I have been reading much research on "midlife" crisis. I do not like the term myself as research starts to become more entrenched in the term "midlife" and trying to pinpoint an age rather than looking at it as what I see as a personality flaw/disorder that causes a person to suddenly act in a way completely different from previous ways.
One thing that the research does state is that the MLCer is prone to cycling in and out of depression for years prior to the "crisis". If you read the threads, many have dealt with issues prior to BD. They may not have been as strong, but they were there.
There seems to be a triggering mechanism- loss of job, death of parent/close friend, or another tragic event which triggers the crisis into full bloom. The MLCer or personality-stressed person, then becomes focused on avoiding and changing their current life. The prime change being their current relationship. They seek something new and something different. Very interesting that most but not all choose someone younger than themselves. There is also a large amount of narcissistic behavior which is evident by the very lack or concern about the LBSer's feelings or needs as well as leaving friends and even children to pursue their new "life".
Much talk is centered on making themselves "happy" or it is their time to have "fun". I would say that if you were to do an analysis, the number that enter a full blown crisis like those that we read about is probably very low: one to two percent of the population. But one out of a hundred does add up.
I have tried to avoid reading the stages or putting to much thought into the stages. Not because I don't believe in them, but rather the focus of the LBSer becomes focused on hyper analysis on every tiny behavior thinking they are out of replay or in withdrawal or ready to reconnect that it creates false hope and expectations on the part of the LBSer rather than looking at themselves and getting a life which is much better tactic then trying to figure out where they are in the tunnel.
All comes to pass with time and when everything falls into place, you can look back and see the transitions as clear but you cannot see them at all as you make the journey.
This is all based upon the words of Ready. It is not scientifically sound but based upon what I have read on the various threads, the words of the newbies, the words of those that have been on the site for a long time, and the words of those that actually reconciled with their loved ones.