We don’t yet know for sure. It is probably a combination of several different things, which will be different for different people.
Similar genetic ‘risk factors’ are involved in whether someone develops schizophrenia, severe depression or bipolar disorder. There are also environmental risk factors, and these can interact with genetic risk factors to increase or decrease someone's risk of developing these conditions.
For example, someone might have genetic risk factors that mean they are more likely to develop schizophrenia. However, if they grow up or live in a stable and positive environment this may protect them from developing a serious mental illness.
Having a parent with the condition
Having a parent with a serious mental illness like schizophrenia is the strongest known risk factor for someone developing a serious mental illness. Children with a parent who has a serious mental illness have a 1 in 3 chance of developing a serious mental illness themselves.
Although only 1 in 100 people get schizophrenia, about 1 in 10 people with schizophrenia have a parent with the illness.
An identical twin has exactly the same genetic make-up as their other twin, down to the smallest piece of DNA. If one identical twin has schizophrenia, their twin has about a 50:50 chance of having it too.
Non-identical twins have a different genetic make-up to each other. If one of them has schizophrenia, the risk to the other twin is just slightly more than for any other brother or sister. These findings are much the same even if twins are adopted and brought up in different families.
|Relatives with schizophrenia||Chance of developing schizophrenia|
|None||1 in 100|
|1 parent||1 in 10|
|1 identical twin (same genetic make up)||1 in 2|
|1 non-identical twin (different genetic make up)||1 in 8|
Brain scans show that there are differences in the brains of some people with schizophrenia – but not in others. Where this is the case, it may be that parts of the brain have not grown normally because of:
- a problem during birth that stops the baby’s brain from getting enough oxygen
- a virus infection during the early months of pregnancy.
Drugs and alcohol
Sometimes, street drugs seem to bring on schizophrenia.
Amphetamines can give someone psychotic symptoms, but they usually stop when they stop taking the amphetamines. We don’t yet know whether these drugs, on their own, can trigger off a long-term illness, but they may do if someone is vulnerable.
Some people start using drugs or alcohol to cope with symptoms, but this can make things worse.
The heavy use of cannabis seems to double the risk of developing schizophrenia. New research has shown that the stronger forms of cannabis, such as skunk, may increase this risk.
It’s more likely if someone starts using cannabis in their early teens.
If someone has smoked it frequently (more than 50 times) during their teens, the effect is even stronger – they are 6 times more likely to develop schizophrenia.
Difficulties often seem to happen shortly before symptoms get worse. This may be a sudden event like a car accident, bereavement or moving home. It can be an everyday problem, such as difficulty with school. Long-term stress, such as family tensions, can also make it worse.
At one time people thought that communication problems in the family could cause schizophrenia. This doesn’t seem to be the case. However, if someone has schizophrenia, family tensions can certainly make it worse.
A difficult childhood
As with other mental disorders, schizophrenia is more likely if someone was deprived or physically or sexually abused as a child.
A few people with schizophrenia do become violent – they usually hurt themselves but sometimes hurt other people. This can be caused by feelings of persecution or voices telling them to do it – often a combination of the two. It is much more likely if the person has used drugs or alcohol.
When thinking about the causes of developing schizophrenia, it is important to remember that lots of different things are involved, and that no one risk factor causes schizophrenia.