“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
Integration in its most basic form occurs any time that information is processed. When an individual incorporates a fact into their understanding of their self or an event into their understanding of their personal history, that's integration. Dissociation can be seen as a failure of integration. When an individual is struggling with depersonalization or derealization, they're having difficulty processing relevant information about their self or environment in real time. When an individual has dissociative amnesia, their memory of the traumatic or stressful event(s) are kept separate from their other memories and may be accessible only through dissociative flashbacks. When an individual has dissociative identity disorder (DID) or other specified dissociative disorder subtype 1 (OSDD-1), information is stored in separate dissociated parts, known as alters.
Every individual who has been through trauma must integrate to some extent as part of healing. This means accepting that the trauma occurred, making it part of one's personal narrative, and making it accessible in a way that does not cause intense re-experiencing of trauma elements. In doing so, an individual may have to accept thoughts, feelings, and urges associated with their trauma. For example, an individual with posttraumatic stress disorder (PTSD) may find that as they integrate their trauma history into their personal narrative, they have to also process feelings of helplessness, betrayal, fear, or anger. In terms of structural dissociation, the individual has to integrate the emotional part(s) associated with their trauma, and that means having to take ownership of everything that the part(s) contained.
For individuals with DID or OSDD-1, however, some or all of their parts are likely to go beyond simple containers of traumatic materials. For individuals with DID, some alters may have strongly developed independent senses of autonomy and self. The individual must then make the choice of to what extent they want to integrate their alters as part of their healing. Again, some degree of integration is inevitable. The individual must integrate traumatic materials in order to heal from PTSD. As well, enough integration between alters must occur to allow for easy communication and a lack of dissociative amnesia between parts. The individual must be able to take responsibility for all of the system's actions, and all alters in the system should work together towards the same goals.
Some systems choose to stop there, at what the ISST-D calls resolution. In this case, they may retain any number of independently acting alters. Reasons for choosing not to fully integrate can include: feeling that full integrate is unnecessary; not understanding what integration actually entails and being afraid of "losing" their alters; uncertainty over how to navigate the world as one integrated person; being used to having alters around for company, entertainment, or support; alters having their own unique relationships that they're hesitant to lose; alters wanting to remain separate for their own sakes; or the individual not wanting to lose attention, support, or a sense of being unique that they feel is associated with remaining dissociated. Unfortunately, even some individuals who might otherwise want to integrate can find it impossible to integrate all of their parts or to maintain one integrated personality over time or when faced with stress. This can occur when an individual is in a highly stressful or unsafe environment, can't bring themself to fully accept their trauma history, can't access treatment from professionals who are knowledgeable about DID, or can't afford to continue treatment or when symptoms from comorbid personality or other disorders interfere. Some question if integration is even permanently possible, and many therapists are less insistent on full integration as the only possible treatment for DID than they were in the past. Even if a therapist tries to push their client to integrate, permanent integration cannot be forced, and an integration that occurs before the system was ready for it is very likely to fall apart.
That said, even individuals with DID who are vocal about their desire not to integrate are likely to spontaneously integrate some alters and fragments as they process and heal, many systems willingly integrate down to a few alters (including well known systems such as Robert Oxnam), many individuals who are fully integrated are convinced that integration is the best option, and some studies (such as this one) do suggest that complete integration is the option most likely to result in long-term stability and healing. In order to integrate two or more alters (which the ISST-D refers to as "fusion," with "final fusion" referring to a complete integration of all dissociated parts), the individual needs to take ownership of all thoughts, feelings, memories, urges, skills, and other traits that were previously associated with those parts of the self. Integration is complete when there are no subjective differences between the parts involved; only one sense of self remains.
Integration of alters can be experienced in different ways. For fragments, integration may simply entail other alters being able to access what those parts held without a switch being necessary. There may be no major difference in how other alters perceive themselves or the world. Even with more developed alters, one alter may seem to integrate into the other so that the resulting part retains the identity of one of the alters involved but gained some of the skills, traits, preferences, or views of the other. Another possibility is that the integration of two or more alters may lead to the creation of a "new" alter that contains some combination of traits from the parts that integrated. This alter may feel like all or none of the alters involved but is regardless an acknowledgment that what the alters held no longer needs to be kept separate. Finally, an integration may indicate a shift that has already occurred in the system. For example, if an alter primarily held acceptance of same sex desires, that alter may no longer be perceived as separate as the system as a whole moves towards accepting their sexuality.
It must be noted that not every trait that an alter held will be experienced in the resulting part in the exact same way as it was prior to integration. Traits such as gender identity, sexuality, or religion might differ between parts, and the individual will need to figure out for themself how they feel in relation to these and other points of conflict. Some preferences that alters had may be muted when no longer contained in relative isolation; for example, the integration of an alter who really loved hard rock is unlikely to completely change the musical preferences of an individual who loves classical music, but the individual might find that the integration results in a greater tolerance for rock music or widens the range of music that they enjoy. Some traits may be lost entirely, such as an alter's unhealthy ability to ignore pain at the expense of respecting one's physical limits. Finally, some skills or abilities that alters excelled at may require additional practice as an integrated individual before they can be fully expressed.
Even when some traits are lost, integration that was not forced or rushed should not feel like a loss. Healthy integrations feel like what they are: an acceptance of aspects of oneself that one wasn't previously able to fully accept. Some individuals do need to take some time to mourn the loss of experiencing an alter as separate, but others experience integration as joyous! Alters especially may want to integrate so that they no longer miss out on so much of the system's life, so that their emotional range is no longer limited, or so that they can consistently contribute to the system's functioning and safety. Overall, integration leads to a more stable and well rounded individual who has consistent access to all parts of themself. As the individual learns to connect with all of their thoughts, feelings, and behaviors, they will learn to rely less on dissociation, and their general dissociative symptoms will decrease. An individual who is fully integrated and has achieved final fusion may be less vulnerable to increased dissociation or splitting into new parts as a result of future stress.
Finally, it must be noted that even if an individual successfully integrates, it is possible for the integration to temporarily dissolve during times of stress or conflict. It's important to keep in mind that this is a normal part of the progression of integration and is not a bad sign or a setback. Before final fusion, integrations might fall apart if the parts involved weren't completely ready yet, if a stressor arises that one alter in particular was created to handle, or if the parts involved need influence from other parts in order to remain stable in their unity. After an attempt at final fusion, it might take some time for the individual to become used to living as one integrated identity. Like everything else, learning a new way of viewing oneself and learning how to rely on responses other than dissociation take practice! Treatment should continue for a while after a system has achieved final fusion in order to support this process, and an individual can also briefly return to therapy at any point in order to address temporary lapses in integration, an increase in dissociation in response to new stressors, or any other problem.
Here is a blog post from a therapist who specializes in trauma and dissociation and who believes that integration is neither necessary nor always helpful.
Here is an article by an individual with DID who wants to integrate. It details the difference between wanting alters to disappear (which is impossible) and accepting them fully (which is necessary for integration).
Here is an article by an individual who had DID and chose to integrate. It details what integration is and why the author views it as the best goal for healing.