Which disorder is characterized by the appearance of multiple personalities in the same person?

Dissociative Identity Disorder (DID) is characterized by the presence of two or more distinct personality states or an experience of possession. Each personality, or "alter," may have its own unique behavior patterns, memories, and ways of perceiving the world. This disorder is often a result of severe trauma during early childhood, such as extreme, repetitive physical, sexual, or emotional abuse, which leads the individual to create these alternate identities as a coping mechanism to dissociate from the traumatic experiences.

Individuals with DID may be unaware of other personalities and can experience gaps in memory concerning everyday events, personal information, and traumatic experiences. This dissociation helps them manage stress and maintain psychological stability in the face of overwhelming psychological pain.

In contrast, Dissociative Amnesia involves memory loss that is more specific and doesn't include the creation of new identities. Major Depression is primarily characterized by persistent feelings of sadness and loss of interest, and Conversion Disorder involves neurological symptoms that cannot be explained by medical conditions. Thus, DID is specifically linked to the development of multiple distinct identities within the same individual, which makes it the correct choice for this question.