Aрpellant McMorris was convicted of strong-armed robbery in Wisconsin state court. He now appeals from the denial of his petition for federal habeas corpus relief. 28 U.S.C. § 2254 (1976). McMorris challenges the Wisconsin procedure for the admission of polygraph evidence and alleges that the exclusion of exculpatory polygraph evidence denied him a fair trial in violation of his rights under the United States Constitution. We reverse and remand for further proceedings.
At petitioner’s trial in state court, the prosecution presented only one witness, the victim, who identified the petitioner as the pеrson who had grabbed him from behind, taken his wallet and pushed him to the ground. On cross-examination the victim conceded that it was dark at the time of the attack and that the whole incident was over very quickly. The victim also conceded that he had never seen the defendant before and that he was unable to give a description of his assailant. The petitioner took the stand and denied any involvement in the crime, claiming that the victim was mistaken in his identification of the petitioner as his assailant.
Realizing the importance of credibility in such a case, the petitioner sought to take a polygraph examination prior to trial. Evidence of a polygraph test taken by a defendant is admissible in Wisconsin under the conditions set forth in
State v. Stanislawski,
(1) That the district attorney, defendant and his counsel all sign a written stipulation providing for defendant’s submission to the test and for the subsequent admission at trial of the graphs, and the examiner’s opinion thereon on behalf of either defendant or the state.
(2) That notwithstanding the stipulation the admissibility of the test results is subject to the discretion оf the trial court, i. e., if the trial judge is not convinced that the examiner is qualified or that the test was conducted under proper conditions he may refuse to accept such evidence.
(3) That if the graphs and examiner’s opinion are offered in evidence the opposing party shall have the right to cross-examine the examiner respecting:
(a) the examiner’s qualifications and training;
(b) the conditions under which the test was administered;
*460 (c) the limitations of and possibilities for error in the technique of polygraphic interrogation; and
(d) at the discretion of the trial court, any other matters deemed pertinent to the inquiry.
(4) That if such evidence is admitted the trial judge should instruct the jury that the examiner’s testimony does not tend to prove or disprovе any element of the crime with which a defendant is charged but at most tends only to indicate whether at the time of the examination defendant was telling the truth. Further, the jury members should be instructed that it is for them to determine what corroborative weight and effect such testimony should be given.
In order to comply with Stanislawski, trial counsel for the petitioner contacted the prosecutor seeking a Stanislawski stipulation. After some initial indecision, the prosecutor refused to enter into the stipulation but gave no reason for his refusal. The request was renewed shortly before trial but the prosecutor again refused without explanation.
After petitionеr was convicted, he filed a post-conviction motion in state court challenging the exclusion of the polygraph testimony. A hearing was held before the trial judge at which evidence was introduced as to a polygraph examination taken by petitioner after he was convicted. The results of the polygraph examination supported petitioner’s claim that he had not committed the robbery. The trial court concluded that the polygraph evidence was both reliable and probative, “an important piece of evidence for the jury to consider and weigh in arriving at its verdict” in light of the evidence received at trial. The trial court reluctantly denied any relief, however, on the basis of Wisconsin law governing the admission of polygraph evidence. The Wisconsin Supreme Court affirmed in an unpublished per curiam opinion. 2
The defendant then filed for habeas corpus relief in federal court. The district court denied relief because it found that the Wisconsin stipulation requirement did not deny the defendant due process of law. A motion for reconsideration was also denied and the petitioner now appeals from these rulings.
I. The Constraints Imposed by the Due Process Clause
Every state is generally free to exercise its sovеreign prerogative as to the evidence it will admit in its courts. The federal constitution, however, imposes a limited restraint upon state evidentiary rules where exculpatory evidence is excluded by arbitrary state rules.
Washington v. Texas,
A. The Exculpatory Significance of the Defendant’s Polygraph Evidence
It seems obvious that evidence of a favorable polygraph examination taken by the defendant would have been of great importance to the defense of the Wisconsin charge. The entire prosecution case rested on the testimony of the victim as to the identity of his assailant. Given the problems inherent in such an identification, see O’Connor, "That’s the Man”: A Sobering Study of Eyewitness Identification and the Polygraph, 49 St. John’s L.Rev. 1, 26-27 (1974), particularly an identification based upon a short observation in the dark while being knocked to the ground, the state’s case was far from overwhelming. The defendant took the stand and denied any involvement in the crime. His credibility was the primary issue before the jury, and the jury’s verdict reflects its decision not to credit his testimony. Under these circumstances, a polygraph examination that bolstered the defendant’s credibility might easily have been decisive in securing an acquittal. Although we do not wish to become еmbroiled in a “numbers game”, we note that even the most ardent detractors from the validity of polygraph evidence concede *462 a degree of reliability of 70% or higher for properly administered examinations. 7 Further, a large portion of this 30% “error” rate includes tests that yield inconclusive results — results that cannot fairly be described as “erroneous.” 8 Where credibility is as critical as in the instant case, the circumstances are such as to make the polygraph evidence materially exculpatory within the meaning of the Constitution. See Westen, Compulsory Process II, 74 U.Mich.L.Rev. 191, 197 (1975).
