Risk factors in schizophrenia. Season of birth and family history.

The association between the familial risk for schizophrenia and season of birth was studied in 88 schizophrenic patients. An increased risk for schizophrenia and 'spectrum' disorders was demonstrated among the first-degree relatives of winter and spring-born schizophrenic patients. However, patients with a family history of schizophrenia and 'spectrum' disorders did not differ from patients with no family history with respect to season of birth. Season of birth was unrelated to the sex of the patient, birth order, age at onset, or clinical subtypes (paranoid vs non-paranoid, as defined by the RDC, and 'narrow' vs 'broad', as defined by Taylor & Abrams' 1975 criteria). The morbid-risk data support a 'stress-diathesis' hypothesis whereby environmental factors (in this case a seasonally varying viral insult may be implicated) interact with genetic vulnerability to increase the risk for schizophrenia.