People with the disorder are said to have a thin emotional skin and often behave like 2-year-olds, throwing tantrums when some innocent word, gesture, facial expression or action by others sets off an emotional storm they cannot control. The attacks can be brutal, pushing away those they care most about. Then, when the storm subsides, they typically revert to being “sweet and wonderful,” as one family member put it.

In an effort to maintain calm, families often struggle to avoid situations that can set off another outburst. They walk on eggshells, a doomed effort because it is not possible to predict what will prompt an outburst. Living with a borderline person is like traversing a minefield; you never know when an explosion will occur.

A Misleading Label

The name of the disorder was coined in the 1930s, in a misleading reference to the border between neurosis and psychosis. Experts say it has nothing to do with either condition.

Rather, affected individuals seem to be born with a quick and unduly sensitive emotional trigger. The condition appears to have both genetic and environmental underpinnings. Brain studies have indicated that the emotional center of the nervous system  the amygdala  may be overly reactive, while the part that reins in emotional reactions may be underactive.

As children, people who will develop the disorder are often “hyperreactive, hypervigilant and supersensitive,” Valerie Porr, a therapist in New York, said in an interview. Typically they receive a host of misdiagnoses and treatments that are inappropriate and ineffective.