We granted further appellate review in this case to consider the standard to be applied in ruling on a defendant’s motion for a new trial based on the failure of the prosecution to disclose exculpatory evidence in its possession.
At the defendant’s 1978 trial on charges of rape, kidnapping, unarmed robbery, mayhem, and assault and battery, identification of the assailant was the only contested issue. The police arrested the defendant near the crime scene in Cambridge within minutes after the attacker had left the victim. The Cambridge police took photographs of the defendant shortly after his arrest. The victim testified repeatedly that her attacker was clean-shaven. Another witness who identified the defendant as the person he’had seen near the crime scene testified several times that the man he had seen had no moustache. Police witnesses testified that the defendant did not have a moustache on the day of the crimes. The defendant’s wife testified, however, that at the time of the crimes the defendant did have a moustache.
*403
The defendant moved generally for the disclosure of exculpatory evidence. Although he and his counsel knew that photographs had been taken shortly after his arrest, the defendant never specifically requested their production. Certainly, the defense was not hindered by an inability to be specific in any request it might make for the photographs. In October, 1988, approximately ten years after his trial, the defendant wrote the record department of the Cambridge police department for a copy of the photographs taken of him on the night of his arrest. In circumstances not disclosed on the record, the defendant obtained them. The frontal photograph of the defendant’s face shows the defendant with a moustache. The photograph would have aided the defendant in the impeachment of the principal witnesses against him. The Commonwealth grants that the photographs were exculpatory
(Commonwealth
v.
Ellison,
Citing the rule stated in
Commonwealth
v.
Gallarelli,
We shall discuss (1) the reasons why the prosecution must depart from a totally adversary role in cases of this sort, (2) the interrelationship of common law and constitutional principles in the decision of such cases, (3) the role of defense counsel and the relationship of any omissions of defense counsel to the prosecution’s nondisclosures, (4) the significance, in deciding whether any error was reversible error, of the fact that the case was tried to a jury, and (5) the standard to be applied in deciding whether, in particular circumstances, a new trial may or should be ordered because admittedly exculpatory evidence was not provided to the defendant (and thus was not disclosed to the jury). Of all these concerns, the last is the most problematic because, on such a fact-based issue, a universal guide as to when prosecutorial error requires a new trial cannot be fashioned except in general terms and because the outcome of a nondisclosure case may depend on seemingly minor word differences in expressing the standard for measuring prejudice to a defendant’s case. 2
1.
The governing principles.
Due process of law requires that the government disclose to a criminal defendant
*405
favorable evidence in its possession that could materially aid the defense against the pending charges. The Supreme Court of the United States announced the prosecution’s constitutional obligation to disclose material, exculpatory evidence in
Brady
v.
Maryland,
In
United States
v.
Bagley,
There are several forces at work in prosecutorial nondisclosure cases. First, when the question arises posttrial, there is a public interest in the finality of judgments. New trials should not be granted except for substantial reasons. Second, prosecutors, who are agents of the State and often have access to information that defendants may not have, should be encouraged to disclose exculpatory evidence that in fairness defendants should have for their defense. Of course, a prosecutor cannot always know that a particular piece of evidence is or might be exculpatory.
5
A rule that encourages prosecutors
*407
to make pretrial disclosures of obviously or even arguably exculpatory material would not only promote fair trials
6
but would also help to avoid the difficulties of posttrial judicial review. Judges, therefore, should be sensitive to the allowance of motions for the disclosure of specific information claimed to be exculpatory. A prosecutor’s duty, however, extends only to exculpatory evidence in the prosecutor’s possession or in the possession of the police who participated in the investigation and presentation of the case. See
Commonwealth
v.
Daye,
2.
Common law and constitutional principles.
In deciding whether a defendant is entitled to a new trial because the prosecution failed to turn over exculpatory evidence, State judges are not limited to constitutionally based principles. The opinions of the Supreme Court of the United States that we have cited previously in this opinion are concerned with defendants’ Federal constitutional rights. Our opinions in this area have generally applied the Federal constitutional standards without stating explicitly whether the same standards apply under the State Constitution. Cf.
Commonwealth
v.
Daye,
There is no reason why the nondisclosure issue could not be advanced by a motion for a new trial to which the regular principles of Mass. R. Crim. P. 30 (b),
In dealing with common law decisions of a motion judge who was the trial judge, we have deferred in large measure to the judge’s views on motions for a new trial. Id. at 307. There is, therefore, a discretionary range in cases like this within which the trial judge may properly award a new trial, even if a new trial is not constitutionally required and even if we would not have granted a new trial on our own assessment of the record. We have not extended similar deference to the views of a judge who, as was the case here, acted on a new trial motion but was not the trial judge. We have, however, deferred to such a motion judge’s conclusions on testimony presented in association with a new trial motion. See Commonwealth v. Grace, supra. If the new trial claim is constitutionally based, this court will exercise its own judgment on the ultimate factual as well as legal conclusions.
