Under sufficient stress, the Adult can be impaired to the point where the Parent or Child take over inappropriately and reactively. They have been reactivated overwhelmingly by similar situations to the original recordings, and the "I" takes on the identity of the Parent or Child viewpoint - that of imposing on or being imposed on; of dominating commandingly or feeling dominated; of controlling or being controlled; of being cause or being effect; of opposing or being opposed. Most people, in their transactions, compulsively and unknowingly slip into Parent or Child type sub-personalities.
The ego has the task of bringing the influence of the external reality to bear on the id and endeavors to substitute the reality principle for the pleasure principle that reigns supreme in the id.
In Freudian terms, individual behavior is determined by the "id" forces (the unconscious instinctual drives, the uncurbed passion of the child, the raw requirements of primitive thinking), under control of the conscious function of the "ego,", which itself has been modified by the development of an ego-ideal or "Superego", a learned mode of controlling the ego, which may consist of conscious belief systems linked to Subconscious roots.
As Freud says, "The ego is the part of the id that has been modified by the direct influence of the perception-consciousness system. The ego has the task of bringing the influence of the external reality to bear on the id and endeavors to substitute the reality principle for the pleasure principle that reigns supreme in the id. In the ego, perception plays the part which in the id devolves upon instinct".
The Child, then, is the world of feelings and the id; this corresponds to right-brain mental processes.
The Parent is the hypnotic world of commands, the superego that directs his conscience; this corresponds to left-brain verbal belief systems with repressed right-brain emotional and traumatic components, and with energetic roots into the unconscious primal lower-brain.
The Adult is the mature ego of rational thought and free expression; this corresponds to integrated functioning with the left and right brain in un-repressed inter-active communication.
The stereotyped, habituated combination of all these is the "persona", Jung's term for the mask that is one's presumed face to the world. This mask will be switched as the ego reactively identifies with one or other dominant sub-personality from his collection of Parent- or Child-type identities, to meet the perceived demands of his circumstances. He will tend to have a chronic, seemingly permanent mask that he identifies as his personality, as "me!", but also many temporary masks for acute situations, which he may not even realize he possesses and uses, since they are adopted reactively and sub-consciously.