What Causes Schizophrenia?
There is no single organic defect or infectious agent which causes schizophrenia, but a variety of factors increase the risk of getting the illness—among them, genetics and obstetric complications.
Relatives of people with schizophrenia have a greater risk of developing the illness, the risk being progressively higher among those who are more genetically similar to the person with schizophrenia (see Figure I.3). For a nephew or aunt the lifetime risk is about two percent (twice the risk for someone in the general population); for a sibling, parent, or child the risk is about ten percent, and for an identical twin (genetically identical to the person with schizophrenia) the risk is close to 50 percent.
Studies of people adopted in infancy reveal that the increased risk of schizophrenia among the relatives of people with the illness is due to inheritance rather than environment. The children of people with schizophrenia have the same increased prevalence of the illness whether they are raised by their biological parent with schizophrenia or by adoptive parents.
There is evidence implicating several genes in causing schizophrenia, and it is likely that more than one is responsible, either through an interactive effect or by producing different variants of the disorder.
Since identical twins only have a 50 percent risk of developing the illness, we know that genetics alone do not explain why someone gets the illness. Other powerful factors have to play a part; one of these is problems of pregnancy and delivery. The risk for people born with obstetric complications, such as prolonged labor, is double the risk for those born with none. A history of obstetric complications has been found in up to 40 percent of patients with schizophrenia, making it a major risk factor.
The risk of intrauterine brain damage is increased if a pregnant woman contracts a viral illness. We know that more people with schizophrenia are born in the late winter or spring than at other times of year, and that this birth bulge sometimes increases after epidemics of viral illnesses like influenza, measles and chickenpox. Maternal viral infections, however, probably account for only a small part of the increased risk for schizophrenia.