Under some circumstances untruthfulness is admissible to impeach a witness.

See United States v. Safavian, 435 F. Supp. 2d 36, 38-42 (D.D.C. 2006) ("The requirement of authentication or identification as a condition precedent to admissibility is satisfied by evidence sufficient to support a finding that....A thing or matter is sufficiently authenticated to be admissible in evidence if the authenticating evidence submitted to the trial court is sufficient -- strong enough -- to permit a reasonable jury or reasonable trier of fact to conclude that the proffered thing or matter is authentic, that the proffered thing or matter is what its proponent claims it is.

Cf. United States v. Scheffer, 523 U.S. 303, pin? (1998) (Military Rule of Evidence 707 requiring exclusion of polygraph test results in general court-martial not a violation of accused's constitutional right to due....Polygraph test results are insufficiently reliable to be admissible.

Fed. R. Evid. 703 ("[f]acts or data that are otherwise inadmissible" ordinarily "shall not be disclosed to the jury by the proponent of the opinion or inference").Although an expert witness may rely on inadmissible evidence such as hearsay as the basis of his or her expert opinion and testimony, any such hearsay (or other inadmissible evidence) should not be disclosed to the jury unless the trial court determines that the probative value of such hearsay (or other inadmissible evidence) in assisting the jury to evaluate the expert's opinion substantially outweighs the prejudicial effect of disclosing such hearsay (or other inadmissible evidence) to the jury.