Familial and environmental aetiologic factors in paranoid psychosis quantified by a new statistical method.

A new mathematical model for quantification of familial factors was used in this study of psychiatric morbidity among the first-degree relatives of 25 patients with paranoid psychosis. The method provides a measure, a familial score of psychiatric morbidity of practical value in correlation analysis since it is applicable at different numbers of relatives per proband, excludes the influence of systematical errors, and allows the inclusion of background factors. A total of 159 first-degree relatives above the age of 15 were traced, 44 (28%) of whom were found to have a history of psychiatric disorder. The familial score of psychiatric morbidity, which varied between 2.0 and 23.0 in the 25 patients, was not significantly correlated to neurophysiological findings or clinical picture. A highly significant negative correlation between the familial score and parental age suggested that the relative influence of nonfamilial factors on the development of psychiatric disorder may be increased with increasing parental age.