This question reflects a fascinating discussion that has been going on for decades. There is no agreement among philosophers, academics, scholars, scientists, psychiatrists, and clinicians. I have witness fights in professional settings when the opinions are so divided that some individuals that have dedicated years of their lives to the subject get devalued and lose their temper. The opinions are extreme, dived and controversial.
One of the most interesting and polemic points of view among clinicians is the argument that the formation of alters is simply a form of social compliance, possibly to conform with popular and psychiatric conceptions of psychopathology, but usually in response to therapists on the lookout for the disorder (Baynes et al, 2009).
This is true in many cases, and I have observed it myself. Clients that have a propensity to dissociate and are easy to hypnotize, are highly suggestible, and therefore could become victims of abusive therapists that have the need to control and enjoy exercising a significant influence on others. I have seen the clients and the therapists that fall in this description. Most of those clinicians are people that have unhealthy dissociative symptoms themselves and instead of working on integrating themselves, work on “integrating” others by fragmenting them. But I want to believe that this is the exception and not the rule —as many affirm.
But of course, it is difficult to generalize about DID (Dissociative Identity Disorder) and the structure of the alter-systems, because in many cases the presented structures of some people —that speak up about their own experience— can be quite complex, and there is little understanding of the extent of our minds’ power and reach.
The controversy goes beyond the regular conversations because there are individuals that go into talking about entities out of the self that comes and goes freely into a person’s system when the person hosts them (aware or unaware) due to either a low ego, or a lack of will (those highly suggestible individuals are prone to accept their fate without a fight).
I’m open to the possibility that a person can be a host of other minds (or energies) not as a deficit, but as a way to enjoy/experience the extensive possibilities of our existence. I don’t think it’d be fair to deny this option. We can’t really be sure of anything. Even when we have been granted a great cognition, it’s too limited to understand the extent of “reality” since what we think is real, it’s actually not. For example, we think we are standing in a still terrain, right? while in “reality” is that the terrain and everything else is spinning. Our cognition can understand it, but our perception cannot (Braude).
Going back to a more “grounded” and clinical version of DID, and exploring whether it really exists, it’s a proven fact that we suffer from fragmentation after traumatization, especially prolonged, as a way to protect our psyche from confronting a reality that seems unbearable. The typical example is the child that has a part that loves the (abusive) mother because s[h]e depends on her, while another part will grow hating the mother in the way of hating women, or despising people with the mother’s characteristics (unconsciously), or even having a part that mimics the mother as a way to internalize the hate.
Independently of trauma, what I have observed through many years of practicing IFS (Internal Family Systems) is that WE ALL HAVE PARTS. Well-integrated people have parts that know each other, while unintegrated persons have parts that are in conflict and pull into opposite directions creating internal emotional conflicts to the bearer.
Depending on how you frame it, a PART can go from simple emotions to emotional states, to ego states, to alters. You can read about the description of each of the different type of Parts in another post: What is the difference between parts and alters?
Therefore, you could differentiate the nature of the parts before you assign the diagnosis.
For me, the issue is not whether DID exists or not, is that the way is treated tends to fragment the psyche of the client further. Once you believe you have DID, you will think that all your parts are alters and will lose control over the protective qualities of your parts. I have seen how clients that believe they suffer from DID surrender control over the parts instead of gaining awareness and control over their emotions, and therefore, over their actions and responses.