opinion: KENTUCKY v. STINCER
  No. 86-572.
  Argued April 22, 1987
  Decided June 19, 1987
  Blackmun, J., delivered the opinion of the Court, in which Rehnquist, C. J., and White, Powell, O’ConnoR, and Scalia, JJ., joined. Marshall, J., filed a dissenting opinion, in which BRennan and Stevens, JJ., joined, post, p. 748.
  Penny R. Warren, Assistant Attorney General of Kentucky, argued the cause for petitioner. With her on the briefs were David L. Armstrong, Attorney General, and John S. Gillig, Assistant Attorney General.
  Mark A. Posnansky, by appointment of the Court, 479 U. S. 1005, argued the cause and filed a brief for respondent.
  Briefs of amici curiae urging reversal were filed for the State of Arkansas et al. by Steve Clark, Attorney General of Arkansas, and Rodney A. Smolla, joined by the Attorneys General for their respective jurisdictions as follows: Don Siegelman of Alabama, Charles M. Oberly III of Delaware, Robert A. Butterworth of Florida, Charles Troutman of Guam, Jim Jones of Idaho, Linley E. Pearson of Indiana, Neil F. Hartigan of Illinois, Robert T. Stephan of Kansas, William J. Guste, Jr., of Louisiana, Frank J. Kelley of Michigan, Hubert H. Humphrey III of Minnesota, Edwin L. Pittman of Mississippi, William L. Webster of Missouri, Mike Greely of Montana, Brian McKay of Nevada, Stephen E. Merrill of New Hampshire, W. Cary Edwards of New Jersey, Hal Stratton of New Mexico, Lacy H. Thorn-burg of North Carolina, Nicholas Spaeth of North Dakota, Dave Frohn-mayer of Oregon, LeRoy S. Zimmerman of Pennsylvania, James E. O’Neil of Rhode Island, T. Travis Medlock of South Carolina, Roger A. Tellinghuisen of South Dakota, W. J. Michael Cody of Tennessee, Jeffrey Amestoy of Vermont, J’Ada Finch-Sheen of the Virgin Islands, Charles G. Brown of West Virginia, and Donald J. Hanaway of Wisconsin; and for the Appellate Committee of the California District Attorneys Association by Ira Reiner and Harry B. Sondheim.
  Briefs of amicus curiae urging affirmance were filed for the American Civil Liberties Union by George Kannar; and for the National Association of Criminal Defense Lawyers by Nancy Hollander.
  Donald N. Bersoff filed a brief for the American Psychological Association as amicus curiae.
  Justice Blackmun
  delivered the opinion of the Court.
  The question presented in this case is whether the exclusion of a defendant from a hearing held to determine the competency of two child witnesses to testify violates the defendant’s rights under the Confrontation Clause of the Sixth Amendment or the Due Process Clause of the Fourteenth Amendment.
  I
  Respondent Sergio Stincer was indicted in the Circuit Court of Christian County, Ky., and charged with committing first-degree sodomy with T. G., an 8-year-old girl, N. G., a 7-year-old girl, and B. H., a 5-year-old boy, in violation of Ky. Rev. Stat. §510.070 (1985). After a jury was sworn, but before the presentation of evidence, the court conducted an in-chambers hearing to determine if the two young girls were competent to testify. Over his objection, respondent, but not his counsel (a public defender), was excluded from this hearing. Tr. 15.
  The two children were examined separately and the judge, the prosecutor, and respondent’s counsel asked questions of each girl to determine if she were capable of remembering basic facts and of distinguishing between telling the truth and telling a lie. Id., at 15-26. T. G., the 8-year-old, was asked her age, her date of birth, the name of her school, the names of her teachers, and the name of her Sunday school. She was also asked whether she knew what it meant to tell the truth, and whether she could keep a promise to God to tell the truth. Id., at 16-18. N. G., the 7-year-old girl, was asked similar questions. Id., at 20-25. The two children were not asked about the substance of the testimony they were to give at trial. The court ruled that the girls were competent to testify. Respondent’s counsel did not object to these rulings. Id., at 20, 25.
