1/ Here's an updated thread on borderline personality, from a forthcoming book chapter. (I wasn't happy with the earlier version—this one's better)

"The term borderline dates back to a time when psychiatric classification distinguished primarily between neurotic and psychotic

2/ disturbance based on intact versus impaired reality testing. Over time, clinical writers began describing patients on the 'border,' who seemed neither neurotic nor psychotic. The diagnostic construct has evolved but the term borderline remains with us.
People with borderline

3/ personality have been described as 'stably unstable.' There is a pattern of instability in emotional life, self-concept, and relationships. Core features include affect dysregulation, splitting, identity diffusion, projection, projective identification, & insecure attachment.

4/ People with borderline personality have difficulty regulating affect. Their emotions can change rapidly and unpredictably and spiral out of control, leading to extremes of despair, anxiety, agitation, and rage. They experience episodes of deep depression in which they lose

5/ access to any glimmer of hope. They are often filled with rage, and they are prone to destroy relationships with hateful, rage-filled outbursts. Poor impulse control is an ongoing problem and leads to ill-considered actions and self-destructive behavior.

6/ Splitting refers to compartmentalizing good and bad perceptions, feelings, and experiences, leading the person to experience self and others as all good or all bad. (The term dichotomous thinking in Dialectical Behavior Therapy refers to this phenomenon.)

7/ Splitting results in extreme, wildly fluctuating views of self and others, depending on which 'compartment' the person is experiencing. When distressed, people with borderline personality lose the capacity to see others as complex, three-dimensional human beings. Instead, they

8/ become one-dimensional heroes, saviors, villains, and abusers.
The person may see certain people as all good ('good objects') and others as all bad ('bad objects'), or their experience of the same person may swing between contradictory extremes.

9/ This leads to unstable and chaotic relationships. For example, a person with borderline personality may see the clinician as a savior, up until they disappoint. Then they may see the clinician as a 'bad person' and attack them for their callousness or incompetence. Such shifts

10/ from idealization to devaluation are often precipitated by perceived rejection.
Splitting also refers to compartmentalized, contradictory experiences of self. The person may vacillate between experiencing themselves as a good person and experiencing themselves as someone evil

11/ and rotten to the core. Their self-concept depends on which of multiple, contradictory self-representations is being experienced. Shifts between different self-representations bring corresponding shifts in emotional states and keep the person on an emotional rollercoaster.

12/ Affect dysregulation and splitting go hand in hand.
Because disparate self-representations are not integrated into a coherent whole, people with borderline personality have difficulty maintaining a consistent, stable sense of self ('identity diffusion'). Their attitudes,

13/ values, and self-concept are unstable and changeable. They may shift with relationships, circumstances, or emotional state. They person may present in strikingly different ways on different occasions, often to the consternation of clinicians. If they are feeling good,

14/ they may be blithely unconcerned that they were suicidal only days before. If depressed, they may feel no connection to any part of themselves they have ever experienced as positive.
Primitive forms of projection are a hallmark of borderline personality.

15/ Split, disavowed representations of self and others and the feelings associated with them are projected wholesale onto other people with conviction and certainty. The projections often involve intensely negative emotions such as anger, spite, hate, envy, and disgust.

16/ The person regards their projections as facts, not perceptions. It can be disorienting and maddening to others, including clinicians, to be seen and treated repeatedly as someone they are not.
Projective identification takes the defense of projection a step further.

17/ In addition to projecting disavowed parts of themselves, the person works to induce and evoke the feelings they have projected with such vehemence, until the other person begins to feel and act in accord with the projection. Borderline patients are masterful in bringing this

18/ about, although they do not do it consciously. Clinicians describe experiences of not being able to think their own thoughts or feel their own feelings—as if their minds have been colonized by something alien. Under the sway of projective identifications,

19/ clinicians may find themselves filled with hatred for their patient or impelled to cross professional boundaries to rescue them.
The transfer of thoughts and feelings from patient to clinician that occurs in projective identification is not mysterious or mystical.

20/ Observable behavior on the part of the patient pulls, pushes, coaxes, and bullies the clinician into their assigned role, although the clinician may be unaware of this as it is occurring. Generally, countertransference comes first, and understanding emerges after the fact."