It seems that nearly every theory of psychology has its own version of subpersonalities, whether they refer to them by that term or not. The Jungians have complexes. Psychosynthesis has actual subpersonalities, and may be the source of the term. The Freudians have ego states. To be fair, John G. and Helen Watkins (creators of ego state theory and therapy) are not traditional Freudians, but they are firmly entrenched in the psychoanalytic tradition and use many of its terms in their theoretical work.
The description I want to post here is one of the best definitions of subs/parts/ego states that I have read anywhere, revealing many of the fine details of how these other "selves" function in us.
What follows is a lengthy passage from the Watkins' book, Ego States: Theory and Therapy. I've added links where they might be useful.
To understand and deal with ego states one must consider their origin. Often they were first created when the individual was quite young. Accordingly, they think concretely like a child. Adult logic may not reach them, even though they often talk in an adult voice. It is as if they were frozen in time. Covert ego states, unlike the alters in a true multiple personality, require hypnosis for their activation. But if they first originated when the patient was a child, then one should think of them like a small girl who is dressed up in mother's clothes and pretending to be an adult.This is textbook subpersonality theory, and it covers many essential elements important to specific theories within the field of subpersonality work.
Most adults have lost the ability to think concretely like a child, and thus we are at a disadvantage in dealing with child ego states, especially when they are not so differentiated as true multiple personality alters [editor's note: MPD is now called dissociative identity disorder]. States that were created during the subject's adolescence will think like a teenager, rejecting and suspicious of grown-ups and very defensive of their own independence; they don't want to be told what is right or what they ought to do. ... In dealing with child and adolescent states, one should conceptualize to whom one is talking and conduct the interaction accordingly. The clinician who resonates well (Watkins, J., 1978a) will be more successful.
One should be mindful that a child ego state was formed to adapt to the conditions of yesteryear, not today, and that often its attempts to function today result in maladaptation. As one inquires and secures some information about the time and the circumstances when an ego state first appeared, one's approach can be modified accordingly, even though one in confronted ostensibly with a full-grown adult. This modification can be done without embarrassment under hypnosis.
Another characteristic of an ego state is that it was probably developed to enhance the individual's ability to adapt and cope with a specific problem or situation. Thus, one ego state may have taken over the overt, executive position when dealing with parents, another on the playground, another during athletic contests, etc.
In the case of a true multiple personality, the specific situation is usually some very severe trauma such as child abuse. An ego state formed to help in dissociating the pain from the primary alter or to make it easier for the major personality to deal with an abuser without inviting retaliation. It is to be expected that these specific ego states will be reactivated in the transferences of the present, such as to teachers, employers, supervisors, colleagues -- and therapists (Watkins & Watkins, 1990a).
Another common trait is that once created, ego states are highly motivated to protect and continue their existence. The clinician who tries to eliminate a maladaptive state will find to his dismay that the entity probably does not disappear, but that one's intervention has now created an internal enemy who resists therapeutic intervention. Part-persons seek to protect their existence, as do whole-persons. ... It is much easier to modify the motivations of ego states and change their behavior constructively than it is to attempt their total elimination.
A related corollary to this persistence in existence is to realize that the original ego state came into being to protect and facilitate the adaptation of the primary person. It remained because it had a certain amount of success. And from what we have learned about behavior modification we can recognize that it was positively reinforced, first for coming into existence, then for continuing its being. Never mind that now its efforts may be counterproductive. The earliest conditionings hold precedence.
We must recognize the differences in the environment of the past and that of today. One cannot generally induce a state to change into an adaptive stance toward today's adult problems when its earlier struggles are not understood by the treating clinician. Recognition of these traits are essential in the planning and conduct of effective therapeutic maneuvers (such as abreactions).
Often when working with ego states the internal equilibrium of the subject will change in such a way that a new ego state (or one which has been long dormant) will be energized and will make itself known. This state may first manifest itself by slight changes in posture, mannerism, or voice pitch.
Ego states that are cognitively dissonant from one another or have contradictory goals frequently develop conflicts with one another. When they are highly energized and have rigid, impermeable boundaries, multiple personality may result. However, many such conflicts appear between ego states only covertly, and are frequently manifested by anxiety, depression, or any number of neurotic symptoms and maladaptive behaviors. (Watkins & Watkins, 29-30)
One qualm I have with this approach is the felt need for hypnosis. While I am sure that it is a useful tool, it seems to me unnecessary in the majority of cases. More often than not, such standard techniques as empty chair, active imagination, or voice dialogue, among many options, will do the job quite nicely when accessing ego states.
The key points from this passage that I want to highlight:
1) Ego states, or parts, were created when we were young. This being the case, they think like children, concretely.
2) Most ego states arise as a protective measure for the core self. While not as extreme as alters in DID, they serve the same purpose - to protect the fragile and vulnerable self.
3) When ego states were formed, their behavioral patterns were adaptive and useful, or they would not have persisted. While they may not be adaptive in adults, they (the ego states) don't realize that they are not helping, which is why they arose in the first place.
4) When an ego state, or part, "hijacks" the self, it assumes the role of executive in the self system, and we often do not realize we have been hijacked until after the fact.
5) Ego states, or parts, do not give up control easily. They see it as their sacred duty to keep protecting the self from any situation that resembles the one in which it came online. In IFS theory, these ego states are actually protecting much more fragile ego states, called exiles. Until those exiles are healed, the protector states aren't going anywhere. And any attempt to dislodge them will make them even more militant.
6) Ego states, or parts, can be in conflict with each other, especially when one state wants to engage in an activity that another state feels might be to risky in some way (embarrassment, shame, fear, or pain, and so on).
I hope to post one more entry on this topic, looking more closely at the origin of ego states, as theorized by the Watkins.