  Before each of the girls began her substantive testimony in open court, the prosecutor repeated some of the basic questions regarding the girl’s background that had been asked at the competency hearing. Id., at 31-33 (direct examination of T. G.) (questions regarding age, where the witness attended school and Sunday school, and the like); id., at 66 (direct examination of N. G.) (questions regarding age and where the witness attended school). T. G. then testified, on direct examination, that respondent had placed a sock over her eyes, had given her chocolate pudding to eat, and then had “put his d-i-c-k” in her mouth. Id., at 34. N. G., on direct examination, testified to a similar incident. Id., at 69.
  On cross-examination, respondent’s counsel asked each girl questions designed to determine if she could remember past events and if she knew the difference between the truth and a lie. Some of these questions were similar to those that had been asked at the competency hearing. See id., at 38-39, 44-47, 60-63 (cross-examination of T. G.); 71-72, 74-75, 78-83 (cross-examination of N. G.). After the testimony of the girls was concluded, counsel did not request that the trial court reconsider its ruling that the girls were competent to testify. The jury convicted respondent of first-degree sodomy for engaging in deviate sexual intercourse and fixed his sentence at 20 years’ imprisonment.
  On appeal to the Supreme Court of Kentucky, respondent argued, among other things, that his exclusion from the competency hearing of the two girls denied him due process and violated his Sixth Amendment right to confront the witnesses against him. The Kentucky Supreme Court, by a divided vote, agreed that, under the Sixth Amendment of the Federal Constitution and under § 11 of the Bill of Rights of the Kentucky Constitution (the right “to meet the witnesses face to face”), respondent had an absolute right to be present at the competency hearing because the hearing “was a crucial phase of the trial.” 712 S. W. 2d 939, 940 (1986). The court explained that respondent’s trial “might not have taken place had the trial court determined that the children were not competent to testify.” Id., at 941. Two justices, however, dissented, concluding that respondent’s right to confront the witnesses against him was not violated because respondent had the opportunity to assist counsel fully in cross-examining the two witnesses at trial. Id., at 942-944.
  We granted certiorari, 479 U. S. 1005 (1986), to determine whether respondent’s constitutional rights were violated by his exclusion from the competency hearing.
  HH HH
  A
  The Sixth Amendment’s Confrontation Clause provides: “In all criminal prosecutions, the accused shall enjoy the right... to be confronted with the witnesses against him.” This right is secured for defendants in state as well as in federal criminal proceedings. Pointer v. Texas, 380 U. S. 400 (1965). The Court has emphasized that “a primary interest secured by [the Confrontation Clause] is the right of cross-examination.” Douglas v. Alabama, 380 U. S. 415, 418 (1965). The opportunity for cross-examination, protected by the Confrontation Clause, is critical for ensuring the integrity of the factfinding process. Cross-examination is “the principal means by which the believability of a witness and the truth of his testimony are tested.” Davis v. Alaska, 415 U. S. 308, 316 (1974). Indeed, the Court has recognized that cross-examination is the “ ‘greatest legal engine ever invented for the discovery of truth.’” California v. Green, 399 U. S. 149, 158 (1970), quoting 5 J. Wigmore, Evidence § 1367, p. 29 (3d ed. 1940). The usefulness of cross-examination was emphasized by this Court in an early case explicating the Confrontation Clause:
  “The primary object of the constitutional provision in question was to prevent depositions or ex parte affidavits . . . being used against the prisoner in lieu of a personal examination and cross-examination of the witness in which the accused has an opportunity, not only of testing the recollection and sifting the conscience of the witness, but of compelling him to stand face to face with the jury in order that they may look at him, and judge by his demeanor upon the stand and the manner in which he gives his testimony whether he is worthy of belief.” Mattox v. United States, 156 U. S. 237, 242-243 (1895).
  See also Kirby v. United States, 174 U. S. 47, 53 (1899).
  The right to cross-examination, protected by the Confrontation Clause, thus is essentially a “functional” right designed to promote reliability in the truth-finding functions of a criminal trial. The cases that have arisen under the Confrontation Clause reflect the application of this functional right. These cases fall into two broad, albeit not exclusive, categories: “cases involving the admission of out-of-court statements and cases involving restrictions imposed by law or by the trial court on the scope of cross-examination.” Delaware v. Fensterer, 474 U. S. 15, 18 (1985) (per curiam).
