For many years, it was commonly believed that the main cause of borderline disorder was poor or uninformed parenting. It now seems most likely that biological factors, especially genetic ones, are necessary to place a person at risk for developing the disorder and that environmental factors serve to increase the risk of developing specific symptoms and their severity.
For example, psychological and social (environmental) factors, including but not limited to poor parenting, increase the risk of developing the disorder in those people biologically predisposed to it. It appears that no single psychosocial risk factor is responsible by itself for causing the disorder. As stated above, it now appears that a genetic predisposition for developing the disorder is necessary and that environmental factors may increase the risk but are not essential.
Finally, it is generally held that biological and environmental risk factors interact to reach a certain critical level of brain dysfunction in order for the symptoms of borderline personality disorder to become apparent. It appears that this critical degree of disturbance of brain function can be achieved by a large amount of biological risk which then requires only a low exposure to environmental risk factors, or low levels of biological risk factors coupled with high environmental risk, or intermediate levels of both.
Research studies now suggest that 60% of the risk of developing borderline disorder is conveyed by genetic abnormalities. These abnormalities appear to affect the proper functioning of those brain pathways or circuits that serve the behavioral functions of emotion information processing, impulse control and cognitive activity such as perception and reasoning, and the quality of interpersonal relationships. Current research suggests that there is not a single, specific gene for the borderline disorder. It appears that the genes that increase the risk for the disorder may be passed on by people who have the disorder itself, or a related disorder, such as bipolar disorder, depression, substance use disorders, ADHD and posttraumatic stress disorder.
Of all environmental factors that place a person at risk for developing a borderline disorder, those associated with poor or uninformed parenting appear to be the most critical. These include early separation from one or both parents, repeated emotional, physical or sexual abuse by someone within or outside of the family itself, and inconsistent, unsupportive care. Poor parenting can also include failing to protect the child from repeated abuse by the other parent, another member of the family, or an outsider.
It is important to understand that children who have not been exposed to such environmental traumas can still develop the borderline disorder. This suggests that in some people the biological risk of developing the disorder is very high, and may be sufficient in the absence of environmental traumas.