Enormous evidence supports that individuals grow through stages of the development in 1) cognition/thinking, 2) moral decision making and 3) "ego development" among other factors. Quite a few studies, validated across dozens of cultures, draw similar conclusions about how we move from one stage of development to another. Synthesizing hundreds of developmental psychologists, Ken Wilber describes that movement as a three phased process of fusion, differentiation, and integration. Harvard psychologist, Robert Kegan, sketches development of the individual through at least 5 broad stages over a lifetime. But in the movement from each stage to the next, he also saw three phases repeated at every transition: "confirmation, contradiction, and continuity". The most famous researcher of cognitive development, Jean Piaget, used a variety of terms for similar growth processes between stages: assimilation, decentering/accommodation, equilibration.

I dubbed the three stages in the chart above as 1) merging/fusion, 2) e-merging/ differentiation, and 3) integration/reorganization to highlight a process of growth. In this case, a process that couples undergo repeatedly throughout their life together. For example, a couple goes along merrily for a time. Springboarding off that sense of security, one partner decides to enroll in an intense professional training program. The reduced time together stresses their relationship dynamics and they begin to fight more often. The other partner wants more time together. The couple goes through a period of increased conflict eventually resolved as partner number one winds down to the end of e.g., grad school, while partner number two develops more of his/her own interests. The couple even learns more about handling conflict and finally, almost as if a third entity, "the relationship" settles into a new harmonious balance between the two transformed partners. However, in short order their new found financial freedom leads to the decision to have children and then new turmoil.

The harmony at each stage enables growth that leads to a significant life turn or project which ultimately disrupts the harmony that enabled the growth. Now, their relationship dynamics need to reorganize to accommodate the changes/growth/life project. With luck, the couple successfully integrates the changes and settles down-until the next round of growth! And then they're off for another round of merging, e-merging, and re-integrating.

Many "theories" speculate about how couples go through stages though most are not supported by scientific evidence specific to couples. The chart above plays off a much more complex model of Drs. Ellyn Bader and Peter Pearson who distinguish sixteen "stages" of relationship development paralleling a great deal of Margaret Mahler's research on the stages young children go through when "hatching" from their parents from birth to about age three. The mother and child are "symbiotically" connected (fused) initially. The child doesn't differentiate between herself and her mother. Through many twists and turns by three, a healthy child establishes a sense of herself as a separate person though intimately bonded with mom. Similarly, couples typically join at the hip at first and only much, much later figure out how to be inter-independent. Their theory of couples stages doesn't claim to incorporate the research on individual cognitive, moral, and ego development throughout a human life; and there's no reason it should if, in fact, it accounts for all the stages couples go through. It simply takes inspiration from Mahler's descriptions of thecontours of the growth between mother and child, and then looks at the development of couples' relationships through the perspective of those same "contours".

[The following note is included for possible interest to other professionals: Because it relied on the narrow window of early childhood, in my opinion the Bader-Pearson model may be better thought of as an amalgam of stages and phases--the growth process from fusion through differentiation to integration between stages.
Stage models generally hold to a criteria that each more advanced stage "transcend and include" the functionality of the previous stages. It's also reasonable to expect that a couples stage model be consistent with, if not account for, the data subsumed into individual models of development. Certainly, the examples Bader and Pearson give of more advanced couples do generally describe relationships between individuals functioning at higher individual levels of development; but not rigorously nor exclusively. At times some teenagers describe their relationships progressed all the way through some of the characteristics fitting the highest level in Bader/Pearson's model, while people getting together much older in life do not necessarily exhibit the long list of enmeshed characteristics that Bader/Peason list for the first stage of a relationship. In fact, Bader gave separate seminars about how to apply her model when one person is significantly "narcissistic"--though narcissism is frequently considered as an arrest at a developmental stage. Further, a narcissistic person is incapable, by definition, of the relational skills characteristic of a well differentiated compassionate person who could be part of a relationship operating at an advanced level of development.
In contrast, Kegan draws on quite elaborated descriptions of development through the life cycle so that with his model it's instantly clear that a person arrested at the concrete level of thinking (7-12 years old) can't participate at the highly differentiated stage of development he named "Interindividual". It believe it would fruitful to combine Kegan's model of individual stages of development with Bader's way of charting the interactions of couples when each is at a different Kegan stage.]