No one is completely sure what causes this psychological disorder. There is probably no one cause, but rather a variety of factors that contribute to its onset. What is apparent is that a large number of people with the illness have experienced particularly traumatic events or have lived in unstable environments. These environmental factors, along with genetic components, are probably what cause borderline personality disorder.

Between 40% and 70% of those with borderline personality disorder have been exposed to environmental factors that may have contributed to their illness. Many experience physical and sexual abuse as children; even more have been victims of rape and other crimes. But it is hard to measure how these environments may actually have caused the onset of the illness.

Most people with borderline personality disorder have a biological defect in the way they regulate mood and emotion. The amygdala, a device in the brain, helps us to regulate our negative emotions. People with borderline personality disorder have amygdalas that do not function properly. In these cases it is thought that a front portion of the brain may be dampening the effect of the amygdala, causing severe mood swings and abnormal behavior.

Meanwhile, the cause of BPD remains under investigation, and there's no known way to prevent it. Possible causes include:

• 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 and separation from caregivers or loved ones.

• Brain abnormalities. Some research shows 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. Most likely, a combination of these issues results in borderline personality disorder.