"For people with DID, aspects of cognitive function that are normally unified/integrated, like personality, identity, and memory, are fragmented and can become their own separate identities," Lewis-Fernández tells BuzzFeed Health.

People with DID will feel like they're alternating between those multiple identities (each with their own personality traits and memories), which causes them to feel like a coherent, consistent sense of self is missing, he explains. They may feel like there are multiple voices trying to take control in their head, and when a separate identity is present and in control — aka "out" — the person functions with that identity's memories and personality traits.

DID was known as multiple personality disorder until 1994, when it was changed to represent a better understanding of the condition — which is that DID is more of a fragmentation of one's identity, than the development of several random, separate identities. It's under a category in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) that also includes depersonalization disorder and dissociative identity disorder.