A pathological liar is someone who lies frequently and may feel a compulsion to do so. They cant stop lying even when it causes psychological distress, puts them in danger, and creates problems with relationships, work, or other aspects of daily life. Pathological lying tends to start early, in adolescence and young adulthood. It is not in itself a mental health diagnosis, but it is an established concept in psychology. Pathological lying is related to disordered thinking patterns and beliefs, and the word pathological indicates that there is an underlying pathology or illness.

Some mental health conditions may be associated with patterns of pathological lying, including antisocial personality disorder (ASPD), borderline personality disorder (BPD), and factitious disorder or Munchausen syndrome by proxy. People with ASPD may lie for status, resources, or sympathy, while those with BPD may lie to avoid rejection or abandonment.

It is important to note that pathological lying is different from compulsive lying, which is lying for external personal gain in the forms of money, sex, and power. Pathological lying is strictly internal, and people affected by antisocial disorder lie for external personal gain.

Psychopaths experience none of the arousal, stress, and guilt from their deception that pathological liars exhibit. While natural liars are seemingly no different than the average person (apart from their skill in lying), the hallmark of psychopathy is persistent antisocial behavior and lack of empathy and remorse.