As with many mental illnesses, schizophrenia remains, to some extent, shrouded in mystery. There is no single cause for the disease, but experts have long believed that it can be attributed to a combination of genetic and environmental factors. Rather than a "biopsychosocial" model, in which biological, psychological, and social factors are all important, research indicates that schizophrenia follows a "stress-diathesis" model, meaning that people inherit a significant vulnerability, which can be triggered by biological, psychological, or environmental stressors.

Heredity and Environment

Experts have long known that schizophrenia runs in families. Although the disease occurs in 1 percent of the general population, it is seen in 10 percent of people with a first-degree relative who has the disorder. A University of Wales College of Medicine study found that people who have a second-degree relative with the disease also develop schizophrenia more often than the general population, and an identical twin of a person with schizophrenia is most at risk, with a 40 to 65 percent chance of developing the disease.

Research also indicates that there is also a link between schizophrenia and environmental factors. Living in an urban environment, for example, has consistently been identified as a risk factor for the disease, as well as social disadvantage, such as poverty, migration, family problems, unemployment, homelessness, and racial discrimination. Some experts believe that child abuse or trauma can trigger schizophrenia later in life, although it's usually considered a contributing risk factor rather than a primary cause.

Schizophrenia and Substance Abuse

According to the National Institute of Mental Health (NIMH), most research does not indicate that substance abuse causes schizophrenia. However, in 2005, a team of researchers in the Netherlands reviewed five studies and found that the use of marijuana approximately doubled the risk of developing the disease. The studies excluded anyone with a history of psychosis and controlled for the use of other drugs, so they claimed they were able to show the specific effects of cannabis.

A newer study from the Albert Einstein College of Medicine in New York found that heavy use of cannabis caused brain abnormalities that are traditionally associated with schizophrenia. The study used a special type of MRI to study the brains of adolescents and revealed that the abnormalities were most pronounced in schizophrenic subjects who regularly smoked marijuana. According to the researchers, the abnormalities occurred in a brain pathway related to language and auditory functions that's still developing during adolescence.

Experts are quick to point out, however, that the relationship between substance abuse and schizophrenia is a complex one. According to the NIMH, some people who abuse drugs show symptoms similar to those of schizophrenia, and people with schizophrenia may be mistaken for people who are on drugs. In addition, people who have schizophrenia abuse alcohol and/or drugs more often than the general population. The most common form of substance abuse in people with schizophrenia is nicotine; schizophrenics are addicted to the drug at three times the rate of the general population.