The Diagnostic and Statistical Manual, Fourth Edition (DSM-IV), a manual published by the American Psychiatric Association which is used to diagnose all mental disorders, defines personality disorders as patterns of experience and behavior that are drastically different from the norm. In order to warrant a diagnosis of a personality disorder, an individual must exhibit deviant patterns of behavior in at least two of the areas of thinking, mood, personal relations and impulse control. There are ten personality disorders currently identified in the DSM-IV. These disorders are grouped into three major classes, or clusters.

Cluster A, which includes the paranoid, schizoid, and schizotypal disorders, identifies odd or eccentric personality disorders. For example, paranoid personality disorder (PPD) is characterized by persistent feelings of distrust and suspicion against others. A person with PPD may experience constant but unjustified suspicions that others are planning to deceive him or her.

Schizoid personality disorder (SPD) is defined by detachment and lack of interest in relationships with others. An individual with SPD may prefer solitary activities and seem emotionally detached from others. Similarly, schizotypal disorder is also characterized by a need for isolation, but also includes magical or unconventional beliefs. For example, someone with schizotypal disorder may believe he or she has magical powers.

The second group of personality disorders, also known as Cluster B, contains the dramatic and emotional disorders. Antisocial personality disorder (APD), the first disorder outlined in Cluster B, is characterized by an apparent disregard for the safety and feelings of others. Known in earlier DSM editions as a sociopath, an individual with APD may engage in criminal or otherwise reckless behavior with little or no regard for the law.

Borderline personality disorder (BPD), the second member of the Cluster B personality disorders, is indicated by an unstable sense of self, impulsivity, and a tendency towards strict "black-and-white" thinking. Individuals diagnosed with BPD frequently have histories of unstable relationships and engage in self-destructive behavior such as self-mutilation, reckless spending, or binge eating. Histrionic personality disorder, on the other hand, is characterized by extreme theatricality and a constant need to be the center of attention. The last of the Cluster B personality disorders is narcissistic disorder, which is indicated by an inflated sense of self-importance and entitlement.

The remaining three personality disorders outlined in the DSM-IV fall under the Cluster C category. These disorders are classified as fearful or anxious. For example, an individual with avoidant personality disorder may be so afraid of criticism or rejection that he or she withdraws from social situations.

The second of the fearful and anxious personality disorders is known as dependent personality disorder (DPD). Unlike avoidant personality disordered patients, an individual with DPD is intensely dependent on others, feels uncomfortable when alone, and may have difficulty making decisions without the approval or advice of others. Lastly, obsessive-compulsive personality disorder is characterized by a preoccupation with lists and orderliness, often to the point that it interferes with interpersonal relationships.

Although many individuals experience patterns of behavior that are characteristic of personality disorders, a personality disorder can only be diagnosed if these patterns are strong enough to interfere drastically with an individual's daily life. Many personality disorders have high occurrences of comorbidity with other psychological disorders. For example, it is not uncommon for a person with avoidant personality disorder also to suffer from depression or anxiety disorder.