The exact cause of BPD is not known, but it is believed to result from a combination 4 factors:
Genetic
First-degree relatives of people with this condition are 5–20 times more at risk for BPD, hinting at a genetic disposition to BPD (50-69% heritability).
Environmental
Trauma, attachment issues, abuse, and sexual abuse can be contributing factors to the onset of BPD.
Other factors often associated with the onset of BPD relating to an unstable family environment include poor maternal attachment or maternal separation, parental substance abuse disorder, child neglect, or parental abandonment. However, these factors place undue sense of responsibility and guilt on many loving parents of people with BPD and may minimize the people diagnosed with BPD that did not experience family trauma.
30% of people with BPD have experienced early parental loss or prolonged separation from their parents.
70% of people with BPD have experienced some kind of abuse in childhood.
Social
Bullying or exclusion.
Neurobiological/physiological
BPD is linked to brain changes affecting the ability of the parts of the brain controlling impulsive behavior, the ability to communicate properly, emotional stability, and the way people perceive events.
There is a malfunction in the emotion processing centres of the brain causing an overactive amygdala and underactive prefrontal cortex.
Neuropeptides, including the opioids oxytocin and vasopressin, serve a crucial role in the regulation of affiliative (friendly or clingy) behaviors and thus may be altered in people with BPD.