Dr. Murray discussed factors that have been reported to indicate risk for developing schizophrenia in adulthood.

One early risk factor is obstetric complications. Three separate meta-analyses suggest obstetric complications double the risk of schizophrenia later in life. Dr. Murray believes that a combination of genetic predisposition and anoxic birth injury may predispose to changes in the brain that increase vulnerability to later development of schizophrenia.

Another early risk factor for later development of schizophrenia is presence of neuro-developmental abnormalities in childhood. A large birth cohort study in the U.K. observed delayed motor milestones, speech defects, solitary behavior, cognitive decrement, and social anxiety in children who later developed schizophrenia.

Psychotic symptoms at a young age also may indicate risk for developing schizophrenia. In a study conducted by Dr. Murray and colleagues, nearly 800 11-year-old children were asked a series of questions, including "Have other people ever read your mind?" and "Have you ever had messages sent just to you through television or radio?" Of the children who answered either "yes, definitely" or "yes, likely," the rate of schizophrenia was 16-fold higher than normal by the time they reached the age of 26. Also predicting risk of later development of schizophrenia in that study were low IQ and presence of impaired motor development impairment.

Social isolation also has been reported to be a predictor of risk for later diagnosis of schizophrenia. Of 50,000 18-year-old men followed for 15 years by Swedish researchers, 0.4% developed schizophrenia. Those men were more likely to report factors related to social isolation, such as preference for small groups, few friends, or lack of intimate relationship.

Marijuana use is another risk factor for development of schizophrenia. In one study, 18-year-olds who reported smoking marijuana more than 50 times had a six-fold higher rate of schizophrenia compared with subjects who reported that they had never smoked marijuana.

Residing in an urban location is also associated with risk of developing schizophrenia. One study reported a 4-fold higher rate of schizophrenia in persons born in urban Copenhagen than in persons born in rural Denmark. Consistently, a study to be published the Archives of General Psychiatry reports higher rates of psychotic disorders in urban and rural areas (5.7 percent and 2.4 percent, respectively).