The exact causes of schizophrenia are unknown, but research suggests that a combination of physical, genetic, psychological and environmental factors can make people more likely to develop the condition.

Current thinking is that some people may be prone to schizophrenia, and a stressful or emotional life event might trigger a psychotic episode. However, it's not known why some people develop symptoms while others don't.

Increased risk

Things that increase the chances of schizophrenia developing include:

Genetics

Schizophrenia tends to run in families, but no one gene is thought to be responsible. It's more likely that different combinations of genes make people more vulnerable to the condition. However, having these genes doesn't necessarily mean you will develop schizophrenia.

Evidence the disorder is partly inherited comes from studies of twins. Identical twins share the same genes. In identical twins, if one twin develops schizophrenia, the other twin has a one in two chance of developing it too. This is true even if they are raised separately. 

In non-identical twins, who have different genetic make-ups, when one twin develops schizophrenia, the other only has a one in seven chance of developing the condition.

While this is higher than in the general population (where the chance is about 1 in a 100), it suggests genes are not the only factor influencing the development of schizophrenia.

Brain development

Studies of people with schizophrenia have shown there are subtle differences in the structure of their brains. These changes aren't seen in everyone with schizophrenia and can occur in people who don't have a mental illness. But they suggest schizophrenia may partly be a disorder of the brain.

Neurotransmitters

These are chemicals that carry messages between brain cells. There is a connection between neurotransmitters and schizophrenia because drugs that alter the levels of neurotransmitters in the brain are known to relieve some of the symptoms of schizophrenia. 

Research suggests schizophrenia may be caused by a change in the level of two neurotransmitters: dopamine and serotonin. Some studies indicate an imbalance between the two may be the basis of the problem. Others have found a change in the body's sensitivity to the neurotransmitters is part of the cause of schizophrenia.

Pregnancy and birth complications

Research has shown that people who develop schizophrenia are more likely to have experienced complications before and during their birth, such as a low birth weight, premature labour, or a lack of oxygen (asphyxia) during birth. It may be that these things have a subtle effect on brain development.

Triggers

Triggers are things that can cause schizophrenia to develop in people who are at risk. These include:

Stress

The main psychological triggers of schizophrenia are stressful life events, such as a bereavement, losing your job or home, a divorce or the end of a relationship, or physical, sexual, emotional or racial abuse. These kinds of experiences, though stressful, do not cause schizophrenia, but can trigger its development in someone already vulnerable to it.

Drug abuse

Drugs do not directly cause schizophrenia, but studies have shown drug misuse increases the risk of developing schizophrenia or a similar illness.

Certain drugs, particularly cannabis, cocaine, LSD or amphetamines, may trigger symptoms of schizophrenia in people who are susceptible. Using amphetamines or cocaine can lead to psychosis and can cause a relapse in people recovering from an earlier episode.

Three major studies have shown teenagers under 15 who use cannabis regularly, especially "skunk" and other more potent forms of the drug, are up to four times more likely to develop schizophrenia by the age of 26.

Want to know more?

• Mind: mental health effects of street drugs
• Mind: what causes schizophrenia?
• Rethink Mental Illness: causes of schizophrenia
• Royal College of Psychiatrists: cannabis and mental health