DIMINISHED RECALL AND THE COHORT EFFECT OF MAJOR DEPRESSION - A SIMULATION STUDY

Several large-scale epidemiological surveys have reported increasing l ifetime rates of psychopathology among recently born cohorts. In the c ase of Major Depressive Disorder (MDD) younger cohorts tend to manifes t higher lifetime prevalences of the condition than older cohorts, at any given age. In some studies, cohort differences are so large that t he youngest cohort exceeds the lifetime prevalence of the oldest cohor t well before passing through their total period of risk. The data on lifetime prevalences that support the existence of secular differences , however, has typically been collected in cross-sectional studies. Th us, individuals are interviewed at a single point in time and asked to recall all prior psychopathology. Due to poor recall, this design may greatly underestimate earlier experiences. In fact, cohort difference s are not limited to MDD; similar results have been reported for a wid e range of psychopathology, suggesting methodological problems at data collection. We have conducted a simulation study to examine the magni tude of annual rates of forgetting that could produce the secular tren ds reported for MDD. Small, but constant annual rates produce striking , 'cohort effect-like' curves. The rates needed to reconstruct the rep orted effect are compatible with published values for test-retest stud ies of lifetime recall of MDD. This simulation study does not rule out the existence of a cohort effect in some psychiatric disorders, but s tresses the possible limitations of using cross-sectional studies to i nvestigate secular trends.