Questioning Kindling: An Analysis of Cycle Acceleration in Unipolar Depression

The kindling hypothesis for depression predicts that as the number of recurrences rises, the intermorbid interval (i.e., wellness period) between successive recurrences decreases in length. Studies with unipolar and bipolar samples have supported this premise. However past research may have been subject to a potential statistical artifact known as Slater's fallacy. This fallacy maintains that intermorbid intervals may appear to be shortening solely due to the fact that individuals with consistently shorter intermorbid intervals become a larger proportion of the remaining sample with each recurrence. When correcting for this potential bias, researchers have rejected the hypothesis for a progressive shortening of intermorbid intervals for bipolar patients. In the present study, I evaluated Slater's fallacy specifically for unipolar depression and assessed an alternative to kindling: that individuals who are highly recurrence-prone have shorter intermorbid periods between recurrences compared to individuals who are not highly recurrence-prone, in general and even specifically following the first lifetime episode. I also examined the variability in individual trajectories to determine whether subgroups of individuals may display particular trajectories in intermorbid intervals. Results indicated that cycle acceleration was not supported after correcting for Slater's fallacy. Further, individuals did not significantly vary in their trajectories for change in intermorbid interval length over time. Finally, an alternative model was supported, wherein highly recurrence-prone individuals exhibit significantly shorter intermorbid intervals than non-highly recurrence-prone individuals, on average and even following their very first depressive episode.