Changes in the risk of schizophrenia in Scotland: Is there an environmental factor?

Various studies have suggested that there may be an environmental factor in schizophrenia acting before or at birth but with delayed effects. Evidence that the risk of developing schizophrenia varies randomly with the year of birth would help confirm the existence of such a factor. Data from the Sco ttish Health Service Information and Statistics Division, comprising all fi rst admissions for ICD-9 schizophrenia in the years 1963-91, were used to e stimate the lifetime risk of developing schizophrenia for each year of birt h from 1900 to 1969, In the period after 1928 the lifetime risk steadily de clined. The rate of decline was greater in females. The random fluctuations in the risks in females did not change systematically, but there was a sig nificant decline in the variability of these fluctuations in males. These r andom fluctuations suggest a possible role for randomly varying environment al influences around the time of birth. Our findings suggest a possible dim inution in the role of such environmental influences in schizophrenia among males in Scotland in the years 1929 to 1969.