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The exact cause of schizophrenia is unknown, but most experts support what is known as the stress vulnerability model theory of schizophrenia.

The stress vulnerability model

The stress vulnerability model theory of schizophrenia states that every individual has a certain vulnerability to schizophrenia which is determined by a combination of biological, psychological and environmental factors. A stressful or traumatic incident can sometimes trigger the symptoms of schizophrenia in particularly vulnerable people.

For somebody with a high vulnerability to developing schizophrenia, it may only take a relatively moderately stressful event, such as losing a job, to trigger the condition.

However, it may require a significant stressful or traumatic event, such as a bereavement, to trigger schizophrenia in a person who has a lower vulnerability to the condition.

This does not explain what causes the initial vulnerability to schizophrenia, but a number of theories have been suggested, some of which are outlined below.

Schizophrenia and genetics

There is a great deal of scientific evidence that certain people can have an increased vulnerability to schizophrenia as a result of the genes that they inherit from their parents.

However, exactly what genes are involved, and how they are passed down through families, is still unknown, but there is strong evidence to suggest that a vulnerability to schizophrenia can run in families.

For example, if one of your parents has a history of schizophrenia, your chance of developing the condition is one in 10. This risk factor is 10 times higher than that of somebody with no family history of schizophrenia.

If you have an identical twin with schizophrenia, who shares the same genetic code as you, your risk of developing schizophrenia is one in two.

Schizophrenia and dopamine

Researchers believe that dopamine plays an important role in the development of schizophrenia. Dopamine is a neurotransmitter, one of many chemicals that your brain uses to transmit information from one brain cell to another.

Dopamine is associated with feelings of pleasure and reward. For example, when you experience something enjoyable, such as sex, the level of dopamine in your brain increases.

In someone with schizophrenia, it is thought that either the levels of dopamine in their brain become too high, or that their brain is particularly sensitive to the effects of dopamine.

The elevated dopamine levels can interrupt the specific pathways of your brain that are responsible for some of its most important functions such as memory, emotion, social behaviour and self-awareness. The disruption to these important brain functions may explain the psychotic symptoms of schizophrenia, such as hearing voices and delusional thinking.

The role of dopamine in schizophrenia is based on the fact that medicines that are known to reduce the effects of dopamine in the brain also reduce some of the symptoms of schizophrenia.

In addition, illegal drugs that are known to increase the levels of dopamine in the brain, such as cannabis, cocaine, and amphetamines, can trigger similar symptoms of psychosis to those that are often experienced by somebody with schizophrenia.

Schizophrenia and early development

The developmental theory of schizophrenia suggests that the condition is the result of an infection that interferes with the early development of the brain. It is believed that infections that occur while a woman is pregnant could have the potential to cause problems with her child's brain development much later on in life.

For example, one study found that children who are born to mothers who experienced a herpes infection during pregnancy, were six times more likely to develop schizophrenia than other children.

Brain imaging studies have also shown physical differences in the structures of the brain in people with schizophrenia compared to those without the condition.

Risk factors

A number of risk factors have been identified for schizophrenia, some of which are outlined below.

Illegal drug use

All illegal drugs carry a risk, but cannabis users are particularly vulnerable to developing schizophrenia. Regular cannabis users are twice as likely to develop the condition compared with people who do not use cannabis.

Heavy users of strong herbal cannabis, known as skunk, are thought to be six times more likely to develop the condition compared with non-users. A heavy user is defined as someone who uses cannabis at least once a day.

Being brought up in an urban environment

Rates of schizophrenia are higher among people who were born in urban environments such as a city or a town. This may be due to the fact that urban environments can be more stressful places to live compared to rural environments.

An alternative explanation is that mothers who live in urban environments are more likely to be exposed to an infection during pregnancy.

Being Afro-Caribbean

The rates of schizophrenia in the Afro-Caribbean community are higher than in other ethnic groups. It is not known whether this is due to genetic reasons, or if it could be the result of social pressures that are sometimes caused by being a member of an ethnic minority.

Being an immigrant

Rates of schizophrenia tend to be higher among people who immigrate to the UK. This may be due to the social pressures of trying to adapt to a new and unfamiliar environment. 

Previous stressful or traumatic life events

People who have experienced stressful or traumatic events, such as the death of a parent or a car accident, have a higher risk of developing schizophrenia than those who have not.

However, a major life event that most people would consider positive, such as winning the lottery, can also trigger schizophrenia in some vulnerable people.

view information about Schizophrenia on www.nhs.co.uk »

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