In this case, we are in as good a position as the motion judge to assess the trial record. Id. The defendant argues only a violation of constitutional rights and does not assert any State common law right. We shall return to the question of common law relief in this case when we discuss the standard for measuring prejudicial error because we should not feel bound by a party’s election to assert only constitutional rights and thereby decline to raise plausible nonconstitutional grounds for relief.
*410 3. The role of defense counsel. In many instances of prosecutorial nondisclosure of exculpatory evidence, defense counsel may have failed in his obligation to provide adequate representation. As a general rule, the omissions of defense counsel (a) do not relieve the prosecution of its obligation to disclose exculpatory evidence and (b) may provide the defendant with an independent claim of an unconstitutional denial of the effective assistance of counsel.
In this case, the defendant, represented by new counsel, argued below and argues to us that his trial counsel negligently failed to move specifically for the production of the photographs and that that omission requires that he have a new trial. It is true that, for some unclear reason, the defendant moved only generally for the production of exculpatory evidence and not specifically for the production of the photographs. The motion judge did not order a new trial, however, on the ground of ineffective assistance of counsel. He had no need to, and did not, make findings or rulings concerning either the quality of defense counsel’s performance or the possible impact on the verdict of any failure of defense counsel to meet the constitutionally minimum level of competence. Defense counsel did argue to the jury that the prosecution’s failure to produce the photographs warranted an inference that they show that the defendant did have a moustache on the night of the crimes. That argument pales, however, in its likely impact on a jury when compared with the frontal photograph’s candid, objective, and hence irrefutable demonstration on the same subject. We shall not, and need not, pursue the ineffective assistance of counsel argument to its conclusion. Perhaps an adequate explanation why counsel did not move specifically for the production of the photographs lies outside the record. 9
*411 4. The role of the jury. In dealing with a claimed failure of the prosecution to disclose exculpatory evidence to a defendant, this court differs from the Supreme Court of the United States in another significant way. In our judgment, the judge’s task is to decide what effect the omission might have had on the jury. The law of the Commonwealth thus preserves, as well as it can in the circumstances, the defendant’s right to the judgment of his peers. The issue, we think, is not what, if any, impact the late disclosed evidence has on the judge’s personal assessment of the trial record. After all, the goal is to determine what would have happened if the prosecution had fulfilled its pretrial duty to disclose the exculpatory evidence and the jury had seen the photographs. 10
The Supreme Court of the United States, on the other hand, seemingly assigns to the judge the task of assessing the consequences of the prosecutor’s omission. See
United States
v.
Agurs,
*412
5.
The standard of
review.- We come finally in our analysis to the definition of that degree of prejudice or possible prejudice that warrants or requires the granting of a new trial because the prosecution improperly failed to deliver exculpatory evidence to the defense. We have already noted (in part 2 above) that we grant special deference to a decision on a motion for a new trial of the judge who was also the trial judge. We are not concerned here with the standard of review an appellate court would apply in such a situation. We are also not concerned here directly (but see note 9 above) with the standard to be applied when the prosecution has denied the defendant specifically requested exculpatory evidence. In that situation, a defendant need only demonstrate that a substantial basis exists for claiming prejudice from the nondisclosure. See
Commonwealth
v.
Gallarelli, supra
at 20-21;
Commonwealth
v.
Wilson,
Our concern is with the standard to be applied when the defendant has made no request or, as here, has made only a general .request for exculpatory evidence. The Supreme Court opinions refer to the “materiality” of the exculpatory evidence. In this opinion we have generally avoided using the word “material” to describe the importance of evidence to the new trial claim. If the evidence is exculpatory, it is also material in the general evidentiary sense that it is material (and relevant) to an issue in the case. In its opinions in the Agurs and Bagley cases, the Supreme Court has not used the word- “material” in its ordinary, evidentiary sense. To that Court, evidence is material in the sense that its absence mattered or might have mattered in the trial of the case. We prefer to refer in standard common law terms to the degree of prejudicial effect, rather than materiality, of the improper nondisclosure of exculpatory evidence.
Our common law standard for the granting of a new trial and our standard for measuring the consequences of the ineffectiveness of defense counsel provide the appropriate guide
*413
for determining whether the nondisclosure of evidence requires the allowance of a new trial motion. See
Commonwealth
v.
Daye,
The application of the common law standard that we have adopted requires a careful review of the trial court proceedings to determine whether there is a substantial chance that the jury might not have reached verdicts of guilty if the undisclosed evidence had been introduced in evidence. 11 In terms of the case before us, the issue is whether the defendant was denied a substantial factual basis for contending to the jury that the victim misidentified him.