  In the first category of cases, the Confrontation Clause is violated when “hearsay evidence [is] admitted as substantive evidence against the defendant],” Tennessee v. Street, 471 U. S. 409, 413 (1985), with no opportunity to cross-examine the hearsay declarant at trial, or when an out-of-court statement of an unavailable witness does not bear adequate indications of trustworthiness. See Ohio v. Roberts, 448 U. S. 56, 65-66 (1980). For example, in Roberts, we held that an out-of-court statement by an unavailable witness was sufficiently reliable to be admitted at trial, consistent with the Confrontation Clause, because defense counsel had engaged in full cross-examination of the witness at the preliminary hearing where the statement was made. Id., at 70-73. In California v. Green, supra, the Court concluded that the Confrontation Clause was not violated by admitting a declar-ant’s inconsistent out-of-court statement “as long as the de-clarant is testifying as a witness and subject to full and effective cross-examination” at the trial itself. 399 U. S., at 158.
  The second category involves cases in which the opportunity for cross-examination has been restricted by law or by a trial court ruling. In Davis v. Alaska, supra, defense counsel was restricted by state confidentiality provisions from questioning a witness about his juvenile criminal record, although such evidence might have affected the witness’ credibility. The Court held that the Confrontation Clause was violated because the defendant was denied the right “to expose to the jury the facts from which jurors . . . could appropriately draw inferences relating to the reliability of the witness.” 415 U. S., at 318. Similarly, in Delaware v. Van Arsdall, 475 U. S. 673 (1986), defense counsel was precluded by the trial court from questioning a witness about the State’s dismissal of a pending public drunkenness charge against him. The Court concluded: “By thus cutting off all questioning about an event. . . that a jury might reasonably have found furnished the witness a motive for favoring the prosecution in his testimony,” the trial court’s ruling violated the defendant’s rights under the Confrontation Clause. Id., at 679.
  Although claims arising under the Confrontation Clause may not always fall neatly into one of these two categories, these cases reflect the Confrontation Clause’s functional purpose in ensuring a defendant an opportunity for cross-examination. See Lee v. Illinois, 476 U. S. 530 (1986). Of course, the Confrontation Clause guarantees only “an opportunity for effective cross-examination, not cross-examination that is effective in whatever way, and to whatever extent, the defense might wish.” Delaware v. Fensterer, 474 U. S., at 20 (emphasis in original). This limitation is consistent with the concept that the right to confrontation is a functional one for the purpose of promoting reliability in a criminal trial.
  B
  The Commonwealth argues that respondent’s exclusion from the competency hearing of the two children did not violate the Confrontation Clause because a competency hearing is not “a stage of trial where evidence or witnesses are being presented to the trier of fact.” Brief for Petitioner 22. Cf. Gannett Co. v. DePasquale, 443 U. S. 368, 394 (1979) (Burger, C. J., concurring). Distinguishing between a “trial” and a “pretrial proceeding” is not particularly helpful here, however, because a competency hearing may well be a “stage of trial.” In this case, for instance, the competency hearing was held after the jury was sworn, in the judge’s chambers, and in the presence of opposing counsel who asked questions of the witnesses. Moreover, although questions regarding the guilt or innocence of the defendant usually are not asked at a competency hearing, the hearing retains a direct relationship with the trial because it determines whether a key witness will testify. Further, although the preliminary determination of a witness’ competency to testify is made at this hearing, the determination of competency is an ongoing one for the judge to make based on the witness’ actual testimony at trial.
  Instead of attempting to characterize a competency hearing as a trial or pretrial proceeding, it is more useful to consider whether excluding the defendant from the hearing interferes with his opportunity for effective cross-examination. No such interference occurred when respondent was excluded from the competency hearing of the two young girls in this case. After the trial court determined that the two children were competent to testify, they appeared and testified in open court. At that point, the two witnesses were subject to full and complete cross-examination, and were so examined. Tr. 38-58 (cross-examination of T. G.); id., at 71-84 (cross-examination of N. G.). Respondent was present throughout this cross-examination and was available to assist his counsel as necessary. There was no Kentucky rule of law, nor any ruling 'by the trial court, that restricted respondent’s ability to cross-examine the witnesses at trial. Any questions asked during the competency hearing, which respondent’s counsel attended and in which he participated, could have been repeated during direct examination and cross-examination of the “witnesses in respondent’s presence. See California v. Green, 399 U. S., at 159 (“[T]he inability to cross-examine the witness at the time he made his prior statement cannot easily be shown to be of crucial significance as long as the defendant is assured of full and effective cross-examination at the time of trial”).
