The cause of borderline personality disorder is not well-understood. It may be a result of an imbalance of chemicals in the brain called neurotransmitters, which help regulate mood. Mood is also influenced by genetic and environmental influences.

Borderline personality disorder is 5 times more common among people whose parents or siblings have the disorder.⁴ It is also seen more often in families who have other mental health conditions such as antisocial personality disorder, substance abuse problems, and mood disorders like depression.³

People who develop this disorder often have experienced significant childhood trauma, such as sexual, physical, or emotional abuse; neglect; or early loss of or separation from a parent. When this trauma is combined with certain personality traits, such as reacting poorly to stress or having problems with anxiety, the risk for developing borderline personality disorder increases.

Although experts know that people with borderline personality disorder have impaired function in parts of the brain, it is still not clear whether the problems are caused by-or are a consequence of-the disorder.⁵