Pars Pro Toto Thinking and Borderline Personality Disorder

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Robert and Megan are newlyweds. One night, Megan tells Robert how lucky she is to have met him, how he is everything she ever wanted, her "knight in shining armor." The next day, Robert arrives home from work 10 minutes later than usual, and he has forgotten to pick up the gallon of milk Megan asked him to stop for on the way home. Immediately, Megan explodes with rage. "Where were you? I bet you were sleeping with your ex! I knew it! You don't care about me. You forget everything I ask. I should have never married you."

Pars pro toto is Latin for "a part (taken) for the whole." In human psychology, pars pro toto representations characterize a defense mechanism called splitting. At different points in time, the self or the other person is seen as “all good” or “all bad,” but rarely, if ever, as being comprised—as all human beings are—of both good and bad qualities. The good or bad parts stand for the whole; they are not integrated, and the patient oscillates chaotically between these two extremes.

In the example above, Megan engages in pars pro toto thinking. Her husband, who just the night before was the love of her life, suddenly becomes an evil monster, neglectful of her every need and someone who cannot be trusted. Her warm, loving feelings from the night before could not be integrated with her disappointment that Robert was a few minutes late and forgot to pick up what she asked him to at the store. Immediately, she jumps right to accusations and sees him simply as a bad person.

Given the intensity of Megan's rage, her fear of abandonment, and splitting, it is likely that she suffers from a condition called borderline personality disorder (BPD). Pars pro toto thinking is a hallmark feature of this personality disorder, which is marked also by instability in the sense of self, suicidal thinking and gestures, self-destructive and paradoxical behavior, and chaotic and tumultuous interpersonal relationships.

Splitting was first identified by the British psychoanalyst Melanie Klein, and later work by Otto Kernberg (1975) greatly advanced our understanding of this defense mechanism and its relevance as a feature of severe personality disorders. Kernberg views BPD as fundamentally a disorder of identity, tracing the roots of the condition to the very young infant's inability to integrate self and object representations, often, but not always, due to abuse or neglect.

To preserve an image of a good mother, which all children want and need, the child partitions off the good from the bad, resulting in a split psychological structure marked by the pars pro toto thinking that is characteristic of the disorder. As the child grows into an adult, this mode of thinking permeates their object relationships and leads to great interpersonal difficulties.

The successful treatment of borderline psychopathology necessitates that the patient eventually develops an integrated sense of self and other—a realization that both they and other people are inevitably comprised of both good and bad elements and that this is a normal part of human life. That is, the patient must come to understand that no person is either "all good" or "all bad," that all of us have both assets and liabilities and that every person has failed others at some point in some way.

Psychotherapy must also allow the patient to internalize a healthier internal object which allows them to hold and soothe themselves during periods of aloneness (Adler, 1985).

Treatment for BPD includes general psychodynamic psychotherapy, dialectical behavioral therapy, transference-focused psychotherapy (developed by Kernberg and colleagues), and mentalization-based therapy. Psychopharmacology, or medication treatment, is generally ineffective (Leichsenring and colleagues, 2023). With effective therapy, often over several years, many patients with BPD reach meaningful levels of recovery.