  Moreover, the type of questions that were asked at the competency hearing in this case were easy to repeat on cross-examination at trial. Under Kentucky law, when a child’s competency to testify is raised, the judge is required to resolve three basic issues: whether the child is capable of observing and recollecting facts, whether the child is capable of narrating those facts to a court or jury, and whether the child has a moral sense of the obligation to tell the truth. See Moore v. Commonwealth, 384 S. W. 2d 498, 500 (Ky. 1964) (“When the competency of an infant to testify is properly raised it is then the duty of the trial court to carefully examine the witness to ascertain whether she (or he) is sufficiently intelligent to observe, recollect and narrate the facts and has a moral sense of obligation to speak the truth”); Capps v. Commonwealth, 560 S. W. 2d 559, 560 (Ky. 1977); Hendricks v. Commonwealth, 550 S. W. 2d 551, 554 (Ky. 1977); see also Thomas v. Commonwealth, 300 Ky. 480, 481-482, 189 S. W. 2d 686, 686-687 (1945); Comment, An Overview of the Competency of Child Testimony, 13 No. Ky. L. Rev. 181, 184 (1986). Thus, questions at a competency hearing usually are limited to matters that are unrelated to the basic issues of the trial. Children often are asked their names, where they go to school, how old they are, whether they know who the judge is, whether they know what a lie is, and whether they know what happens when one tells a lie. See Comment, The Competency Requirement for the Child Victim of Sexual Abuse: Must We Abandon It?, 40 U. Miami L. Rev. 245, 263, and n. 78 (1985); Comment, Defendants’ Rights in Child Witness Competency Hearings: Establishing Constitutional Procedures for Sexual Abuse Cases, 69 Minn. L. Rev. 1377, 1381-1383, and nn. 9-11 (1985).
  In Kentucky, as in certain other States, it is the responsibility of the judge, not the jury, to decide whether a witness is competent to testify based on the witness’ answers to such questions. Whitehead v. Stith, 268 Ky. 703, 709, 105 S. W. 2d 834, 837 (1937) (question of competency is one for court, not jury, and if court finds witness lacks qualification, “it commits a palpable abuse of its discretion” should it then permit witness to testify); Payne v. Commonwealth, 623 S. W. 2d 867, 878 (Ky. 1981); Capps v. Commonwealth, 560 S. W. 2d, at 560. See 2 Wigmore § 507, p. 714 (citing cases). In those States where the judge has the responsibility for determining competency, that responsibility usually continues throughout the trial. A motion by defense counsel that the court reconsider its earlier decision that a child is competent may be raised after the child testifies on direct examination, see, e. g., In re R. R., 79 N. J. 97, 106, 398 A. 2d 76, 80 (1979) (at close of State’s case, defense attorney moved that 4-year-old boy be declared incompetent on basis of actual testimony given by boy), or after direct and cross-examination of the witness. See, e. g., Reply Brief for Petitioner 12 (“If, during trial, there arises some basis for challenging the judge’s competency determination, the judge may be asked to reconsider,” referring to respondent’s motion to that effect, Tr. 126-127). Moreover, appellate courts reviewing a trial judge’s determination of competency also often will look at the full testimony at trial.
  In this case both T. G. and N. G. were asked several background questions during the competency hearing, as well as several questions directed at what it meant to tell the truth. Some of the questions regarding the witnesses’ backgrounds were repeated by the prosecutor on direct examination, while others — particularly those regarding the witnesses’ ability to tell the difference between truth and falsehood — were repeated by respondent’s counsel on cross-examination. At the close of the children’s testimony, respondent’s counsel, had he thought it appropriate, was in a position to move that the court reconsider its competency rulings on the ground that the direct and cross-examination had elicited evidence that the young girls lacked the basic requisites for serving as competent witnesses. Thus, the critical tool of cross-examination was available to counsel as a means of establishing that the witnesses were not competent to testify, as well as a means of undermining the credibility of their testimony.
