Everyone lies a little, tells only part of the truth, or leaves out relevant facts. A smaller proportion of people lie a lot, and some people lie to the extent that their listeners cannot ever trust their words. Some of these people are referred to as pathological liars.
All official psychiatric diseases are listed in the DSM-IV TR, the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders. Pathological lying is not currently a diagnosis in this psychiatry bible, but some argue it should be. Their reasoning is some people seem to be unable to control their lying just as some people are unable to control their mood, stealing, or worry, in depression, kleptomania, and anxiety respectively. However, since pathological lying is not in the DSM-IV, it is impossible to truly define pathological lying in a way accepted by all mental health professionals. However, there are some common definitions that can be agreed upon.
Dike, Baranoski, and Griffith (1) define a pathological liar as being one whose lies are persistent, pervasive, disproportionate, and do not seem to be for the purpose of getting a reward. In other words, “normal” people lie to avoid fighting (no, honey, that dress doesn’t make you look fat), get some reward (I expect returns of 20% if you invest your money with me), or look better (it wasn’t me). Sometimes, it has been suggested, they have lost the ability to distinguish truth from fiction. Other times pathological liars realize they are lying, but on some level cannot tell the truth.
Pathological liars can be spotted only by a pattern observed over time. They will lie about little things and big ones, they will change their stories, and they will act defensively when confronted. Exaggerations are common, as are one-upping the stories and experiences of others. Their lies are often ridiculous to the listener, but often the liar believes the lies as they speak them. If they have a really exciting event that is true, such as a big win at a competition, or their car catching on fire while they are in it, they may go to the extreme in documenting the event to prove their veracity.
Usually lies are impulsive rather than well thought out, which makes catching the lies easy sometimes, but pathological liars are very good at manipulating the situation and person they are lying to, as well as distracting others from their lies by leaving the situation or becoming angry the listener is questioning their story. The pathological liar will then rewrite history in their head, making themselves the victim, the lie true, or the fight never to have happened.
There are many diseases in the DSM-IV that overlap with these definitions. Narcissism, Factitious Disorders, and Borderline Personality Disorder all include a certain amount of compulsive lying, in which the individual is, to a certain extent, unable to tell the truth. More research is needed to determine if pathological lying should be a diagnosis in itself, or if it is simply a symptom of a larger problem. Regardless, pathological liars need treatment in order to refrain from lying and to be trusted by others.
(1) Dike CC, Baranoski M, Griffith EEH: Pathological lying revisited. J Am Acad Psychiatry Law 33:342–9, 2005
References
http://pn.psychiatryonline.org/content/38/1/24.1.full
http://healthmad.com/mental-health/how-to-spot-a-pathological-liar/
http://www.jaapl.org/cgi/content/full/33/3/350
http://wiki.answers.com/Q/How_can_you_tell_if_someone_is_a_pathological_liar
