663.1Polygraphs, Plethysmography, and Witness Credibility

Last Updated: 12/01/23

Key Concepts

  • Testimony about the administration or results of a polygraph examination (“lie detector test”) is inadmissible at trial, but officers may still use the test for investigative purposes, and any statements made by the defendant during or after the test are not inadmissible simply because they were made in connection with a polygraph.
  • An expert may not offer an opinion that another witness is generally credible or that the witness’s testimony about a particular matter should be believed, but the expert may testify about related issues that have a bearing on the witness’s credibility (such as the witness’s mental health), if the expert is qualified to do so.
  • Penile plethysmography testing, which measure a male subject’s arousal response to sexual stimuli, has been held inadmissible under Daubert and Rule 702.

Polygraph Examinations

Expert testimony (as well as lay testimony) about the administration or results of a polygraph examination, commonly known as a “lie detector test,” is inadmissible in North Carolina courts because the test has not been deemed scientifically reliable. See, e.g., State v. Grier, 307 N.C. 628 (1983) (results of polygraph testing are inadmissible at trial, even if the parties stipulate to its admissibility); State v. Fleming, 350 N.C. 109 (1999) (judge properly prohibited defendant from offering polygraph test results); see also United States v. Scheffer, 523 U.S. 303 (1998) (military court’s evidence rule that polygraph results were inadmissible in court-martial proceedings did not unconstitutionally abridge defendant’s right to present a defense). There are no North Carolina cases addressing the admissibility of polygraph examination testimony in light of the 2011 amendment to Rule 702 adopting the Daubert standard, but given the more rigorous inquiry conducted under Daubert, it is highly unlikely that the appellate courts would reach a different conclusion today. See generally State v. McGrady, 368 N.C. 880 (2016) (While the amendment “did not change the basic structure of the inquiry” under Rule 702(a), it “did change the level of rigor that our courts must use to scrutinize expert testimony before admitting it.”).

Even though the test results themselves are inadmissible, law enforcement officers are still permitted to use polygraph testing for investigative purposes. It is not uncommon for suspects to confess or make incriminating statements during the test or after being confronted with unsatisfactory results of a test, and any such statements or admissions obtained during or after the polygraph testing are not inadmissible merely because they were made in connection with a polygraph, as long as the actual test and results are not presented to the jury. See State v. Harris, 315 N.C. 556 (1986) (defendant’s confession made after failing a polygraph was admissible, and the witness’s related testimony at trial about “[t]he administration of the polygraph test was merely an event bearing on the total circumstances surrounding defendant's inculpatory statement.”); State v. Singletary, 75 N.C. App. 504 (1985) (“under the totality of these circumstances, the trial court did not err in refusing to suppress the incriminating statements solely because a polygraph machine was involved”); see also State v. Payne, 327 N.C. 194 (1990) (“Throughout the investigation and trial, the defendant consistently contended that the shooting was accidental. It was not until after the polygraph examination that the defendant admitted planning to kill his wife.”); but see State v. Willis, 109 N.C. App. 184 (1993) (although polygraph test was not directly mentioned before jury, polygraph examiner’s opinion testimony that defendant was lying in answering questions was improperly admitted because opinion testimony was based on polygraph test results).

Practice Pointer

If it’s not admissible, what’s the point?
In practice, despite longstanding concerns about the scientific reliability of polygraph examinations, these tests can still be quite useful in certain situations. For example, the results of the test (in conjunction with all other evidence in the case) may help prosecutors or law enforcement officers in deciding whether a potential suspect is really just a witness, or whether it would be appropriate to allow a co-defendant to plead guilty and testify against the primary  defendant. (“I swear – I was there, but I didn’t participate…”). Additionally, the process of administering the test itself and the subject’s fear that it will uncover any deception may encourage that person to answer the questions honestly. Finally, the test also provides a useful mechanism for investigators to press the person on certain answers during any follow-up questioning. (“The test results showed clear signs of deception in your answer to question #4 – so now why don’t you tell us what really happened?”)

Credibility of Witnesses

Expert testimony on the credibility or inherent truthfulness of another witness is not admissible. See State v. Heath, 316 N.C. 337 (1986) (holding that an expert’s testimony was improper for this reason); State v. Aguallo, 318 N.C. 590 (1986) (citing Heath and holding that the trial court erred by allowing a pediatrician to testify that a rape victim was “believable”); State v. Green, 209 N.C. App. 669 (2011) (stating the rule, but holding that in this case the expert’s testimony regarding the defendant’s blood alcohol level did not constitute impermissible opinion testimony); see also G.S. 8C-1, Rule 608 (Evidence of character and conduct of a witness) and Rule 405(a) (Methods of proving character). Therefore, it is error to allow an expert to testify that he or she “believes” a witness and to state the reason for this belief. See State v. Teeter, 85 N.C. App. 624 (1987) (“the admission of Ms. Cromer's testimony that she believed Robin and the reason for her belief was error”).

