By Dr. Seth Meyers

Most people encounter a difficult person and call them “crazy” or “awful,” but there are actually clinical terms that therapists use to describe them.

If a person demonstrates a pattern of difficult, frustrating behavior over time, there is a good chance that the individual in question has what therapists call a “personality disorder.”

What is a personality disorder?

Common personality disorders include Narcissistic Personality Disorder and Borderline Personality Disorder, among several others. What’s interesting is that the most difficult type of person to deal with or date isn’t a person who fits one of these disorders perfectly.

The most difficult person to date is the one who has a few characteristics of each of the challenging personality disorders, so the individual has a “mix” of many personality disorders.

According to the Diagnostic and Statistical Manual of Mental Disorders, there are a few personality disorders that are called “Cluster B” personality disorders, and these are the most serious of all.

RELATED: How To Know If An Undiagnosed Personality Disorder Is Ruining Your Relationship

If a person has a mix of traits from three of the Cluster B personality disorders — Narcissistic, Borderline, and Antisocial — that individual will usually have a hard time having a consistent and reasonably happy relationship.

(Note that I’m not saying that these individuals with this personality type are bad people. If they get treatment and deal with their issues, because we all have some, they can be decent men and women and can have functional relationships because they were willing to do the work on their issues.)

The narcissistic traits that annoy and anger others the most include a pronounced sense of superiority, denial of their own accountability when things go wrong, and a difficulty thinking about others’ feelings.

Borderline traits, on the other hand, include cycling between extreme emotions in the same hour or even minute, constantly changing their opinion about others as all good or all bad, and manipulation.

Finally, antisocial traits include a lack of empathy or remorse, lacking a moral compass, and lying to get what they want.

Most people who have personality disorders don’t fit every single criterion of each disorder.

The most difficult type of person to have any kind of relationship with —​ whether platonic, romantic, or even professional — is the person who displays some traits from each of these three serious personality disorders.

There is no doubt that the most dangerous personality disorder of all is the antisocial type because those who fit the disorder don’t feel bad about hurting others.

However, most difficult people you encounter won’t be that severe. They may display a few antisocial traits but they won’t have all the characteristics required to justify the full diagnosis.

RELATED: 15 Signs Someone Has A Narcissistic Personality Disorder — And How To Understand Them

Here's an example of a person with traits of several personality disorders mixed in one.

The antisocial type doesn’t care about established rules. Antisocial individuals will break any rule as they see fit, and they will be extremely calculating to make sure that others don’t see just how many rules or laws they violate. In a word, antisocial types are usually sneaky.

Add to that component a person who has an enormous ego and thinks they’re better than everyone else (the narcissistic component), and you have a difficult person.

If that person has traits of borderline personality, too — say, manipulation and casting out friends or lovers if they make a wrong move because now they’re “all bad” — you are talking about someone with whom you can’t have any kind of successful relationship (until they get mental health treatment).

Most people might imagine that therapists or psychologists can identify these people from miles away, but the truth is that it takes anyone — doctors included — a while to get to know someone.

Men and women who have Cluster B traits are extremely calculating individuals, and they will work had to hide parts of themselves that they don’t like or that they don’t want you to see.

These Cluster B types often work had to present the opposite image to new people, trying to seduce you and to appear as appealing as possible. It is often not until many months later that you start to see some of the hidden traits leak and you start to see the pathological side of who that person is.

As soon as you detect a true pattern of these Cluster B traits — especially a sense of superiority, lying and breaking or changing the rules, or extreme emotionality — tread carefully.

Trying to having any type of a functional relationship with someone with this mix of traits will almost always fail until that individual acknowledges the problem and gets the mental health services they need.

RELATED: 16 Signs You Love Someone With A Serious Personality Disorder

Dr. Seth Meyers is a licensed clinical psychologist, author, and TV guest expert. He practices in Los Angeles and treats a wide range of issues and disorders and specializes in relationships, parenting, and addiction.

This article was originally published at eHarmony. Reprinted with permission from the author.