I'd never had a good handle on borderline personality disorder before, it had always sounded like a vague, fall-between-the-cracks description of difficult people who didn't quite fit the textbook definitions of other disorders. (That impression was fostered partly by its name.)
But I've been reading about it, and thinking about it, and now see it as a very distinct, recognizable syndrome. And, as always, having known a couple of people who clearly have the disorder has made it come much more alive for me.
If you're unfamiliar with the disorder, you might mistake borderlines for sociopaths, because of their ability to self-righteously justify themselves and their unbridled fury when they feel they've been crossed. Or, you might mistake them for narcissists, for the same reason.
But there are some major differences between sociopaths and borderlines. Borderlines only become combative when they think they're right. (Of course, their ability to convince themselves they're in the right is often astounding.) Sociopaths often have that same feeling of righteousness; but they will follow a self-serving course of action even when they know it's wrong.
The end result may be the same; but with borderlines, it's less a matter of willful evil, and more as if they have a near-psychosis that convinces them they are in the right. (This "psychosis," of course, never seems to convince them they are in the wrong, which, admittedly, works out quite conveniently for them.)
Another difference: borderlines aren't gleefully evil the way sociopaths are: they don't, for instance, lie purely for the pleasure of fooling others. Nor are they casually sadistic the way sociopaths are.
Borderlines don't have the same overwhelming ego of sociopaths, in fact may often have the opposite: poor self-images. And they tend to be forthright about their insecurities -- whether about their looks, their intelligence, or even their character.
Borderlines tend to see things in black and white. They are often completely incapable of seeing another's point of view. This is a trait they share with narcissists. (Try having a political discussion with a borderline sometime.)
Borderline personalities are extremely emotionally unstable. As the National Institute for Mental Health says:
Borderline personality disorder (BPD) is a serious mental disorder marked by a pattern of ongoing instability in moods, behavior, self-image, and functioning. These experiences often result in impulsive actions and unstable relationships. A person with BPD may experience intense episodes of anger, depression, and anxiety that may last from only a few hours to days.
Here, too, borderline personality disorder resembles another syndrome, bipolar disorder, in its extreme mood swings. It's certainly easy to mistake one for the other. But I've known people with both, and there seems to be a certain self-serving aspect to the borderline personalities I've known that's missing in bipolars.
Also, with the bipolars, there seems to be almost a certain orderliness to their moods: they're either up or down, and when in either of those states, their behavior can be fairly easily predicted. With borderlines, they can go veering off into a different mood based on an innocuous comment, at a moment's notice.
And, while a bipolar may stay depressed for days or even weeks, they will generally not stay angry for days, and are generally not filled with grudges. Borderlines have a lot more free-floating rage; sometimes they seem like walking temper tantrums in search of a cause. (And their anger is all the scarier because it is so consuming, and uninhibited in its expression.)
Another thing most of the literature on borderlines emphasizes is that they have an intense fear of abandonment. This may not be apparent at first, but one of the ways it can manifest itself is by the borderline trying to drive a wedge between their partner and anyone else they perceive to threaten their relationship.
One of the subtexts in most descriptions of BPD is that there seems to be an undercurrent of hysteria running through a lot of the behavior described. Sure enough, females are diagnosed with the disorder three times as often as men.
People who are in a relationship with borderlines often end up with a modified version of Stockholm Syndrome, tiptoeing around and constantly trying to mollify them in order to avoid confrontation and the inevitable fury that follows.
With the bipolars I've known, even though they are at times hard to deal with, I've always been left with the sense that underneath it all, they're generally decent people. I haven't necessarily gotten that sense with the borderlines; this may be partly why the syndrome is classified as a "Cluster B personality disorder."