“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
Splitting is the act of creating a new alter. There are conflicting theories as to how this occurs, neither of which alone adequately describe all types of splitting but the combination of which does. In the first theory, splitting occurs when an existing alter (or, in the beginning, a terrified and traumatized child) denies what is happening to them, denies and rejects their own memories, thoughts, emotions, perceptions, or reactions, or denies that they are the individual present at a given time (convinces themself that someone else is present and taking whatever experience they cannot handle). This is the older theory and accurately describes the creation of alters who feel that they came from an existing alter and have visibly taken traits, memories, perceptions, opinions, skills, talents, or a function from the other alter in a way that may or may not change the original alter. For example, if Alter A is confronted by trauma that they were previously unaware of, they may split off Alter B not only to hold that trauma but also to protect them from any further realization of the trauma and possibly to hold their own traits or actions that they now associate with the trauma (for example, anger, knowledge of the system's parents, physical strength, putting the body to bed at night). In this scenario, Alter B may be more or less similar to Alter A and may or may not see Alter A as their source or primary charge.
The second theory of how alters are created is that of structural dissociation. According to this theory, alters are created when an existing part cannot integrate new materials (memories, strong emotions, perceptions, attachment styles) within itself and so those new materials must form their own self state. This is the newer theory and appears to accurately describe how DID is first created and how new alters are formed in response to new trauma, high levels of stress, or other overwhelming experiences. Alters that form this way start from scratch and do not have the benefit of beginning with another alter's traits as a foundation. On the other hand, this can allow for high levels of specialization. According to the theory of structural dissociation, alters created in response to trauma are emotional parts (EP) while those created in response to daily life are apparently normal parts (ANP).
In any case, additional alters are the result of extreme stress. The mind even for individuals with dissociative identity disorder (DID) does not like to be fractured. It is true that some individuals who have become used to using splitting as a coping mechanism may seem to split at the drop of a hat, but many individuals cannot split until a split is strictly necessary for their protection, functioning, or ability to remain hidden as a system.
New splits may or may not immediately be well developed. Alters that take from one or more source alters as they split may have more substance or be able to quickly gain substance, but many new splits will at first be disoriented, depersonalized, and feel hollow, flat, or not fully formed. These alters can be referred to as fragments. Alters become more developed and move beyond their original creation as they experience more, interact with more people, and have the chance to react in new ways.
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.