However, testimony from a qualified expert that another witness does or does not suffer from a mental condition that would impair the witness’s ability to testify truthfully (such as a mental illness that prevents the witness from distinguishing between fantasy and reality, or an injury that impairs the witness’s memory) is not prohibited. See State v. Teeter, 85 N.C. App. 624 (1987) (it was not error for a mental health expert to testify that an adult sexual assault victim who suffered certain mental impairments showed no evidence of a disorder that would impair her ability to distinguish reality from fantasy).

But the party eliciting this type of “mental condition” testimony has to be very careful to avoid crossing the line into asking the expert to “vouch” for the other witness, or express an opinion as to that witness’s overall credibility. See State v. O'Hanlan, 153 N.C. App. 546 (2002) (noting that “the cases dealing with the line between discussing one's expert opinion and improperly commenting on a witness' credibility have made it a thin one,” but finding no error in this case where doctor testified that “the victim's emotional state was consistent with someone who had been sexually assaulted; indeed, a severe sexual assault” but “the doctor was never asked whether he believed the victim was sincere”); State v. Heath, 316 N.C. 337 (1986) (noting that expert could have properly testified about her “opinion as to whether Vickie was afflicted with any mental condition which might cause her to fantasize about sexual assaults in general,” but testimony that directly expressed an opinion about the witness’s “truthfulness” regarding the specific assault at issue in the case were improper)

Practice Pointer

Credibility of child victims of sexual assaults
This issue frequently arises in child sexual assault cases where the defense contends that the victim has either imagined the assault or been coached into giving untruthful testimony. Compare State v. Jackson, 320 N.C. 452 (1987) (expert psychiatrist’s opinion testimony that the child victim was a “truthful person” was improperly admitted) with State v. Kennedy, 320 N.C. 20 (1987) (it was not error to allow an expert to testify that the victim responded to IQ and personality test questions in an “honest fashion” because the testimony was not an expert opinion as to the victim’s character or credibility, but rather went to the reliability of the test itself). For further discussion of these issues, and a summary of several relevant cases, see Evidence Issues in Criminal Cases Involving Child Victims and Child Witnesses,” Jessica Smith, Administration of Justice Bulletin, December 2008. See also the related entry on Expert testimony regarding Child Sexual Assault Exams.

Penile Plethysmography Testing

Penile plethysmography tests a man’s level of sexual arousal by “placing a pressure-sensitive device around a man’s penis” and “presenting him with an array of sexually stimulating images” in order to determine “his level of sexual attraction by measuring minute changes in his erectile responses.” “Penile Plethysmography Testing for Convicted Sex Offenders,” Michael C. Harlow & Charles L. Scott, 35 Journal of the American Academy of Science and the Law 536 (2007). This test is widely used by sex offender treatment programs, and it has been ordered by some courts as a term of supervised release for sex offenders. Id. In some cases, the defense may wish to introduce these test results as evidence at trial – for example, to show that the defendant would not have been sexually aroused by a child, or that he did not engage in the alleged conduct for the purpose of sexual gratification.

There are no post-2011 North Carolina cases addressing the admissibility of this type of testing under Daubert and amended Rule 702, but even under the former version of Rule 702 expert opinion testimony based on this test has been held unreliable and inadmissible. See State v. Spencer, 119 N.C. App. 662 (1995) (no error in excluding proposed expert testimony that the defendant had a normal arousal pattern and that there was no evidence of his being sexually aroused by children; the trial court did not abuse its discretion in finding the defendant’s plethysmograph testing data insufficiently reliable to provide a basis for the opinion testimony).

The Fourth Circuit has likewise held that a trial court did not abuse its discretion by ruling that the penile plethysmograph test does not meet Daubert’s scientific validity requirements. See United States v. Powers, 59 F.3d 1460 (4th Cir. 1995) (holding that the district court did not err by excluding the testimony of a clinical psychologist who would have testified that the results of a penile plethysmograph test did not indicate that the defendant exhibited pedophilic characteristics); see also United States v. Cheever, No. 15-CR-00031-JLK, 2016 WL 3919792 (D. Colo., July 18, 2016) (unpublished) (quoting Powers, and noting that “[t]he penile plethysmograph fails to meet the standards of Daubert” and the test “bear[s] a striking resemblance to such pseudoscientific practices as phrenology and physiogamy. The credibility of the penile plethysmograph is highly disputed in the scientific community as it is susceptible to errors and lacks the standardization that could affirm the reliability of its results.”).

Portions of this entry were excerpted from the North Carolina Superior Court Judges’ Benchbook, “Criminal Evidence: Expert Testimony,” Aug. 2017, Jessica Smith.