One of the most difficult to fathom mental illnesses is the mysterious-sounding borderline personality disorder (BPD). Psychologist Marsha Linehan says that, "Borderline individuals are the psychological equivalent of third-degree-burn patients. They simply have, so to speak, no emotional skin. Even the slightest touch or movement can create immense suffering."

Borderlines are the patients psychologists fear most. As many as 75 percent hurt themselves, and approximately 10 percent commit suicide. They are capable of deep love and profound rage almost simultaneously. They attack their loved ones so unexpectedly that they often ensure abandonment -- which is their greatest fear. When they want to hold, they claw instead. Many therapists have no idea how to treat borderlines.

What defines borderline personality disorder is the sufferers' inability to calibrate their feelings and behavior. When faced with an event that makes them depressed or angry, they often become inconsolable or enraged. This leads to impulsive behaviors -- overeating, substance abuse, suicide attempts or intentional self-injury.

Diagnosis of the condition appears to be sharply on the rise. A study of nearly 35,000 adults found that 5.9 percent had been given a BPD diagnosis. As recently as 2000, the it was believed that only 2 percent of people had BPD.

Is borderline the mental illness of the age? When so many are struggling to keep homes and paychecks, might they have become more sensitized to other kinds of desperation? In a world so uncertain, maybe it's natural to lose one's emotional skin. BPD does have at least one thing in common with a recession -- everyone (at least in the world of psychiatry) talks about it, but no one knows what to do about it.