The Role of Environment
Genes alone cannot account entirely for schizophrenia. It is likely that a combination of factors interacting with an inherited predisposition to the disease will someday explain who becomes ill and who remains healthy. Factors in the environment that produce physiological and/or psychological stress may cause one person to develop the disease while sparing another. Perhaps other genes help some people resist environmental factors that would push another person toward mental illness. At this time, we just don't know.
If genes contribute to schizophrenia, how can one identical twin have it and not the other?
Genes produce all the proteins that determine how the body works. Identical twins are born with the same genes, but after that, things change. Exposure to stress, for example, can influence how the activity of genes is regulated. In other words, personal experience can change how genes work.
It has been suggested that a long list of stress-inducing factors in the environment promote schizophrenia in susceptible individuals. These range from exposure to environmental toxins to viral infections.
Some Proposed Schizophrenia Triggers
Viruses and parasites
Maternal viral infection during pregnancy
Advanced parental age
Development problems before birth
Complications during birth
Adapted from Ronald J. Comer's Fundamentals of Abnormal Psychology.
A parasite called Toxoplasma gondii is suspected of playing a role in at least some cases of schizophrenia. It is a common organism; more than 60 million people in the United States have the parasite in their bodies. Most of them are not affected by this microscopic organism because their immune systems suppress it. Still, the U.S. Centers for Disease Control and Prevention believes it is among the top three causes of death from food-borne illness. The protozoa can be found in undercooked meat and cat feces.
T. gondii infection in humans can produce psychotic symptoms. Haloperidol, an antipsychotic drug, targets the protozoan. This leads some researchers to suspect that the parasite might be one of the reasons these drugs relieve some of the symptoms of schizophrenia. A couple of scientific reports conclude that among the risks in the environment for schizophrenia is exposure to cats during childhood.
Nearly a dozen studies have found significantly elevated levels of antibodies to T. gondii in people with schizophrenia. More research is needed, but it seems plausible that exposure to this parasite might contribute to the development of schizophrenia in some susceptible individuals.
There is no evidence for a specific “schizophrenia virus” in the environment. There is a fair amount of evidence, however, that a virus or viruses can contribute to development of the disease. Viruses could work in combination with genes that increase susceptibility to the disease. Or they might interact with other environmental factors that promote its development. Multiple studies indicate that pregnant women who get influenza during the first half of their pregnancy give birth to children who are more likely to develop schizophrenia than children whose mothers did not suffer from the flu.
A woman exposed to a significant amount of stress during the first six months of her pregnancy has an increased risk of giving birth to a child who will develop schizophrenia later in life. This phenomenon was demonstrated in children born to Dutch women who lived through the invasion of their country by the German army in World War II.