I hold that part of the appeal of many characters, particularly in the Gotham City stable, is that they represent natural desires or human defense mechanisms "gone wild". The Joker can be viewed as the very useful, very human "sense of humor" gone very, very bad. The Catwoman can be seen as personifying self-interest taken to the point of amorality, the Riddler as intellectual curiosity fallen into obsession, and the Penguin as ambition rotting into ruthlessness. To some degree, this applies even to Batman, who can be seen as the natural desire for justice taken to the extreme (assuming you consider dressing as a bat and hurling boomerangs at people's heads "extreme"; tastes vary).
That's why Two-Face (who's featured in the forthcoming Batman Dark Knight film) has always been one of my favorite characters. He's certainly not the first figure in literature to reflect man's inherent duality; Two-Face was very consciously adapted from R.L. Stevenson's Dr. Jekyll and Mr. Hyde (in fact, it's the book he's reading on the splash page of his first appearance). But he's certainly one of the best, because instead of just representing the "angel and the devil on our shoulders", he also shows the often-difficult process of making a decision based on conflicting impulses. And, you may be interested to know, science backs him up... .
Yesterday, I read "The Conflicted Brain" (by ingenious evolutionary theorist Jon Wilkins, Sante Fe Institute Bulletin, Spring 2008). The article examines the question of how one person -- their brain, really -- can be at odds with itself, and shows that the brain is physically "designed" to be in self-conflict as the result of evolution.
"Brain-imaging studies suggest that different brain regions come into conflict with each other over certain decisions. At the same time, many genes that are expressed in the brain show evidence of having been in a long-term evolutionary conflict with each other. It is possible that when we feel as if we are of two minds, it is precisely because different sets of our own genes have effective control over different regions of our brains, and these different brain regions are exerting antagonistic influences on the decision-making processes. ... In fact, it may turn out to be misleading to talk about the notion of individuals having a single 'self' at all. "You can read the article for more details, but it demonstrates how adaptively useful such an internal split can be. For example, having different areas of the brain competing to control every decision leads to "an escalatory 'arms race' between different brain regions. ... Eventually, we might expect this to produce an increase in overall brain size. In fact, this may have happened; over the past hundred million years, the size of the mammalian brain has increase disproportionately relative to body size."
The downside, however, is that it leaves us more susceptible to "human behavorial dysfunctions, including schizophrenia, ADHD, autism, and bipolar disorder."
As I've said before, it always disappoints me when Two-Face is represented as a strict 'split-personality'. It robs him of most of his power, which comes from his ability to reflect the human attempt to maintain a unified self despite internal conflict. It's not 'Harvey Dent versus Two-Face'; that's too facile, unsophisticated, and doesn't respect the original concept of the character. Harvey Dent, as an identity, wasn't able to reconcile his inner conflicts; he is replaced by Two-Face precisely because Two-Face CAN resolve those conflicts, simply, effectively, and as quickly as you can flip a coin. As I've stated, the reason Two-Face is so hard to cure is because he's not Harvey Dent's problem; he's his solution.
That's what makes him scary; he represents a malfunction of one of our own human defense mechanisms. Like many Batman villains, he's a cautionary figure, his physical deformities symbolic of the personality deformities he represents. Two-Face isn't a Jason Voorhees, an external monster come to attack us; he's the monster we fear lurking within us all...
"As humans, we routinely engage in a wide variety of self-destructive behaviors. We cheat on our diets. We don't exercise. We smoke and gamble and get addicted to a wide range of substances. It is perhaps time to stop thinking of the human brain as evolution's crowning achievement and the physical embodiment of the 'self'. Rather, our brains are casualties of million of years of internal conflict. Every decision we make is argued out by at least two distinct evolutionary 'selves'. We may eventually discover that multiple personality disorder is simply the most extreme manifestation of a dynamic that governs even the most mundane behaviors in each of us."