Why do some people develop schizophrenia rather than others? Risk factors are the characteristics that may make a person more likely than others to develop a disorder like schizophrenia.

Schizophrenia is caused by a complex interaction between biological and environmental factors. Thus, both nature and nurture are involved in the disorder. The “nature” side has been supported by heredity studies. Indeed, ample research suggests that schizophrenia runs in families.

Family studies are conducted by identifying target individuals called probands. In this case, researchers study families with schizophrenia patients and determine how many family members have the disease. People with schizophrenia are much more likely to have biological relatives with schizophrenia than are people without schizophrenia. For people without schizophrenia, the likelihood that they will have a family member with the disorder is less than 1 percent, whereas approximately 10 percent of the first-degree relatives of individuals with schizophrenia also have the disorder. Three percent of second-degree relatives (first cousins), who share only 12.5 percent of their genes, also have schizophrenia.

Twin Studies
Family members usually share more than genes; they share a common environment.  In order to determine the impact of genes and environment, researchers must become creative in their experimental design. One way in which researchers can disentangle genes from environment is through twin studies.

Twin studies are studies comparing the concordance for a disorder in identical (monozygotic) to fraternal (dizygotic) twins. Identical twins share 100 percent of their genes, whereas fraternal twins share only 50 percent of their genes, the same as nontwin siblings. If schizophrenia were entirely determined by genetics, we would expect all identical twins to be concordant for the disorder, meaning if one twin had schizophrenia the other would have it as well. It turns out that this is not the case. Although the concordance rates are significantly higher for identical twins compared to fraternal twins, far less than 100 percent of identical twins with schizophrenia are concordant for the disorder.

If a member of an identical twin pair has schizophrenia, his or her twin has a 28 percent chance of also having the disease. In contrast, if a member of a fraternal twin pair has schizophrenia, his twin has only a 6 percent chance of also having the disease. This suggests a very significant nongenetic (or environmental) influence on the development of schizophrenia.

Heather Barnett Veague, Ph.D.
Heather Barnett Veague attended the University of California, Los Angeles, and received her Ph.D. in psychology from Harvard University in 2004. She is the author of several journal articles investigating information processing and the self in borderline personality disorder. Currently, she is the Director of Clinical Research for the Laboratory of Adolescent Sciences at Vassar College. Dr. Veague lives in Stockbridge, Massachusetts, with her husband and children.

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