James Rohrer and John Bump, co-defendants below and appellants in this court, were convicted of possession with intent to distribute cocaine, in violation of 21 U.S.C. § 841(a), and conspiracy to distribute cocaine, in violation of 21 U.S.C. § 846. The district court denied their motions for a new trial and they filed timely appeals. We affirm.
I.
FACTS
The government’s case rested primarily on the testimony of Stephen Green, a figure in a Bay Area drug network, who testified against the defendants after the government agreed to seek a reduction in his sentence, imposed as a result of his earlier conviction on drug charges. He described seven sales of cocaine involving Rohrer and five involving Bump. The government introduced hotel and car rental receipts, airline tickets, and customs records to support Green’s testimony, but the only corroboration for the appellants’ involvement were records of long-distance calls made to Rohrer and Bump during the alleged conspiracy.
Appellants attacked Green’s credibility. Three defense witnesses testified to Green’s heavy use of drugs and its debilitating effects. On cross-examination Green admitted his past drug usage and to having experienced blackouts during the conspiracy period.
Another government witness, paid informant William Northcutt, testified against Rohrer in return for reduction of charges against him in another drug case. North-cutt, like Green, had a history of drug use and dealing. Northcutt stated that he had twice obtained cocaine from Rohrer and on other occasions negotiated unsuccessfully for drugs. The only corroboration for Northcutt’s charges were records of calls he made to Rohrer’s phone number during the conspiracy period. Rohrer countered Northcutt’s testimony by presenting against him a number of reputation witnesses, including his ex-wife and his sister.
Against Bump, the government introduced a weighing scale and several boxes of baggies seized from Bump’s home on November 2, 1981, after his arrest. This seizure came fifteen months after the last offense on which he was charged. The items tended to support Green’s charge that Bump kept a scale and baggies at home to use in distributing cocaine.
II.
QUESTIONS ON APPEAL
Rohrer and Bump first raise a variety of objections to jury instructions. Next they object to admission of Green’s cooperation agreement with the government and of a sketch he drew of his drug distribution network, as well as to the exclusion of expert testimony on the effects of drug usage. They also assert that the court abused its discretion in striking post-trial affidavits that they feel demonstrate juror misconduct. Bump, on his own behalf, contends that the trial court abused its discretion in admitting the property seized in his home and in refusing to grant a limiting instruction when it ordered a rereading of testimony that pertained only to Rohrer. Finally, both Rohrer and Bump argue that even if this court holds harmless each of the alleged errors in isolation, the cumulative impact of these errors in a case resting largely upon uncorroborated accomplice testimony requires reversal. We find none of these arguments persuasive.
III.
JURY INSTRUCTIONS
Jury instructions must be analyzed in the context in which they are given — as part of the whole trial.
United States v. Abushi,
*432 Appellants have not made that showing here. We have reviewed the jury instructions in their totality and are convinced that each of appellants’ complaints stems from a failure to consider the full instructions. All of the instructions but one are sufficiently free from error to preclude the need for specific mention.
Appellants’ most nearly meritorious contention concerns an instruction that appeared to identify Green’s credibility as the primary issue in this case.
1
An instruction that appears to reduce a criminal case to acceptance or rejection of a government witness’ testimony may impermissibly lead the jury to forget that the defendant cannot be convicted unless guilty beyond a reasonable doubt. This mistake, if not corrected, can effectively deprive the defendant of his right to a jury trial.
See, e.g., United States v. Oquendo,
IV.
EVIDENTIARY RULINGS
A court’s evidentiary rulings will be overturned only for abuse of discretion.
United States v. Patterson,
A. The Cooperation Agreement
Appellants argue that the admission of the “truthful testimony” portions of
*433
Green’s cooperation agreement with the government and the latter’s references to it at trial constituted impermissible vouching for Green. For support they cite
United States v. Roberts,
No such vouching occurred here. The government in no way put its prestige behind the witness; instead it was careful to ask the jury to look to the agreement to determine Green’s motives. And it did not “implicitly” point to evidence outside the record. Neither the passages appellants cite from the trial transcript nor the cases cited in Rohrer’s brief
3
support the claim of vouching. The court received the full text of the cooperation agreement into evidence only after appellants’ extensive impeachment of Green’s motives on cross-examination and their discussion of part of the agreement. There can be no doubt but that the agreement was relevant to material facts at issue and that the court acted correctly in permitting it to be introduced.
See United States v. Rubier,
B. The Prior Consistent Statements
The court admitted as a prior consistent statement a diagram Green had drawn of his distribution ring. Fed.R.Evid. 801(d)(1)(B). Green drew the diagram, which implicated Rohrer and Bump, just prior to signing his cooperation agreement. A prior consistent statement is admissible to rehabilitate a witness only if made before the witness has a motive to fabricate.
United States v. Rodriguez,
*434 Rohrer also objects to the admission of Northcutt’s 1979 discussion of Rohrer’s activities. These statements, unlike Green’s, were properly admitted. Northcutt made them while negotiating for a cooperation agreement in an earlier trial. The motive to fabricate expired when its purpose was served. The statements were introduced to rebut the charge that Northcutt’s present testimony was fabricated to encourage the government to agree to a cooperation agreement affecting this trial. The evil Rodriguez is designed to cure does not exist under these circumstances.
C. Expert Testimony
The refusal by the trial court to allow experts to testify on the effects of drug usage like Green’s was not improper. Although the credibility of a witness, unlike his competency, is a question for the jury, admission of expert psychiatric testimony bearing on credibility lies in the judge’s discretion,
United States v. Bernard,
V.
JUROR MISCONDUCT
Appellants presented the affidavits of two jurors and of two private investigators who conducted post-trial interviews with some jurors. The jurors’ affidavits testify to the jurors’ individual or collective thought processes; the investigators’ affidavits recount the same material second hand. Evidence concerning a jury’s deliberations or a juror’s reasoning are inadmissible to impeach a verdict. Fed.R.Evid. 606(b);
United States v. Freedson,
VI.
BUMP’S ALLEGATIONS
Three days after arresting Bump the government properly seized a scale with
*435
traces of cocaine and seven boxes of baggies in his home. The seizure came fifteen months after the last acts specified in one count against Bump and two and a half years after those in the other. The judge admitted the scale and the baggies as corroboration for Green’s testimony. A government chemist later testified that he had found traces of cocaine on the scale. Bump argues that his possession of these items was not sufficiently close in time to the conspiracy to be admissible as evidence of other crimes or acts. Fed.R.Evid. 404(b);
see United States v. Bailleaux,
We disagree. Possession of these items, even if considered as having been recently acquired, was admissible.
United States v. Uriarte,
Bump also criticizes the judge’s failure to give a limiting instruction when ordering the rereading of testimony that pertained only to Rohrer. Additional instructions are left to the discretion of the judge.
United States v. Tham,
VII.
CUMULATIVE EVIDENCE
Appellants also argue that even if we can find no single reversible error, the cumulative weight of a series of near misses ought to compel reversal in a case such as this. We disagree. While conviction on the uncorroborated testimony of a single accomplice makes a defendant more likely to be prejudiced by errors than if the government had a stronger case,
United States v. Berry,
AFFIRMED.
Notes
. Thus the instruction reproduced in note 1 supra continued:
If after making that determination — that’s what deciding the facts means. Is deciding what the evidence shows and what the credibility of the witnesses suggest to you — if after all of that you’re left with a reasonable doubt about the guilt or innocence of either of the defendants on any charge, it’s your duty to acquit on those charges. If, however, you conclude that there is no doubt on any charge or charges, then as to those charges it would be your duty to convict.
. In
United States v. Carleo,
. The government, noting a split in the circuits over whether the existence of a prior motive to fabricate determines the admissibility or only the weight of the statement, argues that this circuit has not taken a position on this issue. It claims that
Rodriguez
did not decide that the statement
had
to be made before the motive to fabricate. The government misinterprets
Rodriguez,
which established a narrow exception to the hearsay rule for statements “where it affirmatively appears that the prior consistent statement was made at a time when the declar-ant had no motive to fabricate.”
. Appellants’ cases do not indicate otherwise. In
United States v. Hiss,
. The affidavits do not even support all of appellants’ charges. For instance, they demonstrate that the jury foreman had a fear of arrest and incarceration for his drug use but not that he had been incarcerated. And they do not show that the foreman communicated his beliefs about drug use to the jury. Another indication of the unreliability of appellants’ charges is that the foreman refused to sign an affidavit for appellants. We also do not consider the affidavit of counsel for Rohrer because we find his accusation that Green and Northcutt might have been imprisoned together amply refuted by the government’s affidavit.
. An indictment cannot be completely open-ended, see
United States v. Cecil,
. Bump’s other arguments against admission of these items do not merit discussion.