  Because respondent had the opportunity for full and effective cross-examination of the two witnesses during trial, and because of the nature of the competency hearing at issue in this case, we conclude that respondent’s rights under the Confrontation Clause were not violated by his exclusion from the competency hearing of the two girls.
  h-1 hH h — I
  Respondent argues that his rights under the Due Process ■Clause of the Fourteenth Amendment were violated by his exclusion from the competency hearing. The Court has assumed that, even in situations where the defendant is not actually confronting witnesses or evidence against him, he has a due process right “to be present in his own person whenever his presence has a relation, reasonably substantial, to the fulness of his opportunity to defend against the charge.” Snyder v. Massachusetts, 291 U. S. 97, 105-106 (1934). Although the Court has emphasized that this privilege of presence is not guaranteed “when presence would be useless, or the benefit but a shadow,” id., at 106-107, due process clearly requires that a defendant be allowed to be present “to the extent that a fair and just hearing would be thwarted by his absence,” id., at 108. Thus, a defendant is guaranteed the right to be present at any stage of the criminal proceeding that is critical to its outcome if his presence would contribute to the fairness of the procedure.
  We conclude that respondent’s due process rights were not violated by his exclusion from the competency hearing in this case. We emphasize, again, the particular nature of the competency hearing. No question regarding the substantive testimony that the two girls would have given during trial was asked at that hearing. All the questions, instead, were directed solely to each child’s ability to recollect and narrate facts, to her ability to distinguish between truth and falsehood, and to her sense of moral obligation to tell the truth. Thus, although a competency hearing in which a witness is asked to discuss upcoming substantive testimony might bear a substantial relationship to a defendant’s opportunity better to defend himself at trial, that kind of inquiry is not before us in this case.
  Respondent has given no indication that his presence at the competency hearing in this case would have been useful in ensuring a more reliable determination as to whether the witnesses were competent to testify. He has presented no evidence that his relationship with the children, or his knowledge of facts regarding their background, could have assisted either his counsel or the judge in asking questions that would have resulted in a more assured determination of competency. On the record of this case, therefore, we cannot say that respondent’s rights under the Due Process Clause of the Fourteenth Amendment were violated by his exclusion from the competency hearing. As was said in United States v. Gagnon, 470 U. S. 522, 527 (1985) (per curiam), there is no indication that respondent “could have done [anything] had [he] been at the [hearing] nor would [he] have gained anything by attending.”
  The judgment of the Supreme Court of Kentucky is reversed.
  It is so ordered.
  Immediately prior to the competency hearing of the two girls, the prosecutor moved that the charge regarding B. H., the 5-year-old boy, be dismissed because the prosecution did not believe B. H. was competent to testify. Respondent did not object and the court granted the prosecutor’s motion. Tr. 13-14.
  In response to these questions, T. G. stated that telling the truth meant “[d]on’t tell no stories.” Id., at 17.
  N. G. replied that she would “get a whopping” if she told a lie. Id., at 24.
  There is some confusion as to whether T. G. knew what a “d-i-c-k” was, although she spelled the word at trial. Id., at 55-58. It also appears that N. G. may have recanted her testimony somewhat on cross-examination. Id., at 77-78. These facts, however, relate to whether the evidence was sufficient to convict respondent of the crimes charged. The Kentucky Supreme Court concluded that the evidence was sufficient to withstand a motion for a directed verdict of acquittal. 712 S. W. 2d 939, 941 (1986). That ruling is not before us in this case.
  After the two girls testified, the prosecution stated that it also wished to present the testimony of E. T., a 4-year-old boy who allegedly had witnessed the events in question. The court examined E. T. in the courtroom, without the jury present and, apparently, without respondent present. Tr. 87. No objection from respondent regarding his exclusion from this hearing appears on the record. The court ruled that the boy was competent to testify, a ruling to which respondent’s counsel apparently objected. Id., at 109-110. After direct and cross-examination of E. T., defense counsel moved that the court reconsider its previous ruling that the boy was competent to testify. The court declined to rule that he was incompetent. Id., at 126-127.
