Personality disorders are placed into groups or “clusters” because some share similar symptoms or sometimes co-occur.
Cluster A personality disorders tend to be marked by eccentric behavior, limited emotional expression, and social withdrawal, typified by paranoid personality disorder.
Cluster B personality disorders are almost the opposite, characterized by very emotional, dramatic, or erratic behavior, as seen with histrionic personality disorder, borderline personality disorder, antisocial personality disorder, or narcissism.
Cluster C personality disorders, on the other hand, are marked more by anxiety and fearful thoughts and behavior, as in obsessive-compulsive personality disorder.