On June 18, 2012, the U.S. Supreme Court affirmed the Illinois Supreme Court, ruling in Williams v. Illinois (No. 10-8505) in a four-justice plurality opinion and stating that the Confrontation Clause does not preclude expert testimony based on facts that are not within the expert's personal knowledge when those facts are not offered for the truth of the matter asserted.  The Court ruled alternatively that, even if the facts had been introduced for their truth, there would be no constitutional violation because the disputed evidence in this case did not implicate the practices the Confrontation Clause sought to stop, those being out-of-court statements implicating a particular person in criminal conduct.

Williams was convicted of rape, and the victim identified him as her attacker both in a lineup and at trial. Part of the trial evidence was testimony from a DNA expert, who testified that she compared Williams's DNA profile with a profile compiled by an accredited laboratory (Cellmark) based on semen deposited during the rape. She testified that the two profiles matched. She said no more about the Cellmark report, which was not introduced as evidence. She testified that using a DNA profile compiled by an accredited laboratory was accepted practice in her field. The trial court overruled Williams's Confrontation Clause objection, ruling that it was proper for the expert to rely on facts of which the expert lacks personal knowledge, and the conviction was affirmed by the Illinois Supreme Court.

In the U.S. Supreme Court, the plurality noted that courts have long permitted experts to testify based on facts not within their personal knowledge. The expert here testified that the Cellmark DNA profile matched the defendant's DNA profile, but she did not assert the truth of the Cellmark profile nor was the Cellmark profile shown to the fact finder or introduced as evidence. Therefore, the plurality concluded, the testimony about the Cellmark profile was not intended to show its truth, and therefore is not subject to Confrontation Clause analysis. The plurality stated that its conclusion is consistent with other recent cases in which the Court held that laboratory tests violated the Confrontation Clause because the laboratory tests in those cases were introduced for their truth.  The plurality noted that the trial process has several safeguards against prosecutorial abuse of this approach, including trial courts' duty to screen or prohibit expert testimony that intended to disclose inadmissible evidence.

Alternatively, the plurality ruled, the expert's testimony would have been admissible even it had been offered for the truth of the Cellmark report. The Confrontation Clause is designed to protect against introduction of out-of-court statements accusing a specific person of criminal conduct by use of formalized statements such as affidavits, depositions, or prior testimony. In this case, the Cellmark report was not compiled as evidence against Williams, but to obtain information that would assist in the apprehension of an unidentified rapist who was still at large. Because Cellmark did not know the identity of the person it was profiling, it could not fabricate a pre-ordained conclusion.

Justice Thomas concurred in the judgment but rejected the plurality's reasoning, concluding that the testimony about Cellmark was offered for its truth. He reasoned instead that the expert's statements were not covered by the Confrontation Clause because the clause applies only to statements bearing "indicia of solemnity" such as depositions and affidavits, not to documents such as the Cellmark report.

Justice Alito delivered the Court's judgment in an opinion joined by Chief Justice Roberts and Justices Kennedy and Breyer.  Justice Breyer filed a concurring opinion.  Justice Thomas concurred in the judgment. Justice Kagan dissented in an opinion joined by Justices Scalia, Ginsburg, and Sotomayor.

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