Approver, or PROVER, in the law of England, is a person who has been an accomplice in the perpetration of a crime, but who is admitted to give evidence against the prisoner. Formerly it was applied to one guilty of treason or felony who confessed on the promise of a pardon, conditional on the conviction of those whom he implicated.
The modern practice is to admit accomplices to give evidence for the prosecution, or, as it is said, to turn Queen's evidence, upon an implied promise of pardon, on condition of their making a full and fair confession of the whole truth. This assurance is generally given by the committing magistrate. The admission, however, of an accomplice to give evidence against his fellows requires the previous sanction of the judges of jail delivery, and a motion is usually made at the trial to the judge for leave to admit the accomplice for that purpose, which leave is not given unless the evidence appears to be necessary, and likely to be corroborated. The testimony of an accomplice is in all cases, however, regarded with just suspicion; and unless his statement is corroborated in some material part by unimpeachable evidence, the jury are usually advised by the judge to acquit the prisoner; and if the accomplice, after having confessed the crime, and being admitted as Queen's evidence, does not satisfy the condition on which he was so received by failing to give full information without equivocation, reservation, or fraud, he then forfeits all claim to protection, and may be tried, convicted, and punished on his own confession. The practice, however, is to direct an acquittal before the accomplice is examined, if he has been joined in the indictment.
The term in the law of Scotland analogous to that of approver is socius criminis, and the principles on which the socius is admitted, and on which his evidence is left to the jury, are the same as in England. But the criminal courts in Scotland give absolute protection to the socius who has been examined, after proper warning from the judge that what he says cannot be used against him. This privilege is altogether independent of the prevarication or unwillingness with which the witness may give his testimony. Justice, indeed, may often be defeated by a witness retracting his previous disclosures, or refusing to make any confession after he is put into the box; but it would be much more put in hazard if the witness was sensible that his future safety depended upon the extent to which he spoke out against his associate at the bar. The only remedy, therefore, in such a case, is committal of the witness for contempt or prevarication, or indicting him for perjury, if there are sufficient grounds for any of these proceedings. Attention was called in parliament to the law of Scotland in this matter, in connection with the case of Jessie M'Lauchlan in 1863.