  Respondent’s exclusion from E. T.’s competency hearing is not before us because the validity of respondent’s absence from that hearing was never raised before the Kentucky Supreme Court. See Brief for Appellant in No. 84-SC-496-I (Ky. Sup. Ct.), pp. 14-17. Thus, not surprisingly, the majority opinion of the Kentucky Supreme Court refers solely to the competency hearing of the two girls.
  Under Kentucky law, deviate sexual intercourse means “any act of sexual gratification between persons not married to each other involving the sex organs of one person and the mouth or anus of another.” Ky. Rev. Stat. § 510.010(1) (1985). First-degree sodomy with a child under 12 is a Class A felony and conviction carries a minimum sentence of 20 years’ imprisonment and a maximum sentence of life imprisonment. §§ 510.070(2) and 532.060.
  As an initial matter, respondent asks us to vacate our grant of certio-rari because, in his view, the decision of the Kentucky Supreme Court rests on ‘“separate, adequate, and independent grounds.’” Brief for Respondent 50, quoting Michigan v. Long, 463 U. S. 1032, 1041 (1983). We decline to do so. In Michigan v. Long, we explained that “when ... a state court decision fairly appears ... to be interwoven with the federal law, and when the adequacy and independence of any possible state law ground is not clear from the face of the opinion,” we shall assume that the state court believed that federal law compelled its conclusion. Id., at 1040-1041. In this case, the Kentucky Supreme Court consistently referred to respondent’s rights under the Sixth Amendment to the Federal Constitution as supporting its ruling. The court gave no indication that respondent’s rights under § 11 of the Bill of Rights of the Kentucky Constitution were distinct from, or broader than, respondent’s rights under the Sixth Amendment.
  One noted commentator has pointed out that the main purpose of confrontation “is to secure for the opponent the opportunity of cross-examination” (emphasis omitted), 5 J. Wigmore, Evidence § 1395, p. 150 (Chadbourn rev. 1974) (Wigmore), with an additional advantage being that “the judge and the jury are enabled to obtain the elusive and incommunicable evidence of a witness’ deportment while testifying” (emphasis omitted). Id., at 153.
  The Court sometimes has referred to a defendant’s right of confrontation as a “trial right.” See Barber v. Page, 390 U. S. 719, 725 (1968); see also California v. Green, 399 U. S. 149, 157 (1970) (“right to ‘confront’ the witness at the time of trial”). In Pennsylvania v. Ritchie, 480 U. S. 39 (1987), a plurality of the Court interpreted the Clause to mean that the right of confrontation is designed simply “to prevent improper restrictions on the types of questions that defense counsel may ask during cross-examination.” Id,., at 52. Thus, the plurality in Ritchie concluded that the constitutional error in Davis v. Alaska, 415 U. S. 308 (1974), was not that state law made certain juvenile criminal records confidential, but rather that the defense attorney had been precluded from asking questions about that criminal record at trial. 480 U. S., at 54. The personal view of the author of this opinion as to the Confrontation Clause is somewhat broader than that of the Ritchie plurality. Although he believes that “[tjhere are cases, perhaps most of them, where simple questioning of a witness will satisfy the purposes of cross-examination,” id., at 62 (Black-mun, J., concurring), he also believes that there are cases in which a state rule that precludes a defendant from access to information before trial may hinder that defendant’s opportunity for effective cross-examination at trial, and thus that such a rule equally may violate the Confrontation Clause. Id., at 63-65.
  His differences with the plurality in Ritchie, however, are not implicated in this case. As is demonstrated below, respondent’s ability to engage in full cross-examination at trial was not affected by his exclusion from the competency hearing, nor was his opportunity to engage in effective cross-examination interfered with by his exclusion. Thus, under either the author’s view or that of the plurality in Ritchie, there was no Confrontation Clause violation in this ease.
  Indeed, a competency hearing may take place in the middle of a trial, as did the hearing of E. T. See n. 6, supra.
  Similar requirements for establishing competency to testify were set forth in Wheeler v. United States, 159 U. S. 523 (1895): “[T]here is no precise age which determines the question of competency. This depends on the capacity and intelligence of the child, his appreciation of the difference between truth and falsehood, as well as of his duty to tell the former.” Id., at 524. See generally 2 Wigmore §§ 505-507.
