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.
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.
There's 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 2 neurotransmitters: dopamine and serotonin.
Some studies indicate an imbalance between the 2 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 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.
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.
Research has shown that teenagers and young adults who use cannabis regularly are more likely to develop schizophrenia in later adulthood.