Background: Family study data from a large community sample of young adults and their first-degree relatives were used to examine three questions regarding the relation between subthreshold depression (SubD) and major depressive disorder (MDD): (a) is there an elevated rate of MDD in the relatives of probands with SubD? (b) does SubD aggregate in the families of probands with MDD and SubD? (c) is the relationship between SubD and familial psychopathology specific to MDD?
Methods: A total of 941 probands were assessed twice during adolescence and then at age 24. Direct and informant information was obtained on 2750 first-degree relatives of 840 probands.
Results: The rate of MDD in the relatives of probands with SubD (24.3%) was significantly lower than the relatives of probands with MDD (31.9%) but was significantly higher than the relatives of probands with no history of mood disorder (NMD; 20.2%). Relatives of MDD, SubD and NMD probands did not differ on rates of SubD (9.0, 9.4 and 9.0%, respectively). Familial aggregation of psychopathology in SubD probands was specific to MDD.
Limitations: The primary limitation may be the absence of a standardized definition and assessment procedure for the SubD category. In addition, the sample size, although large, may have been inadequate to detect smaller associations or the moderating effects of sex.
Conclusion: The results data support the view that SubD occupies a milder position on a continuum with MDD.