  Some States explicitly allow children to testify without requiring a prior competency qualification, while others simply provide that all persons, including children, are deemed competent unless otherwise limited by statute. See B. Battman & J. Bulkley, National Legal Resource Center for Child Advocacy and Protection, Protecting Child Victim/Witnesses: Sample Laws and Materials 43-44 (1986) (listing statutes) (Protecting Child Victim/Witnesses); Bulkley, Evidentiary and Procedural Trends in State Legislation and Other Emerging Legal Issues in Child Sexual Abuse Cases, 89 Dick. L. Rev. 645, 645 (1985). Some commentators have urged that children be allowed to testify without undergoing a prior competency qualification. See Protecting Child Victim/Witnesses, at 38 (proposing sample competency statute according children same rebuttable presumption of competency granted other witnesses); 2 Wigmore § 509, p. 719 (“it must be concluded that the sensible way is to put the child upon the stand to give testimony for what it may seem to be worth”).
  A number of States, however, mandate by statute that a trial judge assess a child’s competency to testify on the basis of specified requirements. These usually include a determination that the child is capable of expression, is capable of understanding the duty to tell the truth, and is capable of receiving just impressions of the facts about which he or she is called to testify. See, e. g., Ariz. Rev. Stat. Ann. § 12-2202 (1982); Ga. Code Ann. §24-9-5 (1982); Idaho Code §9-202 (Supp. 1987); Ind. Code §34-1-14-5 (1986); Mich. Comp. Laws §600.2163 (1986); Minn. Stat. §595.02.Subd. 1(f) (Supp. 1987); N. Y. Crim. Proc. Law § 60.20 (McKinney 1981); Ohio Rev. Code Ann. §2317.01 (1981); see Protecting Child Victim/Witnesses, at 45 (listing statutes). The recent reforms in some States of presuming the competency of young children and allowing juries to assess credibility at trial is not called into question by this opinion. We are concerned solely with those States that retain competency qualification requirements.
  See, e. g., Litzkuhn v. Clark, 85 Ariz. 355, 360, 339 P. 2d 389, 392 (1959) (“[I]t is the duty of the trial judge who has permitted a child to be sworn as a witness, at any time to change his mind upon due occasion therefor, to remove the child from the stand and to instruct the jury to disregard his testimony”); Davis v. Weber, 93 Ariz. 312, 317, 380 P. 2d 608, 611 (1963) (“The right of a trial judge to change his mind [regarding a child’s competency] can hardly be denied”).
  California recently amended its statute governing the disqualification of incompetent witnesses to provide explicitly: “In any proceeding held outside the presence of a jury, a court may reserve challenges to the competency of a witness until the conclusion of the direct examination of that witness.” Cal. Evid. Code Ann. § 701(b) (West Supp. 1987).
  See, e. g., Payne v. Commonwealth, 623 S. W. 2d 867, 878 (Ky. 1981) (review of children’s testimony at trial reveals that trial court’s ruling of competency was appropriate); Hendricks v. Commonwealth, 550 S. W. 2d 551, 554 (Ky. 1977) (“Not only did the trial judge determine that the children were competent to testify, but the transcript of the testimony of these children clearly demonstrates their intellectual ability to observe, recollect and narrate the facts and to recognize their moral obligation to tell the truth”); see also In re R. R., 79 N. J. 97, 113, 398 A. 2d 76, 84 (1979) (“[I]n determining the propriety of the trial judge’s determination, an appellate court need not limit its view to the responses given by the witness during the voir dire examination; instead, it can consider the entire record— including the testimony in fact given by the witness under oath — in order to arrive at its decision”).
  Respondent’s counsel, in fact, did move for reconsideration of the court’s ruling on the competency of E. T. after that young boy had testified and had been subjected to cross-examination. See n. 5, supra.
