A deliberate liar knows he is lying. A pathological liar may not.
Although there is no precise definition, certainly none defined by the woefully inadequate but oft-cited DSM-IV, we can state with certainty that it is a person who tells lies incessantly. We can eliminate those who tell lies in order to avoid extreme persecution. But those who consistently tell lies, whether faced with punishment or not, may be considered pathological. We must keep in mind that "pathological" simply means abnormal, or grossly atypical...who among us has not told a lie? At what point does such behavior become "pathological"?
In addition to the difficultly of distinguishing between the liar and the pathological liar, we must also isolate this mental disturbance. Lying is a characteristic of several other disorders as well, such as conduct disorder (CD) and antisocial personality disorder (APD). CD, like many reports of pathological lying, typically has its onset during adolescence. Other behaviors may include inappropriate aggression, destruction, and serious violations of rules and laws. And, as suggested by some doctors, both pathological lying and CD may be caused by, shall we say, challenging situations in the home. Or by a lack of seratonin, in which case Prozac or Zoloft may help. Along with, of course, expensive sessions of psychotherapy.
You may find Jon Ronson's new book, The Psychopath Test an interesting read, as it deals with individuals who would most likely be considered pathological liars, and the trouble with creating a reliable diagnosis.
"...When generating a lie or a false response, one must know the truth, resist the impulse to answer truthfully, and further generate an alternative but appropriate response." - J.M. Nuñez et. al.
The brain should indeed behave in a different manner when concocting a lie, and these differences should be measureable. Studies have found decreased activity in the thalamus, particularly the right hemithalamus, in one pathological lying patient (Modell et al, 1992), whereas another study found increased activity in the anterior cingulate, dorsolateral prefrontal cortex, and caudate and thalamic nuclei in 20 non-pathological volunteers (Nuñez et al, 2005). Further research in this area is clearly warranted, particularly in the right hypotenuse, a subset of the right hemithalamus (see transaxial brain diagram, right).
For explorations of neurological phenomena associated with pathological lying, see:
Modell JG, Mountz JM, Ford CV: Pathological lying associated with thalamic dysfunction demonstrated by [99mTc]HMPAO SPECT. The Journal of Neuropschiatry and Clinical Neurosciences 1992; 4:442-446 <http://neuro.psychiatryonline.org/cgi/content/abstract/4/4/442>
Nuñez JM, Casey BJ, Egner T, Hare T, Hirsch J: Intentional false responding shares neural substrates with response conflict and cognitive control. NeuroImage 2005: 25:267-277 <http://www.fmri.org/pdfs/Nunezetal2005.pdf>.
Lying can often be to our advantage. And a really good lie is worth twenty shallow truths any day. The best lies, of course, are the ones that the liars themselves believe. Is it possible that we can induce pathological lying in ourselves, or test subjects, by inhibiting our production of seratonin? Could we do this without increasing overall aggressive behavior?
These are the sorts of questions researchers here at Osric University struggle with daily. We hide our multi-trillion dollar facilities within the confines of other well-known universities around the world, and we receive funding from most governments and multinational corporations. When you study at Osric University, you study with the very best!
NOTE: due to the lack of research on pathological lying, we welcome your own personal case studies. Please send your own observations on pathological lying to firstname.lastname@example.org. We cannot diagnose cases via e-mail, nor can we always respond to your messages personally. Please do not use the subject's real name in your observations. Pertinent studies, especially those including specific lies and the context in which they were told, will be published.