I´ve been reading Why him? Why her? by Helen Fisher and it is great in its discussion of personality types and the connection with our dominant brain chemical (dopamine, serotonin, testosterone, or estrogen). My humble opinion is that the sudden personality changes in our spouses is due to changes in the predominant brain chemical and this seems logical to me. If it ties to this and issues of FOO, I would like to know how they find their way out of the mess. Also, if it is based on brain chemistry, 1) we are not the cause 2) we are not the solution. You can google her and take her personality test. Even without your spouse taking it, just by reading the descriptions, you´ll know. In the book she lays out the benefits and also the potential negative traits of each of the 4 types. She postulates that we are a mixture of a primary and secondary type: explorer, builder, negotiator or director. Her research shows that explorers do best with other explorers (dopamine based), builders do best with builders (serotonin based) and negotiators and directors do best when they combine. IT is for sure interesting reading.