Hi Kathy. It is the emotional unstableness and unpredictability that is at the very core of this disturbance. It is often said that what is stable in borderline persons is their instability. They have often difficulties to bond and trust to persons and often change partners. This diagnosis is not used for teenagers, but for adults. Some of the self-destructive behaviours are also seen in persons that have depression without borderline diagnosis. The diagnosis has been around for some time, and the article sums up the diagnosis reasonably well.