Family history and your genetics
Because many mental illnesses tend to run in families, researchers have been searching for specific genes, passed down through generations that may increase a person's chance of developing the illness. However,no evidence for the involvement of specific genes in causing mental illnesses has been found.
How does mental illness cluster in families?
People with a relative with mental illness have been found to have an increased risk of experiencing the condition as well. The likelihood of developing a mental illness may depend on the closeness of the blood relationship with your relative who has experienced a mental illness. For example, the lifetime risk for experiencing schizophrenia is 1%. When someone's second degree relative (e.g. aunt or uncle) has the condition, this risk rises to 2%. If someone's biological parent has schizophrenia, the risk is 13% and if someone's identical twin has the condition, the risk is 48%.
At present, although it does seem that there may be a degree of genetic involvement in developing a mental illness, we do not understand how this works.
What role do environmental factors play?
If mental illness had only a genetic cause, then when one identical twin has a mental illness the other twin should always develop a mental illness as well. This is because identical twins share exactly the same genes. However, studies show that when an identical twin has a mental illness, the other twin does not always develop it too. It is more likely that a variety of factors other than genes are involved.
This might be made clearer by comparing the pattern of development of mental illness to the pattern of development of a physical illness like heart disease. There does seem to be a tendency for heart disease to cluster in certain families, in the same way that mental illness sometimes clusters in families. Although genetic factors are believed to be partly involved in heart disease, other factors are also known to contribute such as smoking or lack of exercise. This is similar to schizophrenia, as environmental factors such as isolation or a stressful life event can trigger the condition in someone who may already be vulnerable. However even if someone is vulnerable it doesn’t mean they are definitely going to develop an illness. It is also not uncommon for someone with no family history of mental illness or heart disease to develop one of these conditions.
