Secular trends in lifetime onset of MDD stratified by selected sociodemographic risk factors.

We used multivariate proportional hazards (Cox) models to investigate the effects of cohort of birth on age of first onset of major depression measured independently at two occasions, about six years apart, in the first degree relatives of probands with major affective illnesses. We estimated the cohort trends in strata defined by sociodemographic and other measures, to see if the cohort trends are the same across strata. Graphical summaries of the trends reveal a generally consistent pattern of increasing rates and earlier age of onset with successive birth cohorts, across all strata examined. The relatives with a divorced parent had a somewhat delayed secular increase, suggesting either a ceiling effect or an interaction of the two risk factors (recent cohort of birth and divorced parents) such that the combined effect is less than the sum of the individual effects. Otherwise, the cohort effect is persistent and ubiquitous in this sample.