II. The Available State Justifications for Stanislawski.
State and federal evidentiary rules excluding polygraph evidence have usually justified that exclusion by reference to two problems with polygraph evidence. Courts have been concerned with the reliability of the evidence itself as well as with the potential that juries may improperly regard polygraph evidence as infallible.
9
United States v. Alexander,
polygraph tests have moved from the ‘twilight zone’ of Frye to such degree of standing and scientific recognition that unconditional rejection of expert testimony based on polygraph testimony is no longer indicated.
Stanislawski,
A by-product of Wisconsin’s liberal attitude regarding polygraph evidence, how *463 ever, is that Wisconsin is unable to assert many of the traditional objections to polygraph evidence to sustain the two aspects of the Stanislawski stipulation requirement under review here. Other justifications have been argued, however, and we shall now consider the separate justifications asserted for each portion of the stipulation requirement.
A. State Justifications for the Stipulation Requirement
The state justifies the requirement that the district attorney sign a written stipulation to the admission of the results of a polygraph test on the theory that a pre-test stipulation
13
increases the reliability of the test itself. Without a pre-test stipulation as to admissibility, the defendant could continue to take successive tests until one finally produced favorable results. The defendant could also take each test secure in the knowledge that the test results would neither be disclosed nor be used against him should they not turn out to be favorable. “This sense of security diminishes the fear of discovered deception, upon which an effective examination depends.”
United States v. Wilson,
Defendant, however, challenges the validity of this hypothesis with the citation of other academic works that reject the hypothesis or have found it unsubstantiated in laboratory tests. 14 As indicated, we do not wish to weigh the empirical evidence on this claim and, indeed, feel less than fully qualified to do so.
‘It is one of the happy incidents of the federal system that a single courаgeous State may, if its citizens choose, serve as a laboratory; and try novel social and economic experiments without risk to the rest of the country.’ Mr. Justice Brandeis, dissenting in New State Ice Co. v. Liebmann,285 U.S. 262 , 311,52 S.Ct. 371 , 386,76 L.Ed.2d 747 .
Fay v. New York,
B. The Prosecutor’s Refusal to Enter Into the Stipulation
Our inquiry does not end here, however, for the defendant also challenges the refusal of the prosecutor to enter into the stipulation. An implicit aspect of the
Stanislawski
conditions, as construed in the decisions of the Wisconsin Supreme Court in the instant case and in
Lhost v. State,
Thus, the prosecutor in the instant case gave no reason for refusing to enter into the offered stipulation. The effect of this refusal was to preclude the defendant’s use of the results of a polygraph test under any circumstances. Permitting this kind of refusal invites an abuse of the stipulation requirement. Prosecutors occupy an adversarial role and have adversarial responsibilities. Such untrammeled authority to exclude, without explanation, exculpatory defense evidence must be scrutinized carefully. The potential for abuse of an unrestricted veto is a particular problem in cases like the instant one where the prosecution has a relatively weak case and polygraph еvidence might justifiably alter the outcome. 15
The sole justification that we can perceive for giving the prosecutor the power to exclude polygraph evidence is to interpose an additional check on the determination that polygraph evidence is sufficiently trustworthy and capable of evaluation by the jury to be admitted in a particular case.
16
Such an additional check can certainly serve valid purposes. For example, the prosecutor may properly refuse to enter into a stipulation because the defendant proposed to use an unqualified examiner or to take the test under conditions not conducive to reliable results. The prosecutor might also justifiably refuse to enter into a stipulation if he has reason to believe that the defendant is “polygraph proof” because he had managed to evade detection of deceptive responses in a previous examination. The right of an accused to present evidence in his defense must still yield to “established .rules of procedure and evidence designed to assure both fairness and reliability in the ascertainment of guilt or innocence.”
