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.