The value of asking a suspect to take a polygraph

Whether a suspect agrees or refuses to take a polygraph can be used to assess that suspect’s credibility regarding the issue under investigation


Asking a suspect whether they are willing to take a polygraph (“lie-detector”) test can, in most jurisdictions, be incorporated into an investigator’s non-accusatory interview. There’s no legal obligation for the suspect to take a polygraph, and their mere unwillingness to take one cannot be used against the suspect.

Certainly, within the private sector, this topic is regulated by the Employee Polygraph Protection Act of 1988. Additionally, there may be some statutory guidelines on the proposed use of polygraphs even within criminal cases, such as potential prohibitions involving victims of sexual assaults. Before broaching the topic of a polygraph with a suspect, it would be wise for an investigator to review the issue against their department’s policies and even obtain an opinion from their local prosecutor.

Nevertheless, there is value to presenting the question. Whether a suspect agrees or refuses to take a polygraph can be used to assess that suspect’s credibility regarding the issue under investigation by carefully noting both the suspect’s verbal response, as well as their non-verbal behavior.

The question could be posed as follows:

 “Lucas, if it becomes necessary, would you be willing to take a polygraph or lie-detector test to determine whether or not you were involved in (issue) causing that fire at the high school gym last week?”

Experience tells us that deceptive individuals typically are unwilling to accept the invitation. A response of “yes,” however, does not necessarily ensure that the suspect is being truthful. Instead, a follow-up question then becomes necessary.

“Well let me ask you this, Lucas. If you were to take a lie detector test, how do you think that test would come out on you?”

Typically, truthful responses would be more positive or confident in nature:

  • “Great.”
  • “I’ll do fine.”
  •  “It will show I didn’t set that fire.”
  •  “No problem.”

Deceptive responses indicate less than positive or negative responses:

  • “I hope I pass it.”
  • “I think I’ll pass it.”
  • “I should do OK.”
  • “I’m not that sure how it will come out.”
  • “I don’t know.”
  • “With my luck, I’ll probably fail it.”
  • “Those tests aren’t admitted in court and they are just machines and can make mistakes.”

This last response may seem reasonable, but experience teaches that those who anxiously berate the accuracy and efficacy of polygraphs tend to be more deceptive.

For example, I once interviewed a criminal suspect who actually agreed to a polygraph test, but then when asked, “How do you feel you would do?” responded, “I guess we’ll soon find out.” He ultimately admitted his misconduct.

A response reflecting uncertainty over the results of a polygraph test will lead to a natural follow-up question such as:

“Why would you have any doubts about the outcome of a polygraph or lie detector test.”

Generally, truthful individuals will have a more sincere and immediate response:

  • “I just watched a show where two people took the polygraph and failed. It turns out, six months later DNA eliminated them from committing the crime. Two others confessed to the crime! No way will I jeopardize my life with that medieval witchcraft!”
  • “From what I’ve heard, it’s not an exact science. I have no problem giving my DNA or whatever you want to cooperate in this investigation, but no way will I take a lie test.”

Typically, deceptive suspects are not prepared for this follow-up question and responses would generally be less than positive, vague and less than sincere:

  • “Why wouldn’t I do well on it? Uh, my friend took one and it couldn’t even show his name was his.”
  • “Uh, it’s just a machine.”
  • “Well, I’m a pretty nervous person.”
  • “You hear things about it. I just am, uh, I don’t know.”

A similar inquiry can also be presented during the interview in addition to, or in lieu of, the hypothetical polygraph question, and the principles remain the same. In this instance, however, the focus shifts to the suspect’s level of confidence in the outcome of the overall investigation. The direct question is posed as follows.

“Lucas, how do you feel this investigation will come out on you as to whether or not you (issue) caused that fire to the high school?”

Truthful suspects will usually express confidence that the investigation will exonerate them, whereas the deceptive will be less than certain in the content of their response and in the manner that they deliver it.

These are only a few of the many behavior-provoking questions asked during the non-accusatory, non-confrontational Reid Technique of Investigative Interviewing. Obviously, an assessment of truth or deception is based upon the totality of responses, not just the response to any single question, as well as the suspect’s verbal and non-verbal responses, case facts and evidence. Incorporating any or all of the above behavior-provoking questions during the investigative interview will prove to be a valuable asset for any investigator in determining their suspect’s credibility.

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