Chambers,
Although the prosecutor here stated no reasons for his refusal to stipulate to the
*465
examination, the Wisconsin Supreme Court felt that the prosecutor’s refusal was justified because it prevented “collateral” issues surrounding the testimony of an examiner as to the results of a polygraph examination from overly complicating the jury’s decision. But, given the facts of the instant case, such a rationalization is simply not persuasive. This was a quite simple case with only one prosecution witness. The collateral issues attendant on the introduction of polygraph evidence would thus seem not to have been too confusing or complex for the jury to digest while, at the same time, retaining their focus on the direct evidence of the defendant’s involvement in the crime. The critical importance of the credibility determination in this case also renders the polygraph evidence so material that the very limited potential for confusion from the use of the defendant’s polygraph evidence should not be sufficient to block the use of this evidence for unstated and potentially inappropriate reasons. While Wisconsin has reached a contrary conclusion on this question, its judgment as to the precise contours of the stipulation process must still satisfy the Constitution.
See Galloway v. Brewer,
We must stress, however, that ultimately our decision is closely linked to the peculiarities of the Wisconsin stipulation rule. The Wisconsin rule is not
solely
a “consent” rule whereby the parties waive the benefit of a rule of evidence barring the introduction of polygraph evidence.
19
Stipulation rules based on consent principles reflect state judgments that polygraph evidence is too unreliable or too capable of misinterpretation to be admitted at trial but recognize thаt the parties may waive their evidentiary objections.
20
See State v. McNamara,
In essence, therefore, we find the prosecutor’s veto of the test in the instant case to be constitutionally impermissible because he stated no reasons which might be reviewed by the trial court. From all that appears, he was acting solely for tactical reasons in the belief that a test would not be helpful to his case. If the prosecutor refuses and states reasons, it then becomes the duty of the cоurt to determine whether the reasons offered rise above the purely tactical considerations present in a given case. Certainly, no proper state purpose is served by allowing a purely tactical veto power. We do not hold that Wisconsin may not absolutely refuse to admit polygraph evidence, or that a judge may not refuse to admit polygraph evidence in a particular case; we merely hold that it offends due process for the prosecutor, as an adversary, to exercise an unrestricted veto — unrelated on the record to any reasons appropriate to the stipulation requirement — over the significantly exculpatory polygraph evidence in the instant case.
We therefore hold that the refusal of the prosecutor to enter into the stipulation offered here may have violated the petitioner’s right to be tried in accordance with the due process clause. We remand this action to the district court, however, for further proceedings to assess whether the prosecutor had valid reasons for refusing to enter into the stipulation offered by the defendant. If the refusal was unjustified, 21 the petition for writ of habeas corpus should be granted. 22
REVERSED AND REMANDED.
Notes
. The Wisconsin court adopted the fоrmulation of the Arizona Supreme Court set out in
State v. Valdez,
. The State asserted at oral argument that petitioner had somehow waived his due process claim by failing to offer the testimony at trial or by failing to seek an order from the trial judge compelling the prosecutor to enter into the stipulation. The State raised these same alleged procedural irregularities before the Wisconsin Supreme Court. It is clear, however, that petitioners due process claim is properly before us because the state courts reached the merits of the due process claim.
County Court of Ulster County v. Allen,
. U.S.Const. amend. VI. This right applies to the states through the due process clause of the fourteenth amendment.
Washington v. Texas,
. Westen,
The Compulsory Process Clause,
73 U.Mich.L.Rev. 71, 156 (1973).
See also United States v. Nixon,
. In
Hughes v. Mathews,
We would, however, reach the same result with respect to the one infirmity that we find if we testеd the relevance and competence of the excluded polygraph evidence using the
Hughes
approach. Testimony as to the results of a polygraph examination conducted pursuant to a
Stanislawski
stipulation is both relevant and competent evidence under Wisconsin law.
Lhost v. State,
. Defendant has framed his challenge to the Wisconsin stipulation rule in broad due process terms, as was done by the defendant in Chambers, and we shall deal with his contentions in that way. The more correct approach is, however, to analyze the questions under the sixth amendment right to compulsory process although the due process standards are similar if not identical. Westen, Confrontation and Compulsory Process: A Unified Theory of Evidence for Criminal Cases, 91 Harv.L.Rev. 567, 607-08 (1978).
.
E. g.,
Kubis,
Comparison of Voice Analysis and Polygraph as Lie Detection Procedures,
3 Polygraph 1, 39 (1974); Burkey,
The Case Against the Polygraph,
51 A.B.A.J. 833, 856 (1965). By comparison the evidence unconstitutionally excluded in
Washington v. Texas
or
Webb
v.
Texas,
. See, e. g., Wicker, The Polygraph Truth Test and the Law of Evidence, 22 Tenn.L.Rev. 711, 713 (1953) (75 to 80% accurate — 15 to 20% inconclusive — 5% error).
