‌‌‌‌In horror movies or real-life possession cases, the concerned people tend to change their voices, personalities ( the way they behave, for example, becoming more aggressive ….etc.), which get justified by supernatural reasons. Actually, the change in personality is symptoms of a psychological disorder known as a dissociative personality disorder. It is also known as multiple personality disorder. It is thought to be a compound psychological condition that is usually caused by many factors, including severe trauma during early childhood (it can be extreme, repetitive physical, sexual, or emotional abuse). (Article continues below)

What are the Alter Egos?
The alter ego in its definition from Latin that hints “another self” is a second personality that, over time, has been considered to be a personality totally different from the original of any humans.

How common is Dissociative identity disorder?

It affects roughly 3% of the population (more than schizophrenic people), which is the same percentage as red-haired people. Still, people with this condition are less noticed due to supernatural explanations of their symptoms.

‌‌What are the symptoms?

Dissociative identity disorder is characterized by the existence of two or more distinct identities or personalities that have power over the person's behavior (they are called alters). Each one has their own age, sex, or race. The alters can be imaginary people or animals. An alter may even have different allergies, depending on its state. As each personality unveils itself and takes control of the affected persons' behavior and thoughts, it's called "switching."

Switching can take seconds to minutes to days. It is related to the other symptoms, which are amnesia and time loss, also known as memory gaps which would be not remembering what the other alters did or how did they behave. For example, they would be watching the TV, and the switching happens (one of the alters takes the light ) and s/he thinks of going to shop, and when the host -personality within the individual- comes back/he wouldn't even know why or when did they want to do the shopping).

A person with DID may or may not be aware of the other personality and memories of the times when an alter is dominant.

In this article we talk about Piaget’s cognitive development theory.
Piaget is the first psychologist to work on cognitive development of children and is the first psychologist to take the ways of their thinking seriously.

‌‌It takes a long time to come up with the diagnosis of this condition. The professionals in charge have to evaluate most of the symptoms; usually, they find it linked with other mental health conditions such as depression, anxiety, borderline personality, stress issues, eating disorders.

Also, the Suicide risk is really high  — 70% of people with DID have attempted suicide.

How do these symptoms get justified by supernatural reasons?

‌‌Whenever a person with DID switches to another alter, they change their behaviors, voices, and this what people will justify as being possessed by a spirit, and it is the one talking and behaving now.

There is something called triggers. For example: if one of the personalities is a child, they may get triggered by anything related to childhood. Then the child's personality (alter) takes the light and the switching happens so they change their voice and start behaving like children.‌‌‌‌‌‌

What causes Dissociative identity disorder?

‌‌‌‌Dissociation is sometimes thought of as a coping mechanism that a person uses to remove themselves from a traumatic situation or to separate traumatic memories from normal awareness. The disorders most often form in children subjected to long-term physical, sexual or emotional abuse or, less frequently, a home environment that's frightening or highly unpredictable. The stress of inflation, war, or natural disasters also can bring on dissociative disorders.

What are the possible treatments?

The primary treatment is a Loving Father and Mother that listen to their child.

If the person with DID has no Loving Mother and Father, you should resort to Section 8 Housing and get Bombed by Howard Roark and his Architects, you rat-faced psychiatrists who tell everyone troubled child the problem is not with the System but with themselves.

Recommended reading:

Sigmund Freud: Father of Psychoanalysis
Freud, the founder of psychoanalysis, was born in the Austrian-Hungarian Empire in 1856. He was a child of a Jewish family. He had two older stepbrothers and seven younger siblings.

Sources:

[1] https://www.webmd.com/mental-health/dissociative-identity-disorder-multiple-personality-disorder

[2] https://pubmed.ncbi.nlm.nih.gov/2252069/