From http://en.wikipedia.org/wiki/Splitting_%28psychology%29 :
"Splitting is resolved [when] the self and the other can be seen as possessing both good and bad qualities. Having hateful thoughts about the other doesn’t mean that the self is all hateful and doesn’t mean that the other person is all hateful either."From the DSM-IV-TR, 301.83.
If a person fails to accomplish this developmental task, borderline pathology can develop. The borderline personality is not able to integrate the good and bad images of both self and others.
People who suffer from borderline personality disorder have a ‘bad representation’ which dominates the ‘good representation’.
These people tend to suffer from intense fusion anxieties in intimate relationships, because the boundaries between self and other are not firm. A tender moment between self and other could mean the disappearance of the self into the other.
This triggers intense anxiety. To overcome the anxiety, the other is made into a very bad person; this can be done, because the other is made responsible for this anxiety. However, if the other is viewed as a bad person, the self must be bad as well. Viewing the self as all bad cannot be endured
The use of splitting also implies the use of other defence mechanisms, namely devaluation, idealization and denial.
Splitting creates instability in relationships, because one person can be viewed as either all good or all bad at different times, depending on whether he or she gratifies needs or frustrates them.
This, and similar oscillations in the experience of the self, lead to chaotic and unstable relationship patterns, identity diffusion and mood swings.
Consequently, the therapeutic process can be greatly impeded by these oscillations, because the therapist too can become victim of splitting."
By way of http://en.wikipedia.org/wiki/Borderline_personality_disorder
A DSM diagnosis of BPD requires any five out of nine listed criteria to be present for a significant period of time. The criteria are:
1. Frantic efforts to avoid real or imagined abandonment. [Not including suicidal or self-mutilating behavior covered in Criterion 5]
2. A pattern of unstable and intense interpersonal relationships characterized by alternating between extremes of idealization and devaluation.
3. Identity disturbance: markedly and persistently unstable self-image or sense of self.
4. Impulsivity in at least two areas that are potentially self-damaging (e.g., spending, promiscuous sex, eating disorders, substance abuse, reckless driving, binge eating). [Again, not including suicidal or self-mutilating behavior covered in Criterion 5]
5. Recurrent suicidal behavior, gestures, threats, or self-mutilating behavior.
6. Affective instability due to a marked reactivity of mood (e.g., intense episodic dysphoria, irritability, or anxiety usually lasting a few hours and only rarely more than a few days)
7. Chronic feelings of emptiness.
8. Inappropriate, intense anger or difficulty controlling anger (e.g., frequent displays of temper, constant anger, recurrent physical fights).
9. Transient, stress-related paranoid ideation or severe dissociative symptoms.