DID (Dissociative Identity Disorder)
A Brief Explanation by the Author
Just so you know, we don't run and change clothes every ten minutes!
You may not want to know but I will give you an 'as brief as possible' explanation. From a baby until a child is seven or eight years old, the brain relies on the flight instinct to protect itself. After eight onwards, the brain learns the fight instinct. Over all of these formative years, the ego (or sense of self) is developing, forming the personality.
When any person is faced with a trauma, the input goes straight to the reactive brain, which will decides fight or flight, and will act before the thinking brain is even notified of the impending danger. That's how the brain is built in order to allow someone to survive.
But when a child under the age of eight faces a trauma or traumas that they can't run from, and the brain hasn't learned how to fight, there is another go-to response – freeze. The reactive brain determines that to survive, it needs to play dead, like a mouse caught in a cat’s jaws.
It's in the freeze response that dissociation is formed. If it worked and the body survived, the brain goes to the same response time after time. It can't unlearn it. The brain continues to freeze. The ego then splits and splinters into different personalities that have been built to hide the memories so the brain survives reasonably intact and functioning. These personalities are also designed to manage different types of trauma or input, making DID mostly reactive to stimulus.