The exact causes of schizophrenia are unknown. Research suggests a combination of physical, genetic, psychological and environmental factors can make a person more likely to develop the condition.
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 do not.
Schizophrenia tends to run in families, but no single 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 does not necessarily mean you'll develop schizophrenia.
Evidence that the disorder is partly inherited comes from studies of twins. Identical twins share the same genes.
In identical twins, if a twin develops schizophrenia, the other twin has a 1 in 2 chance of developing it, too. This is true even if they're raised separately.
In non-identical twins, who have different genetic make-ups, when a twin develops schizophrenia, the other only has a 1 in 8 chance of developing the condition.
While this is higher than in the general population, where the chance is about 1 in 100, it suggests genes are not the only factor influencing the development of schizophrenia.
Differences in brain development
Studies of people with schizophrenia have shown there are subtle differences in the structure of their brains.
These changes are not seen in everyone with schizophrenia and can occur in people who do not have a mental illness. But they suggest schizophrenia may partly be a disorder of the brain.
Neurotransmitters are chemicals that carry messages between brain cells.
It is thought people with schizophrenia may have different amounts of certain neurotransmitters in their brains.
Medicines that help lower the amounts of certain neurotransmitters, such as dopamine, can help with the symptoms of schizophrenia in some people.
This suggests neurotransmitters play a role in the development of schizophrenia.
Pregnancy and birth complications
Research has shown people who develop schizophrenia are more likely to have experienced complications before and during their birth, such as:
- a low birthweight
- premature labour
- a lack of oxygen (asphyxia) during birth
It may be that these things have a subtle effect on brain development.
Triggers are things that can cause schizophrenia to develop in people who are at risk.
The main psychological triggers of schizophrenia are stressful life events, such as:
- losing your job or home
- the end of a relationship
- physical, sexual or emotional abuse
These kinds of experiences, although stressful, do not cause schizophrenia. However, they can trigger its development in someone already vulnerable to it.
Studies have shown using drugs, particularly cannabis, cocaine, LSD or amphetamines, can increase the risk of developing schizophrenia, psychosis or a similar illness.
It is not clear if using drugs directly causes symptoms in people who are susceptible to schizophrenia, or if they are more likely to use drugs.
If people have previously had episodes of psychosis or schizophrenia, using drugs can cause a relapse or stop symptoms from getting better.
Research has shown that teenagers and young adults who use cannabis regularly are more likely to develop schizophrenia in later adulthood. The risk may be higher when using stronger forms of cannabis.