Personality, its meaning, can be based on a legal consideration, in the sense of a person who has certain rights, and/or is rational. Generally speaking, the personal consciousness or ego that relates outwardly to a particular social order and inward, reflectively, is one's personal identity, i.e., the individual.
When we turn to the subject of personality disorders, which often begin in childhood and continue throughout most of adult life, our interest is not concerned with minor problems such as nervousness or the changes in sensibility and emotional disposition but, instead, with a cognitive distortion which affects, regularly, both the outward side of personality and the inward, reflective side. This can produce a personality in which suspiciousness, oversensitivity, mistrust, and over preoccupation with hidden motives, are enlarged beyond bounds. Given such an enlargement, personalities can suffer from feelings of inadequacy, a lack of humor, an inability to relax, a susceptibility to anger, to name just a few problems.
When going through the door of Buddhism, those who are encumbered with a personality disorder will see much in Buddhism that is not there and too little of real Buddhism. More than often it is those who have a personality disorder that are, paradoxically, Buddhism's most engaged devotees—but not because they wish to follow the Buddha's teaching and his path, but because they want to reform Buddhism to fit within the milieu of their personality. This, I hasten to add, is not uncommon. It is all-too-human when we look at society in general.
In the business world, in which the personality disorder of psychopathy can sometimes be present (see Dr. Robert Hare's book, Snakes in Suits) great companies go belly-up because a psychopath was the CEO. Nor are certain types of personality disorders uncommon in the medical profession—in fact they may play a major role. Dr. John Banja in his book, Medical Errors and Medical Narcissism has certainly turned a bright light on the problem of the narcissistic personality disorder in medicine which can lead to a surprising number of iatrogenic deaths.
Just looking over the history of religious charlatanism which goes as far back as ancient Rome if not much further, religion seems to be the best refuge for someone with a histrionic personality disorder. It is also a suitable refuge for someone with a dependent personality disorder, who is extremely passive wanting others to make decisions for them; who, in addition, suffers from an extreme lack of confidence.
It should at least be clear to a normal beginner that when they enter the sacred precincts of Buddhism, they'd better be on guard for all that glitters is not gold. Indeed, there are many wolves in Zen robes. But also, of those who are common practitioners—and even beginners—many suffer from different kinds of personality disorders.
Buddhism can't cure personality disorders; neither can psychologists nor psychiatrists. Buddhism is and should be for normal, ordinary people who struggle with life having come to the conclusion that deep within them lies the path to truth. These same normal people, since they know in which direction the true lies are not so disposed to mismanage and overreact to arising impressions that eventually turn into suspicion and mistrust. They don't lack empathy either nor are they over-preoccupied with themselves, that part of themselves which is temporal. They can also adapt to change being able to shed what is superfluous for that which is truly real.