PL should be differentiated from other psychiatric conditions that have been associated with deception. This is complicated because lying behaviors that mimic PL have been described in certain personality disorders and in factitious disorder. The core symptoms of those personality disorders—antisocial, borderline, histrionic, and narcissistic—are often apparent. For example, the falsifications that may occur in borderline personality disorder (BPD) are not usually of the elaborate, fantastic, or complicated nature seen with PL. Patients with BPD often lack a consistent self-identity, hold contradictory views of themselves that alternate frequently, often make false threats, and are prone to false accusations of maltreatment and/or abandonment. Conversely, pathological liars do not show the intense affective dysregulation or suicidal behaviors that characterize BPD. In antisocial personality disorder, the lies are often for external gain, and there is a history of conduct disorder in childhood, unlike in PL. Furthermore, the lying behavior in PL covers a wider context than in factitious disorder, in which lying is solely for the purpose of assuming a sick role.
Other conditions that could be confused with PL include malingering, Ganser syndrome, and confabulation. The elaborate and complicated fantasies seen in PL do not occur in Ganser syndrome, where the lies are limited to approximate answers, or in confabulation (which may be part of the descriptive symptoms of Wernicke-Korsakoff syndrome), where falsifications are used to cover memory gaps. Furthermore, there is no organically derived amnesia in PL as exists in confabulation. The feature that differentiates malingering from PL is the motivation for lying; obvious external incentives alone drive the lies in malingering, unlike in PL, where the motivation to lie is less clear.
Delusions should be considered in the differential diagnosis because of controversial propositions that the lies told in PL rise to delusional proportions. Unlike PL, however, delusions are not intentional lies told to deceive, but rather, they are fixed beliefs that happen to be false. The blurring of fact and fiction that occurs in PL is not the same as the absolute conviction that occurs in persons with delusions.
Other diagnostic conundrums
In their seminal report, Healy and Healy1 argued that true PL occurs in the "absence of definite insanity, feeblemindedness, or epilepsy," an opinion that indicates PL is not secondary to another psychiatric disorder. This opinion has not been universally accepted; counter arguments posit that PL is always secondary to a recognizable psychiatric disorder.3 The only mention of PL in DSM is as a non- essential symptom of factitious disorder.
Ironically, lying to assume a sick role is considered important enough to warrant a diagnostic label; but PL, which, like factitious disorder, has an unconscious motive, is not. It is becoming increasingly clear, however, that there are individuals with PL who have no preexisting psychiatric disorder. For example, Judge Couwenberg's psychiatrist expert witness diagnosed pseudologia phantastica. Going back in history, Cleckley8 described the case of a successful and respected man with a doctorate in physics who had pseudologia phantastica in the absence of insanity or psychopathy. Consequently, Dike and colleagues9 have suggested that PL should be categorized as primary PL or secondary PL, depending on the absence or presence, respectively, of a preexisting psychiatric disorder that might be responsible for the lying behavior.
If PL cannot be considered a clinical entity in its own right, could it be seen as a subset of the impulse control disorder spectrum, given the impulsive nature of the lies? Alternatively, does the observation that pathological liars feel compelled to lie repeatedly, or have obsessional falsifications (according to Fenichel10), warrant a consideration of PL as an obsessive-compulsive disorder? A more controversial consideration would be whether there are subtypes of pathological lying that may fit into a special category of delusional disorders— especially in those whose reality testing is suspect. To suggest, however, that PL is a psychotic disorder would seem preposterous to most psychiatrists because individuals exhibiting PL often function well in many spheres of daily living. Although the cause of PL is unknown, there are increasing associations with CNS dysfunction. As noted earlier, 40% of 72 individuals with pseudologia phantastica had a history of CNS abnormalities.4 In another study, single photon emission CT showed right hemithalamic dysfunction in a patient with pseudologia phantastica.11
The most recent study involved the use of structural MRI in 12 individuals identified as "liars."12 The liars group comprised 4 subgroups: malingering group, PL group (PL was defined using the Hare Psychopathy Checklist–Revised), individuals with conning/manipulative behavior, and individuals who met the deceitfulness criteria for DSM IV. The study found a 22% to 26% increase in the prefrontal white matter and a 36% to 42% reduction in prefrontal gray-to-white ratios in the liars group compared with antisocial controls and normal controls. The main flaw of this study was that although half of the liars group had a diagnosis of malingering and only a small number had PL, the liars group was frequently interchanged with the pathological liars as if they were the same. In addition, PL was defined using the Hare Psychopathy Checklist, an indication that the few pathological liars included in the study were those with criminal behavior and psychopathy.
To ascribe observations from this study to PL is therefore problematic and misleading; PL and malingering are different entities, and most pathological liars are not psychopaths. There is no specific psychological test for PL. However, psychological tests would help in elucidating the presence of personality disorders, other major psychiatric illnesses, or malingering. PL has been at the fringes of psychiatric practice for more than a century. It is not surprising, therefore, that it remains ill understood and poorly researched. The increasing interest in the phenomenon in recent years, and the availability of high-tech radiological investigations may reverse this trend and help answer the many questions that have dogged this phenomenon. Despite the fact that psychiatrists are slowly converging on a uniform definition of PL, it remains unclear whether it is a mental disorder or merely behavioral excess. Associated questions involve the treatability of the phenomenon, available treatment modalities, and outcome of treatment. A psychiatrist representing Judge Couwenberg's defense team opined that pseudologia phantastica was treatable with therapy but did not state the basis for his assertion.