Rumination is commonly defined as 'repetitive and recurrent, self-focused negative thinking about past negative experiences and/or negative mood'. It is similar to worry with the main difference being that rumination is to do with events or situations in the past while worry deals with events or situations in the future. Recent studies have shown that rumination is a powerful predictor of persistent post-traumatic stress disorder (PTSD) but it is not understood how rumination contributes to the condition. Two studies of assault survivors one cross-sectional (i.e. people's thoughts and problems here and now) and one over six-months looked at the connections between rumination and PTSD. Both studies found that a compulsion to keep ruminating, the occurence of unproductive thoughts, 'why' and 'what if' type questions and negative emotions before and after rumination were significantly associated with PTSD.
Michael, Tanja ... [et al] - Rumination in posttraumatic stress disorder Depression and Anxiety 24: 307-317