hurt themselves or someone else, or cannot care for themselves, call 911. If you or someone you know is thinking about suicide, or is in emotional crisis, call or text 988 (Suicide and Crisis Lifeline) at any hour. Read more in mental health crisis help. You can also visit our help page.

Education only. This page is general information written from the sources listed. It is not medical, legal or financial advice and does not replace a doctor, therapist or lawyer who knows your situation. How we write and check pages.