. The potential that the jury will improperly evaluate polygraph evidence presents the more important of the two objections. The degree of reliability goes more to the weight of the evidence. Certainly, as noted by the Eighth Circuit, polygraph examinations are reliable enough to pass the usual threshold for “relevant” evidence.
United States v. Oliver,
. The court in
Frye
ruled that scientific evidence, suсh as the results of a polygraph examination, may not be admitted into evidence unless the test has achieved “general acceptance” in its field.
Frye,
. For example, the test given in Frye measured only blood pressure while the modern polygraph test records changes in blood pressure, respiration, pulse and galvanic skin response. Even the results of the technologically rudimentary polygraph test held inadmissible in Frye, however, were reliable enough to correctly show that an innocent man had been wrongly convicted of murder. Wicker, The Polygraph Truth Test and the Law of Evidence, 22 Tenn.L.Rev. 711, 715 (1953).
.
United States v. Ridling,
. Defendant claims that the Wisconsin rule does not require that the stipulation be entered into prior to the defendant’s submission to the test. This is contradicted, however, by a consistent series of comments on this point in the Wisconsin opinions on polygraph evidence.
State v. Schlise,
. Tarlow,
Admissibility оf Polygraph Evidence in 1975: An Aid in Determining Credibility in a Perjury-Plagued System,
26 Hastings L.J. 917, 959 & n.207 (1975); D. Raskin, G. Borland and J. Podlesny,
Validity and Reliability of Detection of Deception
at 21 (National Institute of Law Enforcement and Criminal Justice, June 1978). These works do not completely refute the thesis underlying the pre-test stipulation. Tarlow actually notes in another portion of his article that stipulations may indirectly increase the reliability of polygraph tests by leading the parties to agree on competent examiners.
. We must stress that there was no allegation of bad faith on the part of the prosecutor in the instant case. The Wisconsin Supreme Court in fact noted in its unpublished per curiam opinion that the prosecutor had acted in good faith. Good faith is not necessarily the issue, however. The issue is whether the prosecutor’s adversarial role, and the tactical decisions attendant to it, are constitutionally compatible with an unexplained and unreviewable veto power.
. In
Stanislawski,
the Wisconsin Supreme Court has made the judgment, which we find reasonable, that in most cases properly instructed juries will be able to accord polygraph evidence secured from stipulated examinations its appropriate weight in the trial process. Pri- or to
Stanislawski,
Wisconsin did not allow even the results of
stipulated
polygraph evidence.
LeFevre v. State,
. In other circumstances, valid state interests unrelated to the integrity of the trial process might be sufficient to prevent the use of evidence offered by the defendant.
See Washington v. Texas,
. We are not apparently confronted here with the potential case of rejection of all polygraph evidence in all cases by the district attorney of a particular county. Such a situation would not appear to involvе refusals by the prosecutor to stipulate “for purely tactical reasons”— the subject of this opinion. Nonetheless, this sort of practice may raise both due process and equal protection problems.
But see State v. Mendoza,
If any district attorney in [Wisconsin] is not convinced of the reliability of polygraph testing, and not similarly convinced of the qualifications of the examiner named to do the testing, there will be and can be no polygraph test results admitted in criminal cases in his county while he is district attorney. All he has to do is refuse to stipulate to the test being given and the results of the test admitted. If he wants the door lockеd, he can lock it.
. Admittedly, the Wisconsin rule does have some consensual elements.
See State v. Craft,
. A state with a “consent” stipulation requirement would be able to assert, as a justification for the exclusion of the evidence, its determination that polygraph evidence is too likely to be misused by the jury. The same justification would be available to support a state rule that absolutely barred the use of polygraph evidence. Of course, we express no opinion on whether this justification would be sufficient to sustain the total exclusion of polygraph testimony. We note these alternatives only to point out the limited scope of this decision.
. Even if the prosecutor argues that his refusal was based on otherwise justifiable reasons such as the failure of the defendant to propose using a qualified examiner, the district court should grant the writ of habeas corpus unless it determines that the defendant would have been unable to cure the objection had he been informed of it prior to the Wisconsin trial. Obviously, this problem is unique to the instant case and would presumably not be involved in future cases.
. In the event that Wisconsin would subject petitioner to another trial, the results of the polygraph examination taken by the defendant after his conviction in state court would not be admissible under
Stanislawski
because therе was no pre-test stipulation. The defendant would have the opportunity, of course, to seek a stipulation as to a future polygraph examination. It would be unfair under the particular circumstances of this case, however, for the petitioner to be denied any opportunity to take a new polygraph examination pursuant to a
Stanislawski
stipulation because of the asserted effect of the earlier examination upon the reliability of the subsequent examination.
See Lhost v. State,
