There are constitutional limits on the use of a rape shield law to exclude evidence offered by the defense in a criminal trial to impeach an adverse witness.

United States v. Photogrammetric Data Services, Inc., 259 F.3d 229, 244-245 (4th Cir. 2001) (upholding trial court's admission into evidence of employee's statements as admission by party's agent because (1) "the statements Webb made to the government....A statement by an agent or an employee who is not authorized to speak on behalf of the agent's principal or the employee's employer may nonetheless be treated as the statement of the principal or the employer and thus as a vicarious admission that is exempt from the prohibition against hearsay; but such a statement can be imputed to the principal or the employer only if the statement, if by an agent, is about a matter within the scope of the agent's agency or, if by an employee, the statement is about a matter within the scope of the employee's employment.

Trammel v. United States, 445 U.S. 40, 44-45 (1980) (the Court weakly endorses, if at all, the proposition that the privilege fosters or protects the sanctity of marriage: "The modern justification for this privilege against adverse spousal testimony is its perceived role in fostering the harmony and...The privilege against adverse spousal testimony, or the marital testimony privilege, fosters or protects the harmony and sanctity of the marital relationship.

Fed. R. Evid. 702 advisory committee's note (note on 2000 amendment of Rule 702: "Consistently with Kumho, the Rule as amended provides that all types of expert testimony present questions of admissibility for the trial court in deciding whether the evidence is reliable and helpful").The admissibility of a non-scientific expert field, method, or result under Federal Rule of Evidence 702 or a comparable State rule requires a showing of the reliability of such evidence.