We find this standard to be clearer and fairer than the Agurs general request test, which is whether the undisclosed evidence creates a “reasonable doubt that did not otherwise exist.” United States v. Agurs, supra at 112. Our standard recognizes the role of the jury. It also does not mandate, as a *414 condition of the granting of a new trial, that a judge conclude that a reasonable doubt would have been created if the undisclosed evidence had been before the jury. It is enough that, on a full and reasonable assessment of the trial record, the absent evidence would have played an important role in the jury’s deliberations and conclusions, even though it is not certain that the evidence would have produced a verdict of not guilty.
If the undisclosed evidence is cumulative, if it lacks credibility, or if, in an over-all assessment, it does not carry a measure of strength in support of the defendant, the failure to disclose that evidence does not warrant the granting of a new trial. If, however, the undisclosed evidence is more credible than any other evidence on the same factual issue and bears directly on a crucial issue before the jury, such as the credibility of an important prosecution witness, that evidence would have been a real factor in the jury’s deliberations, and its presence before the jury might have accomplished something material for the defense.
6. Conclusion. From what we have said, it follows that the nondisclosure of the photographs requires the ordering of a new trial. The frontal photograph, of the defendant’s face was authentic and incontestably showed that the defendant had a moustache. The photograph was made by the Cambridge police and, because of its unquestionable weight and reliability, was not “merely cumulative” of the testimony of the defendant’s wife that he had a moustache on the day of the crimes. Although the fact that the defendant had a moustache does not disprove his commission of the crimes, it casts doubt on the identification of the defendant as the attacker because it demonstrates that the victim and other witnesses were undeniably wrong in one aspect of their sworn identification testimony. Consequently the photograph showing the defendant with a moustache would have been a real factor in the jury’s *415 deliberations, and its introduction in evidence might have accomplished something material for the defense.
Order allowing motion for new trial affirmed.
Notes
The defendant generally fit the victim’s description of her attacker, particularly as to the clothes and belt that her attacker was wearing. The heel from a boot of the defendant was found near the crime scene. Shortly after the incident, a man generally fitting the defendant’s description, and identified by one witness as the defendant, was seen running toward a van owned by the defendant.
Although the motion judge correctly described the moustache as “trimmed, full, [and] plainly obvious,” it was not prominent.
For example, the rule previously stated to be applied to the facts of this kind of case is whether the undisclosed evidence
creates
a reasonable doubt that did not otherwise exist.
Commonwealth
v.
Gallarelli,
A third aspect involves situations in which the prosecution knew or should have known that perjurious testimony was offered and did not disclose that fact. There, a new trial is required “if there is any reasonable likelihood that the false testimony could have affected the judgment of the jury.” United States v. Agurs, supra at 103.
In
Davis
v.
Alaska,
In this case, the prosecutor (who is not appellate counsel) must have been, and certainly should have been, aware before trial that the photographs existed.
During a pretrial suppression hearing held on October 10, 1978, the victim testified that her attacker did not have a moustache, had no growth on his face, and was clean-shaven. A police officer testified that he did not recall any moustache, and the man who identified the defendant as the man he had seen near the crime scene and the defendant’s van testified that the man had no moustache. The defendant had a moustache at the time of the hearing. The trial commenced on October 12. On October 16, a police officer testified on cross-examination by the defendant that the defendant did not have a moustache at the time of the crimes, that he had seen a “mug shot” of the defendant taken perhaps the day after the *407 crimes, and that “[o]ne of the night men that work on the third platoon have [síc] it.” On October 17, the defendant’s wife testified that the defendant had a moustache on the day of the crimes.
The prosecution’s nondisclosure of the photographs obviously made it easier for the Commonwealth to meet its burden of proof.
The rule that the Supreme Court stated in the
Bagley
case on its face makes the prosecution’s state of mind irrelevant in deciding whether due process of law requires a new trial for any defendant from whom the prosecution improperly has withheld exculpatory evidence. This change is interesting in light of that Court’s conclusion three years later in
Arizona
v.
Youngblood,
To be sure, in the case now before us, a claim that at the time of trial the defense did not know of the photographs (and hence now could pass the test of newness) seems impossible to sustain (and has not been made). See
Commonwealth
v.
Grace,
If counsel’s performance did fall measurably below accepted standards, the detrimental effect of that omission would be more easily established in this case than in the usual case of a claimed ineffectiveness of counsel. In this case, the harm caused by any negligence of defense counsel would be measured by the difference between the standard of review that would have been applied to the withheld evidence, if defense counsel had made a *411 specific request, and the standard that is applied when defense counsel made no specific request. That standard in Massachusetts is more favorable to a defendant when defense counsel has made a specific rather than a general request for exculpatory evidence that the prosecution failed to disclose.
If we were dealing with a claim of ineffective assistance of counsel based on the failure of defense counsel to present certain evidence to the jury, our approach also would be to consider the impact of that omission on the jury.
We see no need to express this standard in State constitutional terms. Because it is more favorable to defendants than the Federal Constitutional standard, the common law standard will be the controlling one. Cf.
Commonwealth
v.
Charles,
