Determined: A Science of Life without Free Will

by Robert M. Sapolsky

A great demonstration of this is the fact that if someone has the disease, their identical twin, who shares all their genes, has a 50 percent chance of having it as well (versus the usual 1–2 percent risk in the general population). The genetics of schizophrenia, however, are not about a single gene that has gone awry (as compared with classic single-gene disorders such as cystic fibrosis, Huntington’s disease, or sickle cell anemia). Instead, it arises from an unlucky combination of the variants of an array of genes, many of which are related to neurotransmission and brain development.[*] However, the collection of genes does not cause schizophrenia but, instead, increases the risk for it. This is implicit in flipping the finding just mentioned on its head—if someone has the disease, their identical twin has a 50 percent chance of not having it. In a classic gene/environment interaction, getting the disease basically requires a combination of the genetic vulnerability plus a stressful environment. What sort of stress? During fetal life, disease risk many years later is raised by prenatal malnutrition (for example, the Dutch Hunger Winter famine of 1944 greatly boosted the incidence of schizophrenia among individuals who had been fetuses at that time), exposure to any of a number of viruses by way of maternal infection, placental bleeding, maternal diabetes, or infection with the protozoan parasite Toxoplasma gondii.[*]