The short answer
Dissociative identity disorder (DID) means there are two or more distinct parts of the personality, sometimes called identities or alters, that at times take control, with gaps in memory for everyday events.
Other specified dissociative disorder (OSDD) is the diagnosis for significant dissociation that doesn't fully fit DID. The form I see most often, sometimes called OSDD-1, has parts that are less separate, or distinct parts without the memory gaps.
What they have in common
Both usually develop from overwhelming experiences early in childhood, most often within relationships, at an age when the personality hadn't yet come together. Both involve dissociative parts: ways of being that carry their own feelings, memories and reactions. And both are easy to miss. Many people spend years in mental health services with other diagnoses first.
DID is not as rare as people think. Reviews estimate it affects about 1% of the general population, and more people in psychiatric settings (Dorahy et al., 2014).
How they differ
- DID
- Two or more distinct identities that take full control at times, plus repeated gaps in memory for everyday events, personal information or trauma.
- OSDD
- Parts are present and felt but less distinct, or they influence you from the inside without fully taking over. Or there are distinct parts without the amnesia.
ICD-11, used in many countries, has its own name for the second picture: partial dissociative identity disorder. One part runs daily life most of the time, and other parts intrude with thoughts, feelings, urges or voices that don't feel like your own, without fully taking control.
“The question isn't whether the parts are real enough. It's how much they know about each other, and how well they can work together.”
What it can feel like
People with OSDD often describe inner voices or commentary, feelings that seem to come from nowhere, doing something and feeling as if they watched themselves do it, or suddenly feeling much younger. People with DID may also find things they don't remember buying, hear about conversations they don't recall, or lose time.
Many people with either diagnosis function well, at work at least, and work very hard to keep all of this hidden. The effort it takes is often the most exhausting part.
How they're diagnosed
A screening questionnaire such as the Dissociative Experiences Scale (DES-II) can show whether dissociation is worth looking into. The Multidimensional Inventory of Dissociation (MID) and the structured clinical interview SCID-D help tell DID, OSDD and other conditions apart.
A careful assessment matters, because dissociation is often mistaken for psychosis, bipolar disorder or borderline personality disorder. When structured interviews are used, DID can be told apart from these reliably (Dorahy et al., 2014).
Living with parts, day to day
Whatever the diagnosis, a few things tend to help early on. Noticing, without judging, when a different part seems to be present. Grounding in the present through the senses: what you can see, hear and feel right now. Keeping a shared notebook or calendar, so parts that lose time can find their way back into the day. And treating every part, including the angry or frightened ones, as someone who once had a very good reason to exist.
Does the difference change treatment?
Less than people expect. For both, the treatment is phase-oriented: first safety, stabilisation and getting to know the system of parts, then working through traumatic memories with the agreement of all parts, and then integration and building a life that isn't organised around threat. The pace and how much we work with the parts differ, but the direction is the same.
In my work that combines parts work, schema therapy and carefully adapted EMDR. I've written more about that in Is EMDR safe with dissociation? If you recognise yourself here, the label matters less than finding someone who understands dissociation and won't rush you.
References: Dorahy, M. J., Brand, B. L., Şar, V. et al. (2014). Dissociative identity disorder: an empirical overview. Australian and New Zealand Journal of Psychiatry, 48(5), 402–417. American Psychiatric Association (2022). DSM-5-TR. World Health Organization (2019). ICD-11.
