“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 theory of Structural Dissociation works off of the assumption that no one is born with an integrated personality. Instead, infants operate based off of a loose collection of different ego states that handle their different needs- feeding, attachment to a caregiver, exploring the world around them. Over time, these ego states naturally integrate into one coherent and cohesive personality, usually by the ages of 6 or 9. However, childhood trauma disrupts this process. Different ego states are left unable to merge with each other due to conflicting needs, traumatic memories, or learned action paths or responses to trauma. One coherent sense of self cannot form when the primary caregivers of the child are inconsistent, loving one moment and abusive the next, preventing healthy attachment from occurring and instead facilitating disorganized attachment.
Vitally, this failure to integrate is not solely possible in children. Though the extreme disruption of self states that is found in dissociative identity disorder (DID) can only occur before the self has ever fully integrated, trauma that occurs after the integration of the self or that occurs at a young age but doesn't require a full division of self can cause traumatic materials (memories, perceptions, learned reactions, etc) to fail to integrate into the already existing or forming personality. If there is only one trauma, type of trauma, or series of connected traumas, the result may be that one part of the personality remains disconnected from the rest. If there are more traumas or more complex traumas, multiple parts of the self might remain unintegrated to serve as storage for related traumatic materials.
When only one trauma containing part, or emotional part, remains separate from the main personality, or apparently normal part, this is primary structural dissociation. When multiple emotional parts remain separate from the apparently normal part, this is secondary structural dissociation. In the case of DID, where there are both multiple emotional parts and multiple apparently normal parts, this is tertiary structural dissociation.
The following pages on structural dissociation are based primarily on the book "The Haunted Self: Structural Dissociation and the Treatment of Chronic Traumatization" by Onno van der Hart, Ellert Nijenhuis, and Kathy Steele. A less indepth but shorter and free source is this webpage.