Schizophrenia ordinarily appears first in adolescence or young adulthood, although it may not develop until later in life. Thus, people from families with a first- or second-degree relative who has been diagnosed with the disease are at greatest risk as they enter puberty and early adulthood.

Family History

People with parents who have both been diagnosed run a relatively high risk of developing the disease themselves; this risk is estimated to be about 40 percent. A person with an identical twin who has been diagnosed with schizophrenia is at the greatest risk of also developing the disease. Again, genetics is not the sole determinant of illness, however. The interplay among genetic defects, social factors and psychological factors determines final risk. As a result, an identical twin runs a 40 to 60 percent risk of also being diagnosed with schizophrenia. That the risk is not 100 percent underscores the contribution of environmental factors in the development of the disease. 

Other than a genetic predisposition, there are no clearly defined risk factors for schizophrenia. Although schizophrenia strikes men and women in equal numbers, it tends to be more severe among men and may appear somewhat later in women. Because schizophrenia rarely appears after the age of 45, people who surpass this age are at a reduced risk of being diagnosed with the disease.

Methamphetamine Abuse

Interestingly, chronic methamphetamine use may result in a form of psychosis similar to schizophrenia, including symptoms such as delusions of persecution and hallucinations. A condition known as methamphetamine-associated psychosis may linger even after recovery from addiction to the drug, and scientists have tentatively identified numerous genes that may play a role in this disorder; at least six of them are genes suspected to play a role in susceptibility to schizophrenia.