What causes borderline personality disorder?

The cause is unknown at this time, but several theories are being looked at. There is some evidence that inheritance and other biological or biochemical factors may be involved in some people. Psychological factors are also involved for most people. For example, having had childhood trauma (physical, sexual, or emotional abuse or neglect, or prolonged separation) is far more common in people with this disorder than in the general population.

Dr. Marni Feuerman, LCSW, MFT
Marriage & Family Therapy
As with other mental disorders, the causes of borderline personality disorder aren't fully understood.
Most likely, a combination of the factors below results in borderline personality disorder.
Genetics: Some studies of twins and families suggest that personality disorders may be inherited.
Environmental factors: Many people with borderline personality disorder have a history of childhood abuse, neglect, emotional abuse, parental insensitivity and/ or separation from caregivers or loved ones.  There is strong evidence to support a link between these distressing childhood experiences, particularly involving caregivers, and BPD.
Brain abnormalities: Some research has shown changes in certain areas of the brain involved in emotion regulation, impulsivity and aggression. In addition, certain brain chemicals that help regulate mood, such as serotonin, may not function properly.  These studies have shown that people with BPD have differences in both the structure of their brain and in brain function. BPD has been associated with excessive activity in parts of the brain that control the experience and expression of emotion. For example, people with BPD have more activation of the limbic system, an area in the brain that controls fear, anger, and aggression, than people without BPD. This may be related to the emotional instability symptoms of BPD.
Karyn Hall
The exact cause or causes of borderline personality is unknown at this time. Most believe it is the result of a biological vulnerability or predisposition combined with events in the environment. Marsha Linehan, treatment developer of Dialectical Behavior Therapy, proposed the biosocial theory. She suggests that people with BPD are born with a predisposition and then experience an invalidating environment which leads to difficulty with emotion regulation and other issues. Invalidation means that the person's internal experience (thoughts and feelings)  is judged by significant people to be unacceptable or wrong. The person learns they cannot trust their internal experience. Her ideas have not been proven and are only a theory. 

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