Claiming to have been in Vietnam when he wasn't could be an indication of mental illness, or could be an indication of dishonesty.

As for what is known about false confessions:

Types of False Confessions
According to Saul M. Kassin, a professor of Psychology at Williams College and one of the leading researchers into the phenomenon of false confessions, there are three basic types of false confessions:

    * Voluntary false confessions
    * Compliant false confessions
    * Internalized false confessions

While voluntary false confessions are given with no outside influences, the other two types are usually coerced by external pressure.

Voluntary False Confessions
Most voluntary false confessions are the result of the person wanting to become famous. The classic example of this type of false confession is the Lindbergh kidnapping case. More than 200 people came forward to confess that they had kidnapped the baby of the famous aviator Charles Lindbergh.

Scientists say these kinds of false confessions are prompted by a pathological desire for notoriety, meaning they are the result of some mentally disturbed condition.

But there are other reasons that people make voluntary false confessions:

    * Because of feelings of guilt over past transgressions.
    * The inability to distinguish fact from fiction.
    * To help or protect the real criminal.

Compliant False Confessions
In the other two types of false confession, the person basically confesses because they see confessing as the only way out of the situation they find themselves in at the time.

Compliant false confessions are those in which the person confesses:

    * To escape a bad situation.
    * To avoid a real or implied threat.
    * To gain some kind of reward.

The classic example of a compliant false confession is the 1989 case of a female jogger was beaten, raped and left for dead in New York City's Central Park in which five teenagers gave detailed videotaped confessions of the crime.

The confessions were discovered to be completely false 13 years later when the real perpetrator confessed to the crime and was linked to the victim through DNA evidence. The five teenagers had confessed under extreme pressure from investigators simply because they wanted the brutal interrogations to stop and they were told they could go home if they confessed.

Internalized False Confessions
Internalized false confessions occur when, during the course of interrogation, some suspects come to believe that they did in fact commit the crime, because of what they are told by the interrogators.

People who make internalized false confessions, believing they are in fact guilty, even though they have no recollection of the crime, are usually:

    * Younger suspects.
    * Tired and confused by the interrogation.
    * Highly suggestible individuals.
    * Exposed to false information by interrogators.

An example of an internalized false confession is that of Seattle police officer Paul Ingram who confessed to sexually assaulting his two daughters and killing infants in Satanic rituals. Although there was never any evidence that he ever committed such crimes, Ingram confessed after he went through 23 interrogations, hypnotism, pressure from his church to confess, and was provided graphic details of the crimes by a police psychologist who convinced him that sex offenders often repress memories of their crimes.

Ingram later realized that his "memories" of the crimes were false, but he was sentenced to 20 years in prison for crimes he did not commit and which may never actually happened, according to Bruce Robinson, the Coordinator for The Ontario Consultants on Religious Tolerance.

Developmental Handicapped Confessions
Another group of people who are susceptible to false confessions are those who are developmentally handicapped. According to Richard Ofshe, a sociologist at the University of California, Berkeley, "Mentally retarded people get through life by being accommodating whenever there is a disagreement. They've learned that they are often wrong; for them, agreeing is a way of surviving."

Consequently, because of their excessive desire to please, especially with authority figures, getting a developmentally handicapped person to confess to a crime "is like taking candy from a baby," Ofshe says.

Sources:
Saul M. Kassin and Gisli H. Gudjonsson. "True Crimes, False Confessions. Why Do Innocent People Confess to Crimes They Did Not Commit?" Scientific American Mind June 2005.
Saul M. Kassin. "The Psychology of Confession Evidence," American Psychologist, Vol. 52, No. 3.
Bruce A. Robinson. "False Confessions By Adults" Justice: Denied Magazine.