What is a personality disorder? It helps to begin with the visual image of a bell-shaped curve. Using empathy as a personality characteristic, there is the average range of plus or minus one standard deviation (the largest range of the curve).
Everyone within that range, which is the majority of the population, has a normal amount of empathy.
Those, with a standard deviation of greater than one but less than two, has either too much or too little empathy compared to the average (the medium range of the curve in between the largest and smallest sections). More than two deviation points places empathy at the dysfunctional level. This is the point where it becomes part of a disorder (the smallest range of the curve). Do this same procedure for each one of the personality disorder characteristics.