Because many mental illnesses tend to run in families, researchers have been searching for specific genes, passed down through generations (heredity) that may increase a person's chance of developing the illness.
The evidence that heredity plays a role in the development of some forms of mental illness has been discovered by studying identical twins who were raised separately.
Identical twins are genetically identical but have had different nurturing environments and non-identical twin children (related but genetically different) of mothers with different forms of severe mental illness. These different types of twins are then compared in their risk for developing a severe mental illness against the general public who do not have a mental illness.
Evidence from twin studies
If an identical twin that is raised separately from its other twin develops schizophrenia, the chance that the second twin will also develop the condition is 50%, or 1 chance in 2. In non-identical twins however, (who would share on average only half of their genetic makeup) when one twin develops schizophrenia, there is only a 15% chance that the other twin will also develop the disorder.
While this risk is higher than for the general population (who have about 1 chance in 100 or 1%) of developing the condition, it suggests that heredity is not the only factor influencing the development of schizophrenia in these families otherwise twin study results would show a 100% risk between identical twins (you may hear this referred to as concordance).
Many people worry that, because someone in the family has experienced mental illness, they might be next, but being related to someone does not necessarily mean that you will also develop it.
As with many other health problems, a family history of mental illness will increase the risk for other family members, but the degree to which the risk is increased depends on the closeness of the blood relationship to the person affected.
- If there is no known family history, your risk of developing psychosis is 1%
- If a grandparent, uncle or aunt has psychosis, the risk of developing psychosis is 3%
- If a parent, brother or sister has psychosis, the risk increases to 10%
- If an identical twin has psychosis, the risk is 50%.
However it is important to note that while having a relative who has experienced mental illness is a risk factor for developing severe mental illness, a combination of a number of different risk factors is needed to go on to develop psychosis.
You can find out more in our Siblings Network factsheet Genetics and mental illness.