Pseudologia fantastica, mythomania, or pathological lying are three of several terms applied by psychiatrists to the behavior of habitual or compulsive lying. It was first described in the medical literature in 1891 by Anton Delbrueck. Although it is a controversial topic, pathological lying has been defined as "falsification entirely disproportionate to any discernible end in view, may be extensive and very complicated, and may manifest over a period of years or even a lifetime".
Although little has been written about pathological lying, one study found a prevalence of almost 1 in 1,000 repeat juvenile offenders. The average age of onset is 16 years, and its occurrence was found by the study to be equal in women and men. Forty percent of cases reported central nervous system abnormality (characterized by epilepsy, abnormal EEG findings, head trauma, or CNS infection).
The defining characteristics of pseudologia fantastica are:
- The stories told are not entirely improbable and often have some element of truth. They are not a manifestation of delusion or some broader type of psychosis: upon confrontation, the teller can admit them to be untrue, even if unwillingly.
- The fabricative tendency is long lasting; it is not provoked by the immediate situation or social pressure as much as it is an innate trait of the personality.
- A definitely internal, not an external, motive for the behavior can be discerned clinically: e.g., long-lasting extortion or habitual spousal battery might cause a person to lie repeatedly, without the lying being a pathological symptom.
- The stories told tend toward presenting the liar favorably. For example, the person might be presented as being fantastically brave, knowing or being related to many famous people.
Pseudologia fantastica may also present as false memory syndrome, where the sufferer genuinely believes that fictitious events have taken place, regardless that these events are fantasies. The sufferer may believe that he or she has committed superhuman acts of altruism and love or has committed equally grandiose acts of diabolical evil, for which the sufferer must atone, or has already atoned for in her/his fantasies.
Lying is the act of both knowingly and intentionally/willfully making a false statement. Most people do so out of fear. Pathological lying is considered a mental illness, because it takes over rational judgement and progresses into the fantasy world and back.
Excessive lying is a common symptom of several mental illnesses. For instance people who suffer from antisocial personality disorder use lying to benefit from others. Some individuals with borderline personality disorder lie for attention by claiming they’ve been treated poorly.  Pathological lying, on the other hand, can be described as an addiction to lying. It is when an individual consistently lies for no personal gain. The lies are commonly transparent and often seem rather pointless.
There are many consequences of being a pathological liar. Due to lack of trust, most pathological liars' relationships and friendships fail. If the disease continues to progress, lying could become so severe as to cause legal problems, including but not limited to fraud.
Psychotherapy appears to be one of the only methods to treat a person suffering from pathological lying. No research has been performed regarding the use of pharmaceutical medication to treat pathological liars. Some research suggests that certain people may have a “predisposition to lying”.
Pathological lying is a complex phenomenon, differing from other mental illnesses. It has many life-changing consequences for those who must live with the illness. Currently, there is not enough research in the area of pathological lying to guarantee a cure.