Pathological trends in an individual's personality which leads to immature and distorted attitudes of the self and their surroundings. As a result, when faced with stress, they experience severe anxiety and resort to the use of neurotic defense mechanisms.
Classified as a type of hysteria (exhibits some type of physical illness with no physical cause (ie. hysterical blindness)). Includes amnesia, fugue, multiple personality, and somnambulism (sleepwalking).
The person blocks off a part of their life from conscious recognition. They can retreat even further by going away in a fugue state, where they abandon their current life and establishes a new one somewhere else, completely forgetting the previous life for days or even years). Typically the person has an egocentric, immature personality who is forced within an unpleasant situation from which they see no escape.
In the case of Multiple Personality, the person manifests two or more distinct personas. The person may change from one personality to another at periods varying from a few minutes to several years, and cannot remember in one what happened in the other. Sometimes, however, there can be a Co-Conscious personality where one personality can be dominant and function consciously, while the other functions subconsciously. The subconscious personality knows about the conscious one, but not the other way around, and it makes itself known in roundabout ways.
Multiple personality is generally the result of a deep seated conflict between contradictory impulses where a resolution is achieved through a separation of the conflicting sides.
In the Killing Joke and other stories, the Joker does not seem to remember his past, giving a differing account each time he tells it.
In "Going Sane" The Joker seems to be having a fugue state (forgetting his life as the Joker and becoming Joseph Kerr). He is also demonstrating co-conscious personality where J.Kerr becomes the dominant personality and the Joker persona is the subconscious personality. One can wonder whether the J.Kerr persona was ever real, just another fabricated personality by the Joker, or if this persona is normally hiding in the Joker's subconscious.
So to summarize the Joker has:
Antisocial/Narcissistic (Personality Disorder)
Dissociative (Neurotic Disorder)
Schizoaffective (Psychotic Disorder)
In other words: Hat Trick!
But what does it all mean?
The predisposing factors of mania are primarily genetic. The average age when mania starts to develop is about 44 (the majority of cases occur in a range of 25-65) Schizophrenia can also be genetic, but genetic schizophrenia tends to shows up in late adolescence/early adulthood, while schizophrenia caused by trauma occurs later in life. Later onsets of mania and schizophrenia could explain why Joker seems to have gotten crazier with time.
There is a seeming incongruity with his dissociative disorder in that the main dynamics of multiple personality is an exaggeration of conflict between the conforming and non-conforming personalities (as seen in Two-face and Ventriloquist). But all of Joker's personalities are non-conforming. So what the flip? I can think of two possibilities:
a). As in "Going Sane", the Joker has a co-conscious personality and his conforming personality is always in the subconscious, so we never see it. One could ponder whether his obsession with Batman could be the subconscious personality's way of trying to put the dominant personality in the path of someone who can stop him.
b). The Joker was a sociopath prior to becoming "The Joker". Because Sociopaths don't have moral conflicts, all of his personalities reflect this.
What triggered the splits in personality in the first place then? If, having Narcissistic PD, he already subconsciously thinks he is fundamentally flawed, then ending up disfigured to the point where he'd be a social outcast could have caused the complete meltdown of his persona. His evolving personalities might be an attempt to create someone he thinks is acceptable (working within the framework of his craziness), but that never happens, so he is constantly changing.
What do you think?