Definitions for terms in Bowen theory were established in clear and straight-forward language by Dr. Murray Bowen. Thus, these definitions are repeated almost without change in all follow-up publications. I have used Wikipedia versions.
Differentiation of self
Differentiation of self refers to one’s ability to separate one’s own intellectual and emotional functioning from that of the family. Bowen spoke of people functioning on a single continuum or scale. Individuals with “low differentiation” are more likely to become fused with predominant family emotions. (A related concept is that of an undifferentiated ego mass, which is a term used to describe a family unit whose members possess low differentiation and therefore are emotionally fused.) Those with “low differentiation” depend on others’ approval and acceptance. They either conform themselves to others in order to please them, or they attempt to force others to conform to themselves. They are thus more vulnerable to stress and they struggle more to adjust to life changes.
To have a well-differentiated “self” is an ideal that no one realizes perfectly. They recognize that they need others, but they depend less on other’s acceptance and approval. They do not merely adopt the attitude of those around them but acquire their principles thoughtfully. These help them decide important family and social issues, and resist the feelings of the moment. Thus, despite conflict, criticism, and rejection they can stay calm and clear headed enough to distinguish thinking rooted in a careful assessment of the facts from thinking clouded by emotion. What they decide and say matches what they do. When they act in the best interests of the group, they choose thoughtfully, not because they are caving in to relationship pressures. Confident in their own thinking, they can either support another’s view without becoming wishy-washy or reject another’s view without becoming hostile.