“As an undergraduate student in psychology, I was taught that multiple personalities were a very rare and bizarre disorder. That is all that I was taught on ... It soon became apparent that what I had been taught was simply not true. Not only was I meeting people with multiplicity; these individuals entering my life were normal human beings with much to offer. They were simply people who had endured more than their share of pain in this life and were struggling to make sense of it.”
― Deborah Bray Haddock, The Dissociative Identity Disorder Sourcebook
The causes of tertiary structural dissociation are very similar to the causes of secondary structural dissociation. That is, trauma that begins at a young age, lasts for a long time, is perpetrated by a family member or guardian, or involves more extreme disorganized attachment can result in either secondary or tertiary structural dissociation with tertiary structural dissociation resulting from more extreme or long-lasting trauma, in someone with less psychic defenses, or in someone who is more naturally dissociative. Tertiary structural dissociation is theoretically equivalent to dissociative identity disorder (DID), though in reality someone with other specified dissociative disorder-1 (OSDD-1) might fit the criteria while a few individuals with DID might not simply due to their number of parts.
Tertiary structural dissociation refers to the presence of multiple apparently normal parts (ANP) and multiple emotional parts (EP) within an individual. The ANP of tertiary structural dissociation each handle different (though possibly overlapping) aspects of daily life. For example, one ANP might be the host and handle school, relationships, and work. Another ANP might be better at relationships and so assist the first in that regard either knowingly or unknowingly (some believe that ANP always lose time when they switch with each other and are not aware that they are not each other; for some with DID, this may be true) while another might help with a specific subject matter in school or aspect of work. Another ANP might handle the system's family of origin or help to take care of dependent children. Individuals with DID do not have a single ANP that accurately reflects who they would be as a fully integrated personality. Before individuals with DID are aware of their condition, their ANP are highly phobic of their EP, though some ANP might seek out less threatening EP once they understand what the EP are. Because alters can be highly developed and have strong dissociative barriers between them, ANP can often safely interact with certain EP without necessarily experiencing bleed through of traumatic memories, perceptions, or urges.
Strategies that ANP use to avoid the activation of EP can involve amnesia, anesthesia, or limiting the ANP's range of emotions or numbing their emotion intensity. These avoidant behaviors combined with frequent intrusions from EP can drain mental energy and allow for depression, anxiety, or chronic feelings of hopelessness, shame, guilt, or rage. Particularly desperate ANP might engage in self harm or use psychogenic substances in attempts to forcefully tether themselves to the present and prevent EP from intruding. It is likely that emotionally disconnected ANP will struggle to form meaningful relationships with others and will have a poor grasp of their own physical and emotional needs. However, most ANP have a need to appear high functioning and may find it easier to avoid all potentially triggering situations and actions, including those relating to attaching to and trusting others, and throw themselves into work or other non-reflective activities.
As in secondary structural dissociation, the EP of tertiary structural dissociation handle different aspects of the trauma and may contain different memories, different learned responses, different internalized messages, different strong emotions, different attachment patterns, or different personal characteristics. The EP of tertiary structural dissociation are most like those found in OSDD-1. However, these EP are likely to be more complex and well developed than are those typically found in secondary structural dissociation, though some may remain fragmented traumatic containers. EP of those with DID may perceive themselves as belonging to a range of ages and are not always all small children who are frozen in time. EP may handle some aspects of daily life such as exploration, play, or socialization. Some may be very similar to ANP or even resent the implication that holding traumatic materials makes them somehow less developed, rational, or mature!
Individual EP may be focused more on defense through fight, flight, freezing, or submission, on specific emotions, or on specific types of attachment. It's common for EP to present in a stratified manner, with one or more EP presenting with their trauma as the previous EP and their trauma are processed and integrated. Multiple EP are often grouped according to the types of trauma that they experienced, with some EP even having experienced different aspects (sites, sounds, emotions) or being created to contain different defensive actions associated with the same trauma and traumatic memories. Some traumas might be associated with an experiencing part who registered the sensorimotor and affective elements of the trauma and with an observing part who witnessed the trauma from within or during an out-of-body experience. The observing part might present as emotionless and detached and is associated with the concept of an internal self helper.
Both EP and ANP can manifest through either passive influence or through full switches. Time loss and black outs may or may not always accompany full switches depending on the system's degree of co-consciousness. Systems with high degrees of co-consciousness are especially likely to deny each other or each others' memories, though denying each other also occurs for those who think to blame their black outs on a supposed medical condition, iatrogenic or sociocognitive influences, or a naturally poor memory, and denying each others' memories is common for all systems. On the other hand, both ANP and EP may be tempted to deny the body's current physical form or situation or may resent the system's job, spouse, children, or location. ANP and EP may all be avoidant of each other or of internal groups that they do not identify with and may react to these unfavorable alters or groups with shame, blame, or hatred. However, internal friendships and alliances are also common, and some systems may even group based on perceived family units.
Alters may or may not be aware of each other or that they are part of a whole. Alters may experience themselves as completely unique individuals and may view the other alters within their system as either completely separate individuals or experience them with a sense of "me but not me." They may intellectually own the system's shared history and current life and situations, but they may not feel that they have any control over each other and resent being made to feel as if they do. It is vital that systems take responsibility for all of their members, and most strive to do so, but actions that are not harmful but simply preference based may be completely disowned but alters that are not involved. For example, if a vegetarian alter is told that because their host loves meat a part of them does as well, they may be angered or sickened at the perceived accusation that they are allowing part of themself to engage in actions that they do not approve of. While alters may acknowledge that they share a physical brain and body, they may insist that they are no less unique than clones of an individual would eventually become. That some alters are so independent, differentiated, and developed (sometimes referred to as emancipated and elaborated) does raise questions about what constitutes a person.
It is known that individuals with DID may form both additional EP and additional ANP in adulthood depending on their experiences and needs. While new real or perceived trauma is most likely to create new EP, new ANP may result to give birth or take care of children, to handle new work responsibilities, or to replace previous hosts who have grown too worn to continue to manage daily life. New parts are created when new action systems, strong emotions, memories, or perceptions cannot be integrated into the whole of the individual or into any existing part. It must be remembered that no part can contain anything that is not available to the individual; to use a clear and extreme example, a part cannot be created to be a medical doctor if the system does not have medical training! On a more realistic level, a part cannot be created that can automatically heal the system or handle all of the system's current problems if the coping skills and desires to do so are not already present within the system.
Some facts from "The Haunted Self."
Hart, O., Nijenhuis, E. R. S., & Steele, K. (2006). The haunted self: Structural dissociation and the treatment of chronic traumatization. New York: W.W. Norton.