Recurrent and nonrecurrent depression. A family study.

The morbidity risks for unipolar depression were determined from all 763 first-degree relatives of 75 probands with unipolar depression who had been followed up for 12 to 18 years after their first lifetime admission. Significant independent differences were found according to the proband's age at onset and whether the proband had had a single episode or recurrent depression. The lowest morbidity risk in the relatives (3.4%) was associated with single-episode depression with late age at onset in the probands and the highest risk (17.4%) with early age at onset and recurrent depression, other results being intermediate. These findings offer a subclassification of unipolar depression based on age at onset and on whether the illness is single episode or recurrent, and they may explain some of the variability in morbidity risk found in well-conducted studies where these factors were not considered.