What causes schizophrenia? Despite enormous efforts by researchers, this question still defies a simple answer. In the sections that follow, we discuss what is currently known about the etiology of schizophrenia. What is clear is that no one factor can fully explain why schizophrenia develops. The old dichotomy of nature versus nurture is as misleading as it is simplistic. Psychiatric disorders are not the result of a single genetic switch being flipped. Rather, a complex interplay between genetic and environmental factors is responsible.
It has long been known that disorders of the schizophrenia type are “familial” and tend to “run in families.” There is overwhelming evidence for higher-than-expected rates of schizophrenia among biological relatives of index cases; that is, the diagnosed group of people who provide the starting point for inquiry (also called pro-bands). Figure 13.2 shows the percentage risk of developing schizophrenia given a specific genetic relationship with someone who has the disorder. As you can see, there is a strong association between the closeness of the blood relationship (i.e., level of gene sharing or consanguinity) and the risk for developing the disorder. For example, the prevalence of schizophrenia in the first-degree relatives (parents, siblings, and offspring) of a proband with schizophrenia is about 10 percent. For second-degree relatives who share only 25 percent of their genes with the proband (e.g., half-siblings, aunts, uncles, nieces, nephews, and grandchildren), the lifetime prevalence of schizophrenia is closer to 3 percent.
Of course, just because something runs in families does not automatically implicate genetic factors. The terms familial and genetic are not synonymous, and a disorder can run in a family for nongenetic reasons (if I am obese and my dog is also obese, the reasons for this are clearly not genetic!). As we have repeatedly emphasized, the interpretation of familial concordance patterns is never completely straightforward, in part because of the strong relationship between the sharing of genes and the sharing of the environments in which those genes express themselves. Although they are indispensable in providing a starting point for researchers, family studies cannot, by themselves, tell us why a disorder runs in families. To disentangle the contributions of genes and environment, we need twin and adoption studies.