This can be experienced as (but is not limited to) having dissociated parts of oneself, sharing a body with other people, or having multiple selves/personalities.
There is no right or wrong way to be plural. If you are plural, then you are plural.
There are many different explanations out there. Trauma is the most accepted one, but it is not the only one.
The most studied and accepted cause is trauma during childhood. DID and OSDD 1 are trauma based dissociative disorders. If a child experiences trauma before the ages of 6-9 (depending on developmental factors), they may fail to integrate their parts of self into one unified whole. These parts develop their own sense of self. There is often amnesia between them as an adaptive mechanism to protect the brain from trauma.
Trauma is not the only way plurals define their origins. Some believe that they are naturally plural due to neurodivergence. Some attribute theirs to spiritual causes. These plurals may believe they were born with many souls in one body, or that they invited spirits into their bodies.
Some created their system members through imagination and willpower.
Some don't know or don't care to know their origins.
It seems simple- if you have other sentient entities with their own sense of agency in your body, you are plural. Despite this, it can be hard to tell, especially if you have DID or OSDD. These disorders often hidden from those who have them, but it is not impossible to figure out that you have it.
We have written an article about ways you can tell. Check it out here: How To Tell If You Are Plural
We've got you. Here is the link: Plural Terminology
We have a list of resources available on our site: Plural Resources