The short answer is this: a person is 11 times more likely to develop schizophrenia if he or she has a relative with the disorder. It's important to understand what this number means. Many studies show that schizophrenia occurs in 0.2 to 1.1% of people who have no relatives with schizophrenia. People who have a relative with the disorder get it at a rate of 1.4 to 16.2%. In identical twins, if one sibling has schizophrenia, the other has a 31- 78% chance of having the disease. These numbers mean that there is a strong genetic part to schizophrenia.
The fact that this disease has a strong genetic component doesn't necessarily mean that someone with schizophrenia will pass it down to his or her children. There are several ways in which people inherit disease. Huntington's disease, a movement disorder with many psychiatric symptoms, is caused by an error with a single gene that causes the disease in everyone who has the defective gene (in medical jargon, this is called autosomal dominant inheritance). Family members with the disorder can get a blood test to find out if they will get it or not. Schizophrenia is not like this. No specific gene that causes schizophrenia has been isolated and no blood test will prove whether or not a person will develop the condition or pass it down to their children. Researchers believe schizophrenia is a disease in which many genes conspire to cause symptoms. This is not a clear-cut situation, and unfortunately, represents the case for many diseases.
The question of whether schizophrenia puts relatives of patients at higher risk for other psychiatric disorders is a complicated one. There is some consensus that substance dependence and anxiety disorders are not specifically increased in relatives of people with schizophrenia. Recent evidence has suggested that depression is increased in relatives of people with schizophrenia. Definitive studies on these issues have yet to be done.
From my personal experience working with patients, schizophrenia presents itself in all families from all walks of life. The first psychotic break for a patient is often a time of fear and confusion for many family members. Despite the fact that schizophrenia has a strong genetic component, when it occurs, it usually does so as the result of random chance. Even if there are relatives in a family with the disorder, there's no way to predict whether a child will grow up to have schizophrenia, and there is little that can be done to prevent the disorder for emerging. Early detection and proper treatment, however, can have a substantial impact on quality of life. There are many types of schizophrenia and the disease occurs with varying severity. I will discuss these subtypes in a future article.