Environmental Factors

Prenatal Virus Theory

Many theorists now believe that the brain abnormalities mentioned above are caused by exposure to a virus before birth. They propose that the virus enters the fetus’s brain and remains dormant until puberty or young adulthood. Studies have shown that mothers of people with schizophrenia were exposed to the influenza virus during the second trimester more often than mothers of other persons. Also, studies have found antibodies to a particular group of viruses usually found in animals in the blood of 40 percent of people with schizophrenia, suggesting that they had been previously exposed to such viruses. The theory about viruses being a possible cause for schizophrenia is primarily based on circumstantial evidence. The risk for schizophrenia is higher in those born during winter months and in cities, as well as in large families where there are short intervals between siblings (less than two years). These observations suggest that exposure to infectious agents early in infancy may help set the stage for later development of the disease.

Further evidence for this theory comes from recent studies involving twins. In the past, researchers had assumed that, if monozygotic twins were discordant for schizophrenia it was due to environmental factor that happened after birth (since they have the same genes and share the same environment before birth). The problem with this is that not all monozygotic twins do share the same environment. Depending on when the fertilized egg separates into to two, the fetuses can have one placenta or two placentas (the placenta is what connects the baby to the mother), meaning they might not have the same intrauterine environment. The data from these studies estimate that the concordance rate for schizophrenia is 10.7 percent in dichorionic twins and 60 percent in the monochorionic twins. This data provides evidence for a strong interaction between environment and heredity during prenatal development.

Oxygen Deprivation in the Fetus

Several studies have associated schizophrenia and problems surrounding birth, mostly in those that cause oxygen deprivation. Complications associated with increased risk for schizophrenia include prolonged labor, bleeding during pregnancy, short gestation period and low birth weight, and malnutrition in the mother during the first trimester of pregnancy.

Thiamine Deficiency

A normal adult who has been deprived of Thiamine (a vitamin that we get from food) and then suddenly has Thiamine reintroduced in the diet will have brain damage. It is possible that the build-up of toxins that occurs after Thiamine is reintroduced can also cause brain damage in developing fetuses. In the Hunger Winter (a severe food shortage that occurred in The Netherlands when Germany blockaded the country during World War II), the children of mothers who were in their second trimester of pregnancy when the blockade ended (and the mothers suddenly began eating a normal diet) were two times more likely to develop schizophrenia.

Psychological Factors

Parental influences are no longer believed to play any major role in the development of schizophrenia. However, there may be outside pressures and influences that exacerbate or trigger symptoms, particularly in the prefrontal lobes of the brain, which are extremely responsive to environmental stress. Because schizophrenic symptoms usually elicit negative responses from family and acquaintances, it is thought that negative feedback can exacerbate and even trigger symptoms.