  We note once again that the Kentucky Supreme Court held that respondent’s confrontation rights were violated because the competency hearing was a “crucial phase of the trial.” 712 S. W. 2d, at 940. It is true that the hearing was crucial in the sense that respondent may not have been convicted had the two girls been found incompetent to testify. Nevertheless, the question whether a particular proceeding is critical to the outcome of a trial is not the proper inquiry in determining whether the Confrontation Clause has been violated. The appropriate question is whether there has been any interference with the defendant’s opportunity for effective cross-examination. No such interference occurred in this case. Of course, the fact that a stage in the proceeding is critical to the outcome of a trial may be relevant to due process concerns. Even in that context, however, the question is not simply whether, “but for” the outcome of the proceeding, the defendant would have avoided conviction, but whether the defendant’s presence at the proceeding would have contributed to the defendant’s opportunity to defend himself against the charges. See infra, Part III.
  Although respondent perhaps could have been more artful in presenting his due process claim to the Kentucky Supreme Court as clearly founded on the Fourteenth Amendment, he did raise a due process claim to that court, see Brief for Appellant in No. 84-SC-496-I (Ky. Sup. Ct.), pp. 14-17, and the claim therefore is properly before us.
  During the competency hearing of E. T., the judge, the prosecutor, and respondent’s counsel asked the boy several questions regarding the substance of his testimony. Tr. 91-101. As noted above, however, see n. 6, supra, respondent’s exclusion from E. T.’s competency hearing is not before us.
  Counsel for the Commonwealth acknowledged that if a competency hearing “were to exceed its normal scope,” that would “begi[n] to bear a substantial relation to [a defendant’s] opportunity to defend.” Tr. of Oral Arg. 9; see also State v. Howard, 57 Ohio App. 2d 1, 4-5, 385 N. E. 2d 308, 312-313 (1978) (defendant’s presence bears reasonably substantial relation to defense when witnesses give testimony in in-camera hearing identifying defendant as assailant). Although, as noted above, most competency hearings do not focus on substantive testimony, it is not impossible that questions related to substantive testimony could be asked. See Comment, Defendants’ Rights in Child Witness Competency Hearings: Establishing Constitutional Procedures for Sexual Abuse Cases, 69 Minn. L. Rev. 1377, 1384 (1985) (emphasis on testing a child’s memory “suggests that a judge may inquire about the actual sexual assault”); see n. 19, supra. But see Moll v. State, 351 N. W. 2d 639, 643 (Minn. App. 1984) (“[T]he trial court has broad discretion as to the type of question to be put to the child during this preliminary examination, but should not elicit from the child the anticipated testimony concerning the alleged offense, recognizing the suggestibility of young children”).
  Where the competency hearing bears a substantial relationship to the defendant’s opportunity to defend, a court must then balance the defendant’s role in assisting in his defense against the risk of identifiable and substantial injury to the specific child witness. See Brief for American Psychological Association as Amicus Curiae 15-26 (noting that intuitive view that child victims of sexual abuse are particularly vulnerable in legal proceedings may not be correct for all children).
  Contrary to the dissent’s charge, see post, at 754, we do not address the question whether harmless-error analysis applies in the situation where a defendant is excluded from a critical stage of the proceedings in which his presence would contribute to the fairness of the proceeding. In this case, respondent simply has failed to establish that his presence at the competency hearing would have contributed to the fairness of the proceeding. He thus fails to establish, as an initial matter, the presence of a constitutional deprivation.
  Respondent also argues that his Sixth Amendment right to effective assistance of counsel was violated by his inability to consult with counsel during the competency hearing. Brief for Respondent 28-32. Respondent acknowledges that this argument was not raised below, id., at 28, n. 25, but he argues that, as the prevailing party, he may assert any ground in support of his judgment. See Dandridge v. Williams, 397 U. S. 471, 475, n. 6 (1970). The Court has noted, however, that it is “the settled practice of this Court, in the exercise of its appellate jurisdiction, that it is only in exceptional cases, and then only in eases coming from the federal courts, that it considers questions urged by a petitioner or appellant not pressed or passed upon in the courts below.” McGoldrick v. Compagnie Generale Transatlantique, 309 U. S. 430, 434 (1940); see also Heckler v. Campbell, 461 U. S. 458, 468-469, n. 12 (1983) (Court will consider ground not presented to federal court below only “in exceptional cases”). Because the judgment here is that of a state court, and because we do not believe that respondent’s claim of deprival of the effective assistance of counsel qualifies as an exceptional case, we decline to review this claim.
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