Yes, it is possible for a single person to have different personalities or identities within their mind, but it is not referred to as "multiple personality disorder" anymore. The condition is now known as Dissociative Identity Disorder (DID).
Dissociative Identity Disorder is a complex and rare mental health condition characterized by the presence of two or more distinct identities or personality states within one individual. These different identities may have their own unique ways of thinking, feeling, behaving, and even have distinct memories. When one identity is dominant, the person may not have awareness or memory of the experiences of the other identities.
DID is considered a genuine and serious psychological disorder. It is believed to develop as a coping mechanism to deal with severe trauma, often occurring in childhood, such as physical or sexual abuse. The dissociation serves as a way for the person's mind to separate from the overwhelming and traumatic experiences.
It's important to note that DID is a complex and controversial condition. Some professionals question its validity or suggest that it may be overdiagnosed or misdiagnosed. Research in this area is ongoing, and there is still much to learn about the disorder and its underlying mechanisms.
Overall, while the concept of multiple personalities might be depicted in fictional works, Dissociative Identity Disorder is a real and serious condition that requires appropriate diagnosis and treatment by mental health professionals. If you or someone you know is experiencing symptoms that suggest the presence of different identities or memory gaps, it's crucial to seek help from a qualified mental health professional for an accurate assessment and appropriate care.