Perhaps the most significant advantage of a prosecutor's decision to charge several defendants with conspiracy is that he may invoke special procedural rules that apply only to conspiracy cases. The major prosecutorial advantages of conspiracy are that it enables the prosecution to join all the conspirators for trial and to use out-of-court statements of each conspirator against all the others.
Joinder of conspirators for trial, coupled with relaxation of the rules of venue to allow the trial to take place wherever acts in pursuance of the conspiracy have occurred, is a measure designed to promote efficiency and convenience for courts, prosecutors, and witnesses. Where evidence pertaining to all defendants substantially overlaps, joinder avoids multiple trials involving the same issues and evidence. Even where the cases against various defendants are more distinct, the rule is helpful to the prosecution, since such constraining factors as prosecutorial resources and availability of witnesses often dictate a choice between joint trial and dismissal of charges against some of the conspirators.
In some situations, however, joinder may well yield not increased efficiency but rather a profusion of evidence, a multiplication of issues, and consequently much ambiguity and confusion. Moreover, joinder may substantially impair the rights of defendants. Where the jury is asked to hear a large amount of complex evidence, to remember which evidence applies to which defendant, and to make fine discriminations of individual guilt or innocence, there are several problems. First, there is serious danger of guilt by association. Second, conspirators may be hampered in their defense if optimal group strategy conflicts with the best course for an individual. Frequently, defendants attempt to cast the blame on someone else, and end up by convicting one another.
Loosened standards of admissibility of evidence prevail in a conspiracy trial. Contrary to the usual rule, in conspiracy prosecutions any declaration by one conspirator, made in furtherance of a conspiracy and during its pendency (hearsay), is admissible against each coconspirator. For example, in a conspiracy prosecution of a sheriff and a magistrate for extorting money from a coal company, an executive of the company testified that the sheriff told him that the magistrate was his (the sheriff's) agent in the extortion scheme and would pick up the extortion payments (United States v. Vinson, 606 F.2d 149 (6th Cir. 1979)). This testimony would normally be inadmissible because it is hearsay as against the magistrate—that is, it is testimony by the declarant (the witness) of what someone else said (the sheriff), offered to prove the truth of the matter asserted by that other person (that the magistrate was his agent in the scheme). However, since there was enough evidence of a conspiracy, the hearsay testimony was admitted as further evidence of the conspiracy and of the magistrate's participation in it.
The conventional reason for the exclusion of hearsay evidence is that such evidence is thought to be untrustworthy. The witness may report it poorly, either from faulty memory or from motive to misstate, and more importantly, the jury has no means of evaluating the credibility of the declaration unless the original declarant is available for cross-examination. Despite the unreliability of hearsay evidence, it is admissible in conspiracy prosecutions. Explaining this rule, Judge Hand said, "Such declarations are admitted upon no doctrine of the law of evidence, but of the substantive law of crime. When men enter into an agreement for an unlawful end, they become ad hoc agents for one another, and have made 'a partnership in crime.' What one does pursuant to their common purpose, all do, and as declarations may be such acts, they are competent against all" (Van Riper v. United States, 13 F.2d 961, 967 (2d Cir. 1926)).
Thus conspirators are liable on an agency theory for statements of co-conspirators, just as they are for the overt acts and crimes committed by their confreres. Although this theory may explain why co-conspirators are liable for each other's declarations, it does not really dispel the concerns of the hearsay rule regarding the trustworthiness of evidence. By requiring that the declarations be made within the scope of the agency relationship and with intent to advance the objectives of that relationship, the rule excludes declarations made before the agreement or after the termination of the conspiracy as peripheral and hence too unreliable. It thereby creates a nexus between the declarations and the criminal goals of the conspiracy, with whatever assurance of truth that might import.
However, the justification for circumventing the hearsay rule in conspiracy prosecutions is the practical need for such evidence—since conspiracy is a type of crime of which direct evidence is usually unavailable, the choice may be between admitting inferior evidence and admitting no evidence at all. Nevertheless the practice of admitting this evidence conflicts with the policy of the hearsay rule.
The problems encountered in applying the exception to the hearsay rule for co-conspirators were aptly described by justice Robert Jackson in his concurring opinion in Krulewitch v. United States, 336 U.S. 440, 453 (1949):
Strictly, the prosecution should first establish prima facie the conspiracy and identify the conspirators, after which evidence of acts and declarations of each in the course of its execution are admissible against all. But the order of proof of so sprawling a charge is difficult for a judge to control. As a practical matter, the accused often is confronted with a hodgepodge of acts and statements by others which he may never have authorized or intended or even known about, but which help to persuade the jury of existence of the conspiracy itself. In other words, a conspiracy often is proved by evidence that is admissible only upon assumption that conspiracy existed. The naive assumption that prejudicial effects can be overcome by instructions to the jury . . . all practicing lawyers know